Flow-control structures: Difference between revisions
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x=oopsay+1 |
x=oopsay+1 |
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novalue: say |
novalue: say |
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say '───────────────────────────error!─────────────────────────────────' |
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say '*** error! ***' |
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say 'that reference to oopsay (above) will cause control to get here.' |
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say |
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parse source . . fid . |
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say; say 'REXX raised a NOVALUE error in program:' fid |
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say |
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say ' |
say; say 'it occurred on line' sigl |
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say; say 'the REXX statement is:' /*put it on separate line.*/ |
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say |
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say; say 'which code:' condition('C') "error" |
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say |
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say; say "Moral: shouldn't do that." |
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'''output''' |
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...</lang> |
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<pre> |
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───────────────────────────error!───────────────────────────────── |
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that reference to oopsay (above) will cause control to get here. |
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REXX raised a NOVALUE error in program: D:\flow_sig.rex |
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it occurred on line 20 |
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x=oopsay+1 |
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which code: NOVALUE error |
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REXX variable: OOPSAY |
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Moral: shouldn't do that. |
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</pre> |
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=={{header|Ruby}}== |
=={{header|Ruby}}== |
Revision as of 00:51, 19 December 2013
![Task](http://static.miraheze.org/rosettacodewiki/thumb/b/ba/Rcode-button-task-crushed.png/64px-Rcode-button-task-crushed.png)
You are encouraged to solve this task according to the task description, using any language you may know.
These are examples of control structures. You may also be interested in:
In this task, we document common flow-control structures. One common example of a flow-control structure is the goto construct. Note that Conditional Structures and Loop Structures have their own articles/categories.
6502 Assembly
JMP
The jump instruction immediately jumps to any address: <lang 6502asm> JMP $8000 ;immediately JuMP to $8000 and begin executing ;instructions there.</lang> The indirect jump instruction immediately jumps to the address contained in the address: <lang 6502asm> JMP ($8000) ;immediately JuMP to the address in memory locations ;$8000 and $8001</lang>
JSR
The jump to subroutine instruction pushes the address of the next instruction minus one onto the stack and jumps to any address: <lang 6502asm> JSR $8000 ;Jump to SubRoutine</lang> A return from subroutine instruction pops the return address off the stack, adds one, and jumps to that location: <lang 6502asm> RTS ;ReTurn from Subroutine</lang>
BRK
A break instruction causes a non-maskable interrupt (setting the interrupt flag), pushes the current program counter address plus one onto the stack, pushes the flags onto the stack, then jumps to the address in the break vector (commonly at $FFFE and $FFFF): <lang 6502asm> BRK ;BReaK</lang> The return from interrupt instruction pops the flags off the stack, pops the return address off the stack, adds one, and jumps to that location: <lang 6502asm> RTI ;ReTurn from Interrupt</lang>
Ada
goto
<lang ada><<Top>>
Put_Line("Hello, World"); goto Top;</lang>
exit
Exit is used to break out of loops. Exit can be used with a label to break out of an inner loop to an outer loop and its enclosing outer loop <lang ada>Outer:
loop -- do something loop -- do something else exit Outer; -- exits both the inner and outer loops end loop; end loop;</lang>
asynchronous transfer of control
A sequence of operation can be aborted with an asynchronous transfer of control to an alternative: <lang ada>select
delay 10.0; Put_Line ("Cannot finish this in 10s");
then abort
-- do some lengthy calculation ...
end select;</lang> The alternative can be a delay statement or else an entry point call followed by a sequence of operations. The statement blocks at the delay or entry call and executes the sequence of the operation introduced by then abort. If blocking is lifted before completion of the sequence, the sequence is aborted and the control is transferred there.
ALGOL 68
See also Exceptions to see how ALGOL 68 handles transput events.
One common use of a label in ALGOL 68 is to break out of nested loops.
<lang algol68>(
FOR j TO 1000 DO FOR i TO j-1 DO IF random > 0.999 THEN printf(($"Exited when: i="g(0)", j="g(0)l$,i,j)); done FI # etc. # OD OD; done: EMPTY
);</lang>
Multi way jump using labels and EXIT to return result
<lang algol68>STRING medal = (
[]PROC VOID award = (gold,silver,bronze);
award[ 1 + ENTIER (random*3)];
gold: "Gold" EXIT silver: "Silver" EXIT bronze: "Bronze"
);
print(("Medal awarded: ",medal, new line));</lang>
Another use is to implement finite state machines
<lang algol68>STRING final state = (
INT condition; PROC do something = VOID: condition := 1 + ENTIER (3 * random); state1: do something; CASE condition IN state 1, state 2 OUT state n ESAC EXIT state 2: "State Two" EXIT state n: "State N"
); print(("Final state: ",final state, new line));</lang>
ALGOL 68G implements a Refinement Preprocessor to aid with top down code development
<lang algol68># example from: http://www.xs4all.nl/~jmvdveer/algol.html - GPL # determine first generation; WHILE can represent next generation DO calculate next generation;
print next generation
OD.
determine first generation:
INT previous := 1, current := 3.
can represent next generation:
current <= max int - previous.
calculate next generation:
INT new = current + previous; previous := current; current := new.
print next generation:
printf (($lz","3z","3z","2z-d$, current, $xz","3z","3z","2z-d$, previous, $xd.n(real width - 1)d$, current / previous)).</lang>
Sample output:
Exited when: i=13, j=53 Medal awarded: Gold Final state: State Two 4 3 1.33333333333333 7 4 1.75000000000000 11 7 1.57142857142857 etc...
AutoHotkey
<lang AutoHotkey>MsgBox, calling Label1 Gosub, Label1 MsgBox, Label1 subroutine finished Goto Label2 MsgBox, calling Label2 ; this part is never reached Return
Label1:
MsgBox, Label1
Return
Label2:
MsgBox, Label2 will not return to calling routine
Return</lang>
AWK
The [awk] programming language is data driven. However, Awk has break and continue for loop control, as in C.
<lang awk>$ awk 'BEGIN{for(i=1;;i++){if(i%2)continue; if(i>=10)break; print i}}' 2 4 6 8</lang>
BBC BASIC
BBC BASIC has GOSUB and GOTO but they are deprecated. <lang bbcbasic> GOSUB subroutine
(loop) PRINT "Infinite loop" GOTO loop END (subroutine) PRINT "In subroutine" WAIT 100 RETURN</lang>
C
goto
One common use of goto in C is to break out of nested loops. <lang c>int main() {
int i,j; for (j=1; j<1000; j++) { for (i=0; i<j, i++) { if (exit_early()) goto out; /* etc. */ } }
out:
return 0;
}</lang>
C++
goto
<lang cpp>#include <iostream>
int main() {
LOOP: std::cout << "Hello, World!\n"; goto LOOP;
}</lang>
Note that "goto" may also be used in conjunction with other forms of branching.
Exceptions
Exceptions are a way to give control back to a direct or indirect caller in case of an error. Note that throwing exceptions is usually very expensive, therefore they generally should only be used for exceptional situations. <lang cpp>#include <iostream>
- include <ostream>
void foo() {
std::cout << "Going to throw an exception.\n"; throw 7; // almost any object can be thrown, including ints std::throw << "This output will never execute.\n";
}
void bar() {
std::cout << "Going to call foo().\n"; foo(); std::cout << "This will be skipped by the exception coming from foo.\n";
}
void baz() {
try // everything thrown from inside the following code block { // will be covered by the following catch clauses std::cout << "Going to call bar().\n"; bar(); std::cout << "This will be skipped by the exception coming from foo.\n"; } catch(...) // a simple catch-all, but doesn't give access to the thrown exception { std::cout << "An exception occured. I'll just throw it on.\n"; throw; // without an argument, the caught exception is re-thrown } std::cout << "This will not be executed due to the re-throw in the catch block\n";
}
void foobar() {
try { baz(); } catch(char const* s) { std::cout << "If foo had thrown a char const*, this code would be executed.\n"; std::cout << "In that case, the thrown char const* would read " << s << ".\n"; } catch(int i) { std::cout << "Caught an int, with value " << i << " (should be 7).\n"; std::cout << "Not rethrowing the int.\n"; } catch(...) { std::cout << "This catch-all doesn't get invoked because the catch(int) above\n" << "already took care of the exception (even if it had rethrown the\n" << "exception, this catch-all would not be invoked, because it's\n" << "only invoked for exceptions coming from the try block.\n"; } std::cout << "This will be executed, since the exception was handled above, and not rethrown.\n";
}
int main() {
try { foobar(); } catch(...) { std::cout << "The main function never sees the exception, because it's completely handled\n" << "inside foobar(). Thus this catch-all block never gets invoked.\n"; }
}</lang>
COBOL
CALL
This transfers control to a subprogram, with control eventually being returned (provided the subprogram does not terminate the program).
