Conditional Structures
From Rosetta Code
Programming Task
This is a programming task. It lays out a problem which Rosetta Code users are encouraged to solve, using languages they know.
These are examples of control structures. You may also be interested in:
- Conditional Structures
- Exceptions
- Flow Control Structures
- Loop Structures
Here, we document the conditional structures offered by different programming languages. Common conditional structures are if-then-else and switch.
[edit] ActionScript
- See JavaScript
[edit] Ada
[edit] if-then-else
type Restricted is range 1..10; My_Var : Restricted; if My_Var = 5 then -- do something elsif My_Var > 5 then -- do something else -- do something end if;
[edit] case
type Days is (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday); Today : Days; case Today in when Saturday | Sunday => null; when Monday => Compute_Starting_Balance; when Friday => Compute_Ending_Balance; when Others => Accumulate_Sales; end case;
[edit] select
Select provides conditional acceptance of entry calls. Select can also be used to conditionally call an entry
[edit] Conditional Accept
select accept first_entry; -- do something or accept second_entry; -- do something or terminate; end select;
[edit] Conditional entry call
A selective entry call provides a way to time-out an entry call. Without the time-out the calling task will suspend until the entry call is accepted.
select My_Task.Start; or delay Timeout_Period; end select;
The entry Start on the task My_Task will be called. If My_Task accepts the entry call before the timer expires the timer is canceled. If the timeout expires before the entry call is accepted the entry call is canceled.
[edit] ALGOL 68
[edit] IF ~ THEN ~ ELSE ~ FI clause
main: (
INT input := 2;
IF 3 = input THEN
# Do something #
FI;
IF 3 = input THEN
# Do something #
ELSE
# Do something ELSE #
FI
)
One line predicates do not any braces:
IF cond THEN ~ FI;
IF cond THEN ~ ELSE ~ FI;
And these may be mixed:
IF cond THEN
~
IF cond THEN
# multiple expressions #
ELSE
~
FI;
~
FI
Or merged:
IF cond1 THEN ~ ELIF cond2 THEN # multiple expressions # ELSE ~ FI # etc #
[edit] Short-circuit conditionals
The short-circuit evaluation of the ANDIF and ORELSE boolean operators are also often used for control flow.
IF node /= NULL ANDIF next OF node /= NULL ANDIF guarded_test(node) THEN next OF node := next OF next OF node FI
Note that these are not part of the standard library and had to be defined by the user.
Neither the assignment nor guarded_test() will be called IF the previous two conditions aren't met. Other languages such as Pascal don't make that guarantee, and they certainly could not be user defined.
[edit] CASE ~ IN ~ OUT ~ ESAC clause
main: (
INT input := 42;
CASE input IN
~ # Do something, because input = 1 #
,
~ # Do something, because input = 2 #
,
~ # Do something, because input = 3 #
,
# fall through to the next statement IF there is no "OUT" #
OUT
~ # Do something ELSE. #
CASE
)
The "CASE ~ IN ~ OUT ~ ESAC" is directly interchangeable with the "( ~ | ~ | ~ )", hence the above may be written as follows:
main: (
INT input := 42;
( input |
~ # Do something, because input = 1 #
,
~ # Do something, because