CHAIN
This transfers control to the subprogram specified with no return of control. CHAIN
is a non-standard extension
created by Micro Focus and is found in Visual COBOL.
EXIT
EXIT
takes a variety of clauses:
PARAGRAPH
/PERFORM
/SECTION
: Control will be transferred immediately past the end of those blocks.PERFORM CYCLE
: The current iteration of thePERFORM
will stop, and control will go to the clauses of thePERFORM
which will determine if another iteration is to run.PROGRAM
: If used in a called function, execution of it is stopped and control returned to the calling program. If it is used in the main program, the statement is non-functional and ignored.
The PERFORM
/PERFORM CYCLE
clauses can only be used inside of an inline PERFORM
statement.
GOBACK
If used in a called function, control will transferred back to the calling program. If it is used in the main program, the program will be terminated.
GO TO
Basic use: <lang cobol> PROGRAM-ID. Go-To-Example.
PROCEDURE DIVISION. Foo. DISPLAY "Just a reminder: GO TOs are evil."
GO TO Foo .</lang>
A GO TO
can take a DEPENDING ON
clause which will cause program flow to go to a certain paragraph/section depending on a certain value.
<lang cobol> GO TO First-Thing Second-Thing Third-Thing
DEPENDING ON Thing-To-Do
- *> Handle invalid thing...</lang>
The previous example is equivalent to: <lang cobol> EVALUATE Thing-To-Do
WHEN 1
- *> Do first thing...
WHEN 2
- *> Do second thing...
WHEN 3
- *> Do third thing...
WHEN OTHER
- *> Handle invalid thing...
END-EVALUATE</lang>
INVOKE
The INVOKE
statement is used to transfer control to a method of a class/factory object.
PERFORM
The PERFORM
statement can be used to transfer program flow to the specified sections/paragraphs in the subprogram, with control being returned when the end of the last paragraph/section or a relevant EXIT
statement is reached.
<lang cobol> PROGRAM-ID. Perform-Example.
PROCEDURE DIVISION. Main. PERFORM Moo PERFORM Display-Stuff PERFORM Boo THRU Moo
GOBACK .
Display-Stuff SECTION. Foo. DISPLAY "Foo " WITH NO ADVANCING .
Boo. DISPLAY "Boo " WITH NO ADVANCING .
Moo. DISPLAY "Moo" .</lang>
- Output:
Moo Foo Boo Moo Boo Moo
STOP RUN
This immediately terminates the program.
D
goto
<lang d>import std.stdio;
void main() {
label1: writeln("I'm in your infinite loop."); goto label1;
}</lang>
Exceptions
D supports the try/catch/finally mechanism: <lang d>import std.stdio;
class DerivedException : Exception {
this(string msg) { super(msg); }
}
void main(string[] args) {
try { if (args[1] == "throw") throw new Exception("message"); } catch (DerivedException ex) { // We never threw a DerivedException, so this // block is never called. writefln("caught derived exception %s", ex); } catch (Exception ex) { writefln("caught exception: %s", ex); } catch (Throwable ex) { writefln("caught throwable: %s", ex); } finally { writeln("finished (exception or none)."); }
}</lang>
Scope guards
In a complex function, you might need to do cleanup in case of an exception, but it gets out of hand if there are many initialization steps that could fail. Scope guards offer a simplified syntax for try/finally.
There are three scopes you can listen for: exit, which is called unconditionally; failure, which is called if you leave the function via an exception; and success, which is called if you return from the function normally. A statement inside a scope block is only executed if execution reaches the scope block.
For instance: <lang d>import std.stdio;
void main(string[] args) {
scope(exit) writeln("Gone");
if (args[1] == "throw") throw new Exception("message");
scope(exit) writeln("Gone, but we passed the first" ~ " chance to throw an exception.");
}</lang>
If the exception is thrown, then the only text that is written to the screen is "gone". If no exception is thrown, both calls to writeln occur.
scope(failure) and scope(success) work similarly.
E
E does not have goto. The only primitive flow control construct which is not a loop, conditional, or exception is escape, or ejectors.
The basic syntax is
escape ej { ...body... }
Within body variable ej then contains a one-argument function (an ejector) which, if called, immediately returns the provided value from the escape block.
This is a limited form of continuation (it cannot be used after the escape block exits).
Loop break, loop continue, and return-from-middle-of-function are all defined in terms of this basic construct.
Erlang
The one Erlang flow control structure, apart from the ones excluded in the task description, is Exceptions
Forth
CATCH-THROW
Some Forth implementations have goto, but not the standard. It does have an exception mechanism. <lang forth>: checked-array
CREATE ( size -- ) DUP , CELLS ALLOT DOES> ( i -- a+i ) 2DUP @ 0 SWAP WITHIN IF SWAP 1+ CELLS + ELSE 1 THROW THEN ;
8 checked-array myarray
- safe-access ( i -- a[i] )
['] myarray CATCH 1 = IF ." Out of bounds!" 0 THEN ;</lang>
Go
Not covered here::
- Structures involving for, if, switch, continue, break, fallthrough, or panic. As mentioned in the task description, these structures are covered in other tasks.
- Short-circuit operators. These can be considered flow-control structures, but are also covered in other tasks.
- Flow-control functions in the standard library. Many of these are important, but covering them seems beyond the scope of the task.
Goto
Go has goto and labels. The following is an infinite loop: <lang go>func main() { inf:
goto inf
}</lang> Gotos can jump forward or backward within a function but they have some restrictions. They cannot jump into any block from outside the block, and they cannot cause any variable to come into scope.
Function call
Function call works as it does in most languages, transferring execution to to the called function, and returning execution to the following statement upon return from the called function.
The return statement returns from a function. Any function can use a return statement. Functions with return values can only return with a return statement. Functions without return values can return by “falling off” the end of the function.
The defer statement sets a function or method to be executed upon return from the enclosing function. This is useful when a function has multiple returns. The classic example is closing a file: <lang go>import "os"
func processFile() {
f, err := os.Open("file") if err != nil { // (probably do something with the error) return // no need to close file, it didn't open } defer f.Close() // file is open. no matter what, close it on return var lucky bool // some processing if (lucky) { // f.Close() will get called here return } // more processing // f.Close() will get called here too
}</lang>
Goroutines
Goroutines are Go’s take on lightweight threads. A goroutine is started with a go statement, which looks like “go” preceding a function call. When the go statement executes, a new goroutine is created, execution in the new goroutine starts with a call to the function named in the go statement, and execution in the calling goroutine continues without interruption. (The main thread of execution is a goroutine itself.)
The following program prints a mix of 1’s and 0’s. <lang go>package main
import "fmt"
func printOnes() {
for { fmt.Println("1") }
}
func main() {
go printOnes() for { fmt.Println("0") }
}</lang>
A goroutine terminates upon return from the function called in the go statement. Unlike with a regular function call however, it cannot return a value--the calling goroutine has long continued and there is nothing waiting for a return value.
Goroutines may not be able to communicate by returning values, but they have other ways. Principal is passing data through channels. Channel operations affect execution when they yield the processor, allowing other goroutines to run, but this does not normally alter flow of execution. The one exception is when channel operations are used in a select statement. A simple use, <lang go>func answer(phone1, phone2 chan int) {
select { case <-phone1: // talk on phone one case <-phone2: // talk on phone two }
}</lang> Syntax is strongly reminiscent of the switch statement, but rules for flow control are very different. Select will block if no channel operation is possible. If one is possible, it will execute that case. If multiple operations are possible, it will pick one at random.
Process initialization
A complete program must have exactly one function named main, which is called on program start up. In addition, a program can have any number of functions named init. These are called before main, but otherwise in unspecified order.
GW-BASIC
<lang qbasic>10 LET a=1 20 IF a=2 THEN PRINT "This is a conditional statement" 30 IF a=1 THEN GOTO 50: REM a conditional jump 40 PRINT "This statement will be skipped" 50 PRINT ("Hello" AND (1=2)): REM This does not print 100 PRINT "Endless loop" 110 GOTO 100:REM an unconditional jump</lang>
Haskell
In the context of normal, functional-style code, there are no flow-control statements, because explicit flow control is imperative. A monad may offer flow control; what kinds are available depends on the monad. For example, the ExitT
monad transformer lets you use the exitWith
function to jump out a block of statements at will.
<lang haskell>import Control.Monad import Control.Monad.Trans import Control.Monad.Exit
main = do
runExitTMaybe $ do forM_ [1..5] $ \x -> do forM_ [1..5] $ \y -> do lift $ print (x, y) when (x == 3 && y == 2) $ exitWith () putStrLn "Done."</lang>
HicEst
More on HicEst's ALARM function <lang hicest>1 GOTO 2 ! branch to label
2 READ(FIle=name, IOStat=ios, ERror=3) something ! on error branch to label 3
3 ALARM(delay, n) ! n=2...9 simulate F2 to F9 keys: call asynchronously "Alarm"-SUBROUTINES F2...F9 with a delay
4 ALARM( 1 ) ! lets HicEst wait at this statement for any keyboard or mouse event
5 SYSTEM(WAIT=1000) ! msec
6 XEQ('CALL my_subroutine', *7) ! executes command string, on error branch to label 7
7 y = EXP(1E100, *8) ! on error branch to label 8
8 y = LOG( 0 , *9) ! on error branch to label 9
9 ALARM( 999 ) ! quit HicEst immediately</lang>
Icon and Unicon
Prelude about Goal-Directed Evaluation and Generators
Two of the key features of Icon and Unicon that affect program flow are Goal Directed Evaluation and Generators and Expression Failure. Goal Direction uses Generators to produce multiple results as needed and Expression Success and Failure forces the selection of logic pathways within programs.
goto
Does not exist in the Icon or Unicon language.
next
Restarts the enclosing loop. The conditional on the loop is evaluated as normal.
break expr
Default value of expr is the null value &null. This operator breaks out of the enclosing loop, yielding the expression as the result of the loop. Normally loops yield a failure ie no result, so you can write code like this:
<lang icon>
if x := every i := 1 to *container do { # * is the 'length' operator if container[i] ~== y then write("item ", i, " is not interesting") else break a } then write("found item ", x) else write("did not find an item")
</lang> The expression given to break can be another break, which effectively lets you break out of two levels of loop. Finally, the expression given to break can be the next command; for example <lang icon>
break break next
</lang> breaks out of two levels of loop and re-enters the top of the third-level enclosing loop.
return expr
Default value of expr is &null. Apart from the usual meaning of return, if the expr value fails, then the procedure actually fails too, ie does not yield a value. See description of fail keyword. If the expr is capable of yielding more than one result, only the first result is asked for and used.
fail
Causes the the enclosing procedure to terminate without returning value. This is different from returning void or a null value that many other languages do when the code does not return an actual value. For example, in
<lang icon>
x := ftn()
</lang> The value of x will not be replaced if ftn() issues the fail command. If ftn fails, then Goal-Directed Evaluation will also fail the assignment, therefore x is not assigned a new value. If the flow of control through a procedure falls off the end, the procedure implicitly fails.
suspend expr
Default value of expr is &null. Any procedure containing the suspend command will yield a value to the calling code. However the procedure remains in a state of suspended animation ready to be reactivated if the calling code demands another result due to Goal Directed Evaluation. Note that this capability is built directly into the runtime rather than being an artifically constructed behaviour provided by Python or C#'s use of the 'yield' keyword. Every and all expressions may suspend or be involved in a suspending expression without any effort. Behaviourally much closer to Prolog which also supports backtracking as a core part of the language. If the expr is capable of yielding more than one result, then supend (if driven) will progressively yield all of those values.
A procedure can contain several uses of suspend and it's quite reasonable for the procedure to execute many of them in any chosen order.
stop(expr)
Terminate program with prejudice.
error trapping
The keyword &error is normally zero, but if set to a positive value, this sets the number of fatal errors that are tolerated and converted to expression failure; the value of &error is decremented if this happens. Therefore the now-common TRY-CATCH behaviour can be written as:
<lang icon>
&error := 1 mayErrorOut() if &error == 1 then &error := 0 # clear the trap else { # deal with the fault handleError(&errornumber, &errortext, &errorvalue) # keyword values containing facts about the failure }
</lang> Various idiomatic simplifications can be applied depending on your needs.
error throwing
Errors can be thrown using the function
<lang icon>
runerr(errnumber, errorvalue) # choose an error number and supply the offending value
</lang>
IDL
goto
<lang idl>test: ..some code here goto, test</lang>
(This is almost never used)
on_error
<lang idl>on_error, test</lang>
(This resumes at the label test if an error is encountered)
on_ioerror
<lang idl>on_ioerror, test</lang>
(Same as on_error, but for EOFs and read-errors and such)
break
<lang idl>break</lang>
immediately terminates the innermost current loop (or if or case etc)
continue
<lang idl>continue</lang>
immediately starts the next iteration of the current innermost loop
J
Control structures should usually [but not always] be avoided in J. J's primitives already provide iteration and selection.
For example, here's an example of a program which loops over a sequence of integers, multiplying them by two (the result is displayed on the following line): <lang j> 2 * 1 2 3 2 4 6</lang>
That said, J's control structures are documented at http://www.jsoftware.com/help/dictionary/ctrl.htm So, if you want to perform this same operation using a while loop, or a goto, you can do so. It's just... often not a good idea (but sometimes they are indispensable).
Java
"goto" is a reserved keyword in Java; but you cannot use it. There are currently no goto statements.
Java does provide two other statements that provide flow control: break and continue.
break
The break statement can be used to terminate a case clause in a switch statement and to terminate a for, while or do-while loop. In loops, a break can be labeled or unlabeled.
<lang Java>switch (xx) {
case 1: case 2: /* 1 & 2 both come here... */ ... break; case 4: /* 4 comes here... */ ... break; case 5: /* 5 comes here... */ ... break; default: /* everything else */ break;
}
for (int i = 0; i < 10; ++i) {
... if (some_condition) { break; } ...
}
_Time_: do {
for (int i = 0; i < 10; ++i) { ... if (some_condition) { break _Time_; /* terminate the do-while loop */} ... } ...
} while (thisCondition);</lang>
continue
The continue statement skips the current iteration of a for, while, or do-while loop. As with break the continue statement can be labeled or unlabeled to allow iterating a loop level other than the current one in nested loops.
<lang Java>while (condition) {
... if (someCondition) { continue; /* skip to beginning of this loop */ } ...
}
top: for (int 1 = 0; i < 10; ++i) {
... middle: for (int j = 0; j < 10; ++j) { ... bottom: for (int k = 0; k < 10; ++k) { ... if (top_condition) { continue top; /* restart outer loop */ } ... if (middle_condition) { continue middle; /* restart middle loop */ } ... if (bottom_condition) { continue bottom; /* restart bottom loop */ } ... if (bottom_condition) { continue; /* this will also restart bottom loop */ } ... } ... } ....
}</lang>
JavaScript
return
from a function ([1])- loop control with
break [label]
([2]) andcontinue [label]
([3]) - exceptions with
throw
([4]) andtry ... catch ... finally ...
([5])
Lua
Lua has the break
-command to exit loops.
<lang lua>i = 0
while true do
i = i + 1 if i > 10 then break end
end</lang>
Mathematica
Relevant functions are:
TimeConstrained[expr,t] evaluates expr, stopping after t seconds.
MemoryConstrained[expr,b] evaluates expr, stopping if more than b bytes of memory are requested.
Goto[tag] scans for Label[tag], and transfers control to that point.
CreateScheduledTask[expr,t] creates a task that will repeatedly evaluate expr every t second.
Interrupt[] interrupt a computation
Abort[] abort a computation
Quit[] immediately stops all execution and returns to the top level read-eval-print loop
Catch[] prepares for receiving a Throw[] while running a given list of executable expressions
Throw[] causes a non-local jump to a specified Catch[]
MATLAB / Octave
<lang Matlab> try
% do some stuff
catch
% in case of error, continue here
end </lang>
Maxima
<lang maxima>/* goto */ block(..., label, ..., go(label), ...);
/* throw, which is like trapping errors, and can do non-local jumps to return a value */ catch(..., throw(value), ...);
/* error trapping */ errcatch(..., error("Bad luck!"), ...);</lang>
MUMPS
GOTO / G
The GOTO command jumps to a label. If the label is not in the current routine, it is necessary to include the circumflex and routine name. <lang MUMPS>GOTO LABEL^ROUTINE</lang>. This does not affect the subroutine stack, only the program pointer.
<lang MUMPS>GOTO THERE</lang>
HALT / H
Halt and Hang have the same abbreviation, i.e. "H" but (as a mnemonic) Halt takes no arguments. Halt stops the current process, and clears all Locks and devices in Use. On the Cache variant of MUMPS, there is a $HALT special variable that can be set, the value of the $HALT special variable is a routine that is called before cleaning up (in effect, a specialized final error trap).
<lang MUMPS> Read "Do you really wish to halt (Y/N)?",Q#1
IF Q="Y"!Q="y" HALT</lang>
JOB / J
The JOB command starts another MUMPS job starting at a label. If the label is not in the current routine, it is necessary to include the circumflex and routine name. <lang MUMPS>JOB LABEL^ROUTINE</lang>. <lang MUMPS>JOB THERE</lang> This does not affect the subroutine stack, nor the program pointer in the current job. Since MUMPS is a multi-processing (rather than multi-threading) language, the new job is independent of the current job.
<lang MUMPS> JOB LABEL^ROUTINE</lang>
QUIT / Q
Exits a loop, or routine. It decreases the stack level. It can return a value to a calling routine if there is a value after it.
Quit is one of the commands that requires two spaces after it if it is followed in a line by more commands.
<lang MUMPS>FOR I=1:1:1 QUIT:NoLoop DO YesLoop QUIT Returnvalue</lang>
XECUTE / X
eXecute acts as if it were a one line Do command. Its argument must be a string of valid MUMPS code, and it performs that code in a new stack level. There is an implied Quit at the end of each eXecute's argument string.
<lang MUMPS> SET A="SET %=$INCREMENT(I)"
SET I=0 XECUTE A WRITE I</lang>
The above block will output "1".
Nemerle
Flow control statements made available in the Nemerle.Imperative namespace: break, continue, return (to return from somewhere other than the last expression in a function).
Exceptions can also be used to transfer control from a try block to a catch block.
NetRexx
NetRexx doesn't have a GOTO instruction and unlike Rexx the SIGNAL instruction is only used to throw exceptions.
Like Rexx however, NetRexx provides the LEAVE and ITERATE instructions.
LEAVE
The LEAVE instruction causes immediate exit from one or more DO, SELECT or LOOP constructs.
<lang NetRexx>loop xx = 1 to 10
if xx = 1 then leave -- loop terminated by leave say 'unreachable' end</lang>
A name parameter can be provided to direct LEAVE to a specific end of block (as defined by a LABEL option or in the case of a controlled LOOP the control variable of the loop.
<lang NetRexx>loop xx = 1 to 10 -- xx is the control variable
... loop yy = 1 to 10 -- yy is the control variable ... if yy = 3 then leave xx -- xx loop terminated by leave if yy = 4 then leave yy -- yy loop terminated by leave ... end ... end xx
loop label xlabel xx = 1 to 10 -- xx is still the control variable but LABEL takes precidence
... loop yy = 1 to 10 -- yy is the control variable ... if yy = 3 then leave xlabel -- xx loop terminated by leave ... end yy ... end xlabel
do label FINIS
say 'in do block' if (1 == 1) then leave FINIS say 'unreachable' signal Exception("Will never happen")
catch ex = Exception
ex.printStackTrace()
finally
say 'out of do block'
end FINIS
loop vv over ['A', 'B']
select label selecting case vv when 'A' then do; say 'A selected'; say '...'; end when 'B' then do; say 'B selected'; if (1 == 1) then leave selecting; say '...'; end otherwise do; say 'nl selection'; say '...'; end end selecting end vv</lang>
ITERATE
The ITERATE instruction alters flow of control within a LOOP construct. On encountering an ITERATE instruction, execution of the loop body is terminated and control is passed directly back to the top of the loop just as though the last clause in the body of the loop had been executed.
As with LEAVE an optional name parameter can be supplied to direct the instruction to a loop level outside the current level. <lang NetRexx>loop fff = 0 to 9
... loop xx = 1 to 3 ... if fff > 2 then iterate fff ... end ... end fff</lang>
OCaml
An OCaml user can simulate flow control using exceptions:
<lang ocaml>exception Found of int
let () =
(* search the first number in a list greater than 50 *) try let nums = [36; 23; 44; 51; 28; 63; 17] in List.iter (fun v -> if v > 50 then raise(Found v)) nums; print_endline "nothing found" with Found res -> Printf.printf "found %d\n" res</lang>
Oz
Exception handling is documented in other tasks: Exceptions#Oz, Exceptions Through Nested Calls#Oz.
The case
statement can be used for Pattern Matching, but also like a switch statement in C:
<lang oz>case {OS.rand} mod 3
of 0 then {Foo}
[] 1 then {Bar}
[] 2 then {Buzz}
end</lang>
The Lisp-influenced for-loop is very powerful and convenient to use.
As a constraint programming language, Oz has a number of flow control structures which target logic programming. They are typically used to implement new constraint search engines. However, it is also possible to use them for general logic programming.
or
: speculatively executes a number of alternative conditions and blocks until at most one alternative remains valid. Then either fails or commits to the remaining alternative if there is one.
cond
: evaluates a number of conditions in parallel (or in undefined order) and commits to the first alternative that succeeds.
dis
: depreciated
choice
: creates a non-deterministic choice point. In other words, the statement provisionally chooses an alternatives. If the choice turns out to be wrong or if additional solutions to a puzzle are searched, another alternative is chosen.
As an example for choice
, a simple, but stupid way to solve the equation 2*X=18. We assume that the solution is somewhere in the interval 8-10, but we do not quite know what exactly it is.
<lang oz>declare
proc {Stupid X} choice X = 8 {System.showInfo "choosing 8"} [] X = 9 {System.showInfo "choosing 9"} [] X = 10 {System.showInfo "choosing 10"} end 2 * X = 18 end
in
{Show {SearchOne Stupid}}</lang>
Output:
choosing 8 choosing 9 [9]
PARI/GP
Flow control structures include function calling and returning, error
/trap
, next
/break
, alarm
, and the various loops.
Pascal
goto
<lang pascal>label
jumpto;
begin
...
jumpto:
some statement; ... goto jumpto; ...
end;</lang>
exception
<lang pascal>try
Z := DoDiv (X,Y);
except
on EDivException do Z := 0;
end;</lang>
Halt
Halt stops program execution and returns control to the calling program. The optional argument Errnum specifies an exit value. If omitted, zero is returned. <lang pascal>procedure halt(errnum: Byte);</lang>
Exit
Exit exits the current subroutine, and returns control to the calling routine. If invoked in the main program routine, exit stops the program. The optional argument X allows to specify a return value, in the case Exit is invoked in a function. The function result will then be equal to X. <lang pascal>procedure exit(const X: TAnyType)</lang>
Calls of functions/procedures as well as breaks and continues in loops are described in the corresponding tasks.
Perl
goto
Goto is typically looked down upon by most Perl programmers
<lang perl>FORK:
- some code
goto FORK;</lang>
Perl 6
Control exceptions
Control flow is extensible in Perl 6; most abnormal control flow (including the standard loop and switch exits) is managed by throwing control exceptions that are caught by the code implementing the construct in question. Warnings are also handled via control exceptions, and turn into control flow if the dynamic context chooses not to resume after the warning. See [S04/Control exceptions] for more information.
Phasers
Phasers are blocks that are transparent to the normal control flow but that are automatically called at an appropriate phase of compilation or execution. The current list of phasers may be found in [S04/Phasers].
goto
<lang perl6>TOWN: goto TOWN;</lang> Labels that have not been defined yet must be enclosed in quotes.
PHP
goto
Introduced in PHP 5.3, PHP now has a goto flow-control structure, even though most PHP programmers see it as a bad habbit (may cause spaghetti-code).
<lang php><?php goto a; echo 'Foo';
a: echo 'Bar'; ?></lang> Output:
Bar
PicoLisp
As this task asks for the documentation of common flow control structures, we refer here to the online documentation for more complete descriptions and examples.
Relevant functions are:
fork
fork creates a child process
task
task installs a background task consisting of an environment and a list of executable expressions
alarm
alarm schedules a timer, which runs a given list of executable expressions when it expires
abort
abort runs a given list of executable expressions, and aborts processing it if it takes longer than a given time
quit
quit immediately stops all execution and returns to the top level read-eval-print loop, optionally signaling an error
wait
wait delays current processing (optionally to a maximal time) until an optionally given condition evaluates to non-NIL
sync
sync synchronizes with other processes of the same family
protect
protect delays the processing of signals while a given list of executable expressions is executed
catch
catch prepares for receiving a 'throw' while running a given list of executable expressions
throw
throw causes a non-local jump to a specified 'catch' environment
bye
bye exits the interpreter
finally
finally specifies a list of executable expressions, to be run when current processing is done, even if a 'throw' or 'bye' was executed, or an error occurred.
PL/I
<lang PL/I> LEAVE
The LEAVE statement terminates execution of a loop. Execution resumes at the next statement after the loop.
ITERATE
The ITERATE statement causes the next iteration of the loop to commence. Any statements between ITERATE and the end of the loop are not executed.
STOP
Terminates execution of either a task or the entire program.
SIGNAL FINISH
Terminates execution of a program in a nice way.
SIGNAL statement
SIGNAL <condition> raises the named condition. The condition may be one of the hardware or software conditions such as OVERFLOW, UNDERFLOW, ZERODIVIDE, SUBSCRIPTRANGE, STRINGRANGE, etc, or a user-defined condition.
CALL
The CALL statement causes control to transfer to the named subroutine.
SELECT
The SELECT statement permits the execution of just one of a list of statements (or groups of statements). It is sort of like a computed GOTO.
GO TO
The GO TO statement causes control to be transferred to the named statement. It can also be used to transfer control to any one of an array of labelled statements. (This form is superseded by SELECT, above.) [GO TO can also be spelled as GOTO].
</lang>
Pop11
quitloop
quitloop with argument exits from nested loops:
<lang pop11>while condition1 do
while condition2 do if condition3 then quitloop(2); endif; endwhile;
endwhile;</lang>
above quitloop(2) exits from both loops.
goto
goto l transfers control to the label l. goto may be used to exit from nested loops:
<lang pop11>while condition1 do
while condition2 do if condition3 then goto l; endif; endwhile;
endwhile; l:;</lang>
Another use is to implement finite state machines:
<lang pop11>state1:
DO_SOMETHING(); if condition1 then goto state1; elseif condition2 then goto state2; .... else goto stateN; endif;
state2:
....
... ... stateN:
....</lang>
Pop11 goto is a nonlocal one, so "jump out" from a chain of procedure calls:
<lang pop11>define outer();
define inner(n); if n = 0 then goto final; endif; inner(n - 1); enddefine; inner(5); final:;
enddefine;</lang>
This is useful to exit early from successful recursive search, and for exception handling.
go_on
go_on is a multiway jump
<lang pop11>go_on expression to lab1, lab2, ..., labN else elselab ;</lang>
If expression has value K the above will jump to label labK, if expression is not an integer, or if it outside range from 1 to N, then control passes to label elselab. The else part may be omitted (then out of range values of expression cause an exception).
There is a more structured variant of go_on:
go_on expression to lab :
lab 1 : statement1; lab 2 : statement2; ....
endgo_on;
where lab is a prefix chosen by the user.
return
return ends execution of current function. In simplest form it is just:
<lang pop11>return;</lang>
but it is also possible to specify one or more return values:
<lang pop11>return(val1, val2, val3);</lang>
chain
chain has effect of "tail call" but is not necessarily in tail position. More precisely inside proc1.
<lang pop11>chain proc2(x1, x2, x3);</lang>
finishes execution of proc1 and transfers control to the proc2 passing it x1, x2, and x3 as arguments. On return from proc2 control passes to caller of proc1.
Remark: Pop11 does not perform "tail call optimization", one has to explicitly use chain.
PureBasic
Goto
Transfers control to the label referenced. It is not a safe way to exit loops. <lang PureBasic>If OpenConsole()
top: i = i + 1 PrintN("Hello world.") If i < 10 Goto top EndIf Print(#CRLF$ + #CRLF$ + "Press ENTER to exit") Input() CloseConsole()
EndIf </lang>
Gosub & Return
Gosub stands for 'Go to sub routine'. A label must be specified after Gosub where the program execution continues and will do so until encountering a Return. When a return is reached, the program execution is then transferred immediately below the Gosub. Gosub is useful when building fast structured code with very low overhead. <lang PureBasic>X=1: Y=2 Gosub Calc
- X will now equal 7
End
Calc:
X+3*Y Return ; Returns to the point in the code where the Gosub jumped from</lang>
FakeReturn
If the command Goto is used within the body of a sub routine, FakeReturn must be used to correct the stack or the program will crash. <lang PureBasic>Gosub MySub
Lable2:
- The program will jump here, then 'end'
End
MySub: If #PI>3
FakeReturn ; This will simulate the function of a normal "Return". Goto Lable2
EndIf Return</lang>
OnErrorGoto
This will transferee the program execution to the defined label if an error accrue. <lang PureBasic>OnErrorGoto(?MyExitHandler)
X=1: Y=0 z= X/Y
- = a illegal division with zero
Debug "This line should never be reached" End
MyExitHandler:
MessageRequester("Error", ErrorMessage()) End</lang>
OnErrorCall
Similar to OnErrorGoto() but procedural instead. <lang PureBasic>Procedure MyErrorHandler()
;All open files etc can be closed here MessageRequester("Error", ErrorMessage()) End
EndProcedure
OnErrorCall(MyErrorHandler()) X=1: Y=0 Z= X/Y
- This line should never be reached</lang>
Python
Loops
Python supports break and continue to exit from a loop early or short circuit the rest of a loop's body and "continue" on to the next loop iteration. <lang python># Search for an odd factor of a using brute force: for i in range(n):
if (n%2) == 0: continue if (n%i) == 0: result = i break
else:
result = None print "No odd factors found"</lang>
In addition, as shown in the foregoing example, Python loops support an else: suite which can be used to handle cases when the loop was intended to search for something, where the code would break out of the loop upon finding its target. In that situation the else: suite can be used to handle the failure. (In most other languages one is forced to use a "sentinel value" or a special flag variable ... typically set to "False" before the loop and conditionally set to "True" within the loop to handle situations for which the Python else: on loops is intended).
Exceptions
A Python exception is simply any subclass of the built-in BaseException class, or any of its descendents. User defined exception classes are normally descendents of the Exception class (which is, itself, a subclass of BaseException). To "throw" any exception (user defined or otherwise) one uses the raise statement. To capture exceptions one must enclose the code in a try: ... except...: block. Any exception listed in an except block will catch all subclasses of that exception. For example ZeroDivisionError is derived from ArithmeticError. Thus an exception clause for ArithmeticError would catch a ZeroDivisionError (or any other ArithmeticError).
As a consequence of this one must arrange the order of exception clauses such that the more specific exceptions are listed (caught) before their more general base exceptions. Only the first matching exception clause will be executed. An except clause which lists no exceptions will catch all possible exceptions. (This is usually considered to be very poor programming practice because it can hide unintended coding errors).
An exception can be re-raised by simply calling the raise statement without any arguments (from within any exception handler). Thus a function can catch an exception, attempt to deal with it, then, if necessary, throw it it back to the next layer out in a given call stack. Uncaught exceptions will be handled by the interpreter by terminating the program and printing an error message and stack trace.
A custom Exception class is normally declared with the pass statement as no methods of the parent class are over-ridden, no additional functionality is defined and no attributes need be set. Example:
<lang python>class MyException(Exception): pass</lang>
One normally would choose the most similar existing class. For example if MyException was going to be raised for some situation involving an invalid value it might be better to make it a subclass of ValueError; if it was somehow related to issues with inappropriate objects being passed around then one might make it a subclass of TypeError.
In large projects it's common to create an custom application base exception and to have all or most custom exceptions within that application or framework derive therefrom.
To create a "virtual base class" (one which is not intended to be directly instantiated, but exists solely to provide an inheritance to it's derived classes) one normally defines the requisite methods to raise "NotImplementedError" like so:
<lang python>class MyVirtual(object):
def __init__(self): raise NotImplementedError</lang>
It then becomes necessary for any descendants of this class to over-ride the __init__() method. Any attempt to instantiate a "MyVirtual" object directly will raise an exception.
Case 1 - Try, Except
<lang python>try:
temp = 0/0
- 'except' catches any errors that may have been raised between the code of 'try' and 'except'
except: # Note: catch all handler ... NOT RECOMMENDED
print "An error occurred."
- Output : "An error occurred"</lang>
Case 2 - Try, Except <lang python>try:
temp = 0/0
- here, 'except' catches a specific type of error raised within the try block.
except ZeroDivisionError:
print "You've divided by zero!"
- Output : "You've divided by zero!"</lang>
Case 3 - Try, Except, Finally <lang python>try:
temp = 0/0
except:
print "An error occurred."
- here, 'finally' executes when the try - except block ends, regardless of whether an error was raised or not
- useful in areas such as closing opened file streams in the try block whether they were successfully opened or not
finally:
print "End of 'try' block..."
- Output :
- An error occurred
- End of 'try' block...</lang>
Note: Prior to version 2.5 a try: statement could contain either series of except: clauses or a finally: clause but not both. It was thus necessary to nest the exception handling in an enclosing try:...finally: loop like so:
<lang python>try:
try: pass except (MyException1, MyOtherException): pass except SomeOtherException:
finally:
do_some_cleanup() # run in any case, whether any exceptions were thrown or not</lang>
Case 4 - Try, Except, Else <lang python>try:
temp = 1/1 # not a division by zero error
except ZeroDivisionError: # so... it is not caught
print "You've divided by zero."
- here, 'else' executes when no exceptions are caught...
else:
print "No apparent error occurred."
- Output :
- No apparent error occurred.</lang>
Case 5 - Try, Except, break, continue <lang python>i = 0 while 1: # infinite loop
try: temp2 = 0/i # will raise a ZeroDivisionError first. temp = math.sqrt(i) break # 'break' will break out of the while loop except ValueError: # print "Imaginary Number! Breaking out of loop" break # 'break' out of while loop except ZeroDivisionError: print "You've divided by zero. Decrementing i and continuing..." i-=1 # we decrement i. # we 'continue', everything within the try - except block will be executed again, # this time however, ZeroDivisionError would not be raised again. continue # Note that removing it, replacing it with 'pass' would perform the equivalent # see below for a better example
- Output :
- You've divided by zero. Decrementing i and continuing...
- Imaginary Number! Breaking out of loop</lang>
Case 6 - Creating your own custom exceptions, raise <lang python># Let's call our custom error "StupidError"; it inherits from the Exception class
class StupidError(Exception): pass
- Try it out.
try:
raise StupidError("Segfault") # here, we manually 'raise' the error within the try block
except StupidError, details: # 'details' is the StupidError object we create in the try block.
print 'Something stupid occurred:', details # so we access the value we had stored for it...
- Output :
- Something stupid occurred: Segfault</lang>
continue, else in "for" loop
<lang python> i = 101
for i in range(4): # loop 4 times print "I will always be seen." if i % 2 == 0: continue # continue goes back to the loop beginning for a new iteration. print "I'll only be seen every other time." else: print "Loop done" # Output: # I will always be seen. # I will always be seen. # I'll only be seen every other time. # I will always be seen. # I will always be seen. # I'll only be seen every other time. # Loop done
if(__name__ == "__main__"):
main()</lang>
The "with" statement
See [PEP 0343, The "with" statement]
<lang python>class Quitting(Exception): pass max = 10 with open("some_file") as myfile:
exit_counter = 0 for line in myfile: exit_counter += 1 if exit_counter > max: raise Quitting print line,</lang>
The with statement allows classes to encapsulate "final" (clean-up) code which will automatically be executed regardless of exceptions that occur when working "with" these objects. Thus, for the foregoing example, the file will be closed regardless of whether it's more than 10 lines long. Many built-in and standard library classes have "context managers" which facilitate their use in with: code. In addition it's possible to define special __enter__() and __exit__() methods in one's own classes which will be implicitly called by the interpreter when an object is used within a with: statement.
Use cases for with: enabled objects include automated/guaranteed closing of files, release of threading lock objects, commit or rollback of database transactions, and save/restore of any desired state (such as terminal settings when using the curses module, the precision settings when using the Decimal module, or even saving and restoring sys.stdout for temporary redirection). It is a feature that seems to be unique to Python.
Yield expressions
See [PEP 0342, Coroutines via Enhanced Generators]
>>> value = 1 >>> echo = lambda: (yield value) >>> for i in echo(): ... print i ... 1
Racket
exit
Racket's exit quits the whole process, optionally returning an exit code. Note that there is an exit-handler that can be set to intercept such exit attempts.
goto
Racket doesn't have a goto, but like other implementations of Scheme, it adopts the mantra of "Lambda: the Ultimate GOTO" by having all tail calls optimized. This allows writing code that is no different from your average assembly code -- for example, here's a direct translation of Greatest_common_divisor#x86_Assembly into a Racket function:
<lang racket>
- lang racket
- some silly boilerplate to mimic the assembly code better
(define r0 0) (define (cmp r1 r2) (set! r0 (sgn (- r1 r2)))) (define (je true-label false-label) (if (zero? r0) (true-label) (false-label))) (define (goto label) (label))
(define (gcd %eax %ecx)
(define %edx 0) (define (main) (goto loop)) (define (loop) (cmp 0 %ecx) (je end cont)) (define (cont) (set!-values [%eax %edx] (quotient/remainder %eax %ecx)) (set! %eax %ecx) (set! %ecx %edx) (goto loop)) (define (end) (printf "result: ~s\n" %eax) (return %eax)) (main))
</lang>
Exceptions
Racket has exceptions which are used in the usual way, and with-handlers to catch them. In fact, any value can be raised, not just exceptions. For example:
<lang racket> (define (list-product l)
(with-handlers ([void identity]) (let loop ([l l] [r 1]) (cond [(null? l) r] [(zero? (car l)) (raise 0)] [else (loop (cdr l) (* r (car l)))]))))
</lang>
Continuations
Racket has full continuations, of all kinds, including delimited and not. That's plenty of control flow...
And more
Given that Racket has macros, and continuations, and a zillion other features, it is easy to implement new control flow expressions, so any list will not be exhaustive.
REBOL
<lang REBOL>REBOL [ Title: "Flow Control" Author: oofoe Date: 2009-12-05 URL: http://rosettacode.org/wiki/Flow_Control_Structures ]
- return -- Return early from function (normally, functions return
- result of last evaluation).
hatefive: func [ "Prints value unless it's the number 5." value "Value to print." ][ if value = 5 [return "I hate five!"] print value ]
print "Function hatefive, with various values:" hatefive 99 hatefive 13 hatefive 5 hatefive 3
- break -- Break out of current loop.
print [crlf "Loop to 10, but break out at five:"] repeat i 10 [ if i = 5 [break] print i ]
- catch/throw -- throw breaks out of a code block to enclosing catch.
print [crlf "Start to print two lines, but throw out after the first:"] catch [ print "First" throw "I'm done!" print "Second" ]
- Using named catch blocks, you can select which catcher you want when throwing.
print [crlf "Throw from inner code block, caught by outer:"] catch/name [ print "Outer catch block." catch/name [ print "Inner catch block." throw/name "I'm done!" 'Johnson print "We never get here." ] 'Clemens print "We never get here, either." ] 'Johnson
- try
div: func [ "Divide first number by second." a b /local r "Result" ][ if error? try [r: a / b] [r: "Error!"] r ; Functions return last value evaluated. ]
print [crlf "Report error on bad division:"] print div 10 4 print div 10 2 print div 10 1 print div 10 0</lang>
REXX
call
The call statement immediately transfers control to a named subroutine, and the call statement may have any number (or none) parameters. (However, most REXXes have some practical limit to the number of arguments, usually at least 50).
The named subroutine may or may not return a result (which is similar to a return code, but REXX allows character strings as well).
(Also, see function invocation below.) <lang rexx>numeric digits 1000 /*prepare for some gihugeic numbers.*/ ... n=4 call factorial n say n'!=' result exit /*──────────────────────────────────FACTORIAL subroutine────────────────*/ factorial: parse arg x !=1
do j=2 to x !=!*j end /*j*/
return !</lang>
case
(See the select statement below.)
exceptions
(See the signal statement and raising conditions below.)
exit
The exit statement terminates the running (REXX) program and passes control to the invoking program (it could be the shell/host/supervisor program).
If an expression is coded, it normally is used to set the result (if a REXX program) or return code (also called RetCode, RC, completion code, or other such names).
When using the exit with an expressing to pass control to the operating system (i.e., exiting a REXX program), some operating systems require the expression to be a whole number within a certain range (often with a no expression or a [null] expression, which is usually taken to mean a return code of 0).
If the expression is a number, it is normalized to the current numeric digits.
(Also, see the return statement below.) <lang rexx>exit
exit expression</lang>
function invocation
A function invocation (similar to a call) immediately transfers control to a named function (subroutine), and the function/subroutine invocation statement may have any number (or none) parameters. (However, most REXXes have some practical limit to the number of arguments, usually at least 50).
(In REXX, the only difference between a function and a subroutine is that a function returns a result --- that is, some value is returned, which may be null)
The named function/subroutine must return a result (which is similar to a return code, but REXX allows character strings as well).
If no result is returned, REXX generates a syntax error (which may be trapped via the signal on syntax instruction).
(Also, see the call statement above.) <lang rexx>numeric digits 1000 /*prepare for some gihugeic numbers.*/ ... n=4 say n'!=' factorial(n) exit /*──────────────────────────────────FACTORIAL subroutine────────────────*/ factorial: parse arg x !=1
do j=2 to x !=!*j end /*j*/
return !</lang>
iterate
The iterate statement immediately transfer control to the do statement, that is, it (may) iterates (increments or decrements) the named REXX variable (if any) that is specified on the do statement. The iterate statement can also specify which do loop is to be iterated if there is a named variable on the do loop.
(All indentations in REXX are merely cosmetic and are used for readability.} <lang rexx>sum=0
do j=1 to 1000 if j//3==0 | j//7==0 then iterate sum=sum+j end /*j*/
/*shows sum of 1k numbers except those divisible by 3 or 7.*/
say 'sum='sum ... numeric digits 5000 prod=0
do k=1 to 2000 do m=1 to k if m>99 then iterate k prod=prod*m end /*m*/ end /*k*/
say 'prod=' prod</lang>
leave
The leave statement transfer control to the next REXX statement following the end statment of the current do loop. The leave statement can also specify which do loop is to be left if the do loop has a named variable. <lang rexx> do j=1 to 10
say 'j=' j if j>5 then leave say 'negative j=' (-j) end /*j*/
say 'end of the DO loop for j.'
ouch=60 sum=0
do k=0 to 100 by 3 say 'k=' k do m=1 to k if m=ouch then leave k sum=sum+m end /*m*/ end /*k*/
say 'sum=' sum</lang>
raising conditions
(REXX) conditions can be raised by causing some kind of "failure" or triggering event (such as division by zero).
A signal statement must have been issued previous to the event being triggered to enable trapping.
(Also, see the signal statement below.) <lang rexx>... signal on syntax ... y=4 - 4 x=66 say x/y /*divide x by y.*/ say "yup, that's a divide by zero, by gum." exit
syntax: say
/* We can now possibly do some repair work , but most people trap */ /* the condition, display where it happened, the REXX sourceline */ /* (the actual REXX statement), which condition was triggered, */ /* display any other pertinent REXX variables, which line in the */ /* REXX program, and then (usually) exit with some kind of error */ /* message and error code indicator. */ /* Note: the "name" of the REXX program isn't quite accurate, */ /* rather, it is the name that was invoked (called by), which may */ /* be different name than the actual program being executed. */
say '──────────────────────error!─────────────────────────' say 'that division (above) will cause control to get here.' parse source . . fid . say; say 'REXX raised a SYNTAX error in program:' fid say; say 'it occurred on line' sigl say; say 'the REXX statement is:' /*put it on separate line.*/
say sourceline(sigl)
say; say 'which code:' condition('C') "error" say; say 'error code:' condition('D') say; say "Moral: don't do that." exit 13</lang> output
──────────────────────error!───────────────────────── that division (above) will cause control to get here. REXX raised a SYNTAX error in program: D:\OOPSsay.REX it occurred on line 6 the REXX statement is: say x/y /*divide x by y.*/ which code: SYNTAX error error code: Error 42.3: Arithmetic overflow; divisor must not be zero Moral: don't do that.
return
The return statement terminates the running (REXX) program (which could be a subroutine or function) and passes control to the invoking program (it could be the shell/host/supervisor program).
If no internal subroutine or function is active, return is equivalent to exit.
If a subroutine is active (a call was used), control goes to the instruction after the call statement.
If a function is active (a function reference was used) control goes back to the expression evaluation using the value resulting from the return expression.
(Also, see the exit statement above.) <lang rexx>return
return expression</lang>
select
The select statement is used to conditionally test for cases to selectively execute REXX statement(s). <lang rexx>... prod=1 a=7 /*or somesuch.*/ b=3 /* likewise. */
op='**' /*or whatever.*/ ...
select when op=='+' then r=a+b /*add. */ when op=='-' then r=a-b /*subtract. */ when op=='*' then do; r=a*b; prod=prod*r; end /*multiply.*/ when op=='*' then r=a*b /*multiply. */ when op=='/' & b\=0 then r=a/b /*divide. */ when op=='%' & b\=0 then r=a/b /*interger divide. */ when op=='//' & b\=0 then r=a/b /*modulus (remainder). */ when op=='||' then r=a||b /*concatenation. */ when op=='caw' then r=xyz(a,b) /*call the XYZ subroutine*/ otherwise r='[n/a]' /*signify not applicable.*/ end /*select*/
say 'result for' a op b "=" r</lang>
signal
The signal statement can be thought of as a GO TO or JUMP statement, however, on issuance of a signal statement, all active do loops and select structures are terminated. Essentially, that means that there is no real way to re-enter a do loop (or a select structure) once a signal statement is used.
Once a signal statement is executed (or invoked), control passed to the first occurrence of the label specified (in REXX, more than one label with the same name isn't considered an error). The label can be any combination of letters, digits, periods, and some special symbols, the most common are $, #, @, !, ?, and _ (underscore or underbar). Some versions of REXX (CMS, PC/REXX, Personal REXX, TSO, R4, ROO) also allow the cent sign (¢), some of those REXXes support the British pound (currency) symbol (£).
The signal statement is also used to transfer control in case of some specific conditions:
- when an I/O stream (could be a file) isn't ready.
- when the REXX program used a variable that isn't defined.
- when a REXX syntax error occurs.
- when the program is halted (usually via a Ctrl─Alt─Del ──or── PA1 ──or── somesuch.
- when there is a loss of digits (for the newer REXXes).
- when a command executed returns an error return code [other than 0 (zero)].
- when a command executed indicates a failure.
(Also, see raising conditions above.) <lang rexx>... signal on error signal on failure signal on halt signal on lostdigits /*newer REXXes.*/ signal on notready signal on novalue signal on syntax
signal off error signal off failure signal off halt signal off lostdigits /*newer REXXes.*/ signal off notready signal off novalue signal off syntax ... signal on novalue ... x=oopsay+1 exit
novalue: say say '───────────────────────────error!─────────────────────────────────' say 'that reference to oopsay (above) will cause control to get here.' parse source . . fid . say; say 'REXX raised a NOVALUE error in program:' fid say; say 'it occurred on line' sigl say; say 'the REXX statement is:' /*put it on separate line.*/
say sourceline(sigl)
say; say 'which code:' condition('C') "error" say; say 'REXX variable:' condition('D') say; say "Moral: shouldn't do that." output
───────────────────────────error!───────────────────────────────── that reference to oopsay (above) will cause control to get here. REXX raised a NOVALUE error in program: D:\flow_sig.rex it occurred on line 20 the REXX statement is: x=oopsay+1 which code: NOVALUE error REXX variable: OOPSAY Moral: shouldn't do that.
Ruby
return
Return from the currently executing method to the caller.
loop control
Ruby's loop control statements are: break, next, redo and retry
. Break and next are obvious. Redo and retry both restart the current loop iteration, but retry first reevaluates the condition.
They can control while, until, for
loops and iterators.
exceptions
Use raise
to throw an exception. You catch exceptions in the rescue
clause of a begin...end
block.
<lang ruby>begin
# some code that may raise an exception
rescue ExceptionClassA => a
# handle code
rescue ExceptionClassB, ExceptionClassC => b_or_c
# handle ...
rescue
# handle all other exceptions
else
# when no exception occurred, execute this code
ensure
# execute this code always
end</lang> There is also a rescue modifier (example from the Pickaxe book): <lang ruby>values = ["1", "2.3", /pattern/] result = values.map {|v| Integer(v) rescue Float(v) rescue String(v)}
- => [1, 2.3, "(?-mix:pattern)"]</lang>
catch and throw
break
will only break out of a single level of loop. You can surround code in a catch block, and within the block you can throw a string or symbol to jump out to the end of the catch block (Ruby's GOTO, I suppose):
<lang ruby>def some_method
# ... if some_condition throw :get_me_out_of_here end # ...
end
catch :get_me_out_of_here do
for ... for ... some_method end end
end
puts "continuing after catching the throw"</lang>
yield
yield
passes control from the currently executing method to its code block.
SAS
<lang sas>/* GOTO: as in other languages
STOP: to stop current data step */
data _null_; n=1; p=1; L1: put n p; n=n+1; if n<=p then goto L1; p=p+1; n=1; if p>10 then stop; goto L1;
run;
/* LINK: equivalent of GOSUB in BASIC
RETURN: after a LINK, or to return to the beginning of data step */
data _null_; input a b; link gcd; put a b gcd; return;
gcd: _a=a; _b=b; do while(_b>0); _r=mod(_a,_b); _a=_b; _b=_r; end; gcd=_a; return;
cards; 2 15 533 221 8 44
run;</lang>
Tcl
after
The after facility can be used to execute some code at some future time asynchronously, like this
<lang tcl>after 1000 {myroutine x}</lang>
which will call "myroutine" with parameter "x" 1000ms from 'now'; no matter what other code might be running at the time (i.e. "after"; schedules the execution, then returns and continues program flow with the following code).
The scheduled task can be removed from the scheduler for example with
<lang tcl>after cancel myroutine</lang>
(other ways are possible).
The correct way to schedule some regularly recurring task in TCL is to incorporate a self-scheduling at the end of the routine. For example the following will produce a clock whose display is updated once a second:
<lang tcl>package require Tk proc update {} {
.clockface configure -text [clock format [clock seconds]] after 1000 update ; # call yourself in a second
}
- now just create the 'clockface' and call ;update' once:
pack [label .clockface] update</lang>
loop control
Tcl has the break
command to abort the current loop (for/foreach/while) and the continue
command to skip to the next loop iteration.
exception
Tcl's catch
command can be used to provide a basic exception-handling mechanism:
<lang tcl>if {[catch { ... code that might give error ... } result]} {
puts "Error was $result"
} else {
... process $result ...
}</lang> Tcl 8.6 also has a try…trap…finally structure for more complex exception handling. <lang tcl>try {
# Just a silly example... set f [open $filename] expr 1/0 string length [read $f]
} trap {ARITH DIVZERO} {} {
puts "divided by zero"
} finally {
close $f
}</lang>
custom control structures
A novel aspect of Tcl is that it's relatively easy to create new control structures (more detail at http://wiki.tcl.tk/685). For example, this example defines a command to perform some operation for each line of an input file: <lang tcl>proc forfilelines {linevar filename code} {
upvar $linevar line ; # connect local variable line to caller's variable set filechan [open $filename] while {[gets $filechan line] != -1} { uplevel 1 $code ; # Run supplied code in caller's scope } close $filechan
}</lang> Now we can use it to print the length of each line of file "mydata.txt": <lang tcl>forfilelines myline mydata.txt {
puts [string length $myline]
}</lang>
Visual Basic .NET
Goto
This skips the line that changes the value of x to 5.
<lang vbnet> Sub bar2()
Dim x = 0 GoTo label x = 5
label:
Console.WriteLine(x) End Sub</lang>
On Error Goto
This brances in the event of an error. Usually there is an Exit (Sub|Function) to seperate the normal code from the error handling code
<lang vbnet> Sub foo()
On Error GoTo label 'do something dangerous Exit Sub
label:
Console.WriteLine("Operation Failed") End Sub</lang>
This style of code is rarely used.
On Error Resume Next
This performs a sequence of actions. If any action fails, the exception is discarded and next operation is performed.
<lang vbnet>Sub foo2()
On Error Resume Next Operation1() Operation2() Operation3() Operation4()
End Sub</lang>
This style of code is rarely used.
Return / Exit Sub
This shows the classical and modern syntax for exiting a sub routine early.
<lang vbnet>Sub Foo1()
If Not WorkNeeded() Then Exit Sub DoWork()
End Sub
Sub Foo2()
If Not WorkNeeded() Then Return DoWork()
End Sub</lang>
Return value / Exit Function
This shows the classical and modern syntax for exiting a function early. There is an implied variable with the same name as the function. This variable is write-only.
<lang vbnet>Function Foo3()
Foo3 = CalculateValue() If Not MoreWorkNeeded() Then Exit Function Foo3 = CalculateAnotherValue()
End Function
Function Foo4()
Dim result = CalculateValue() If Not MoreWorkNeeded() Then Return result Return CalculateAnotherValue()
End Function</lang>
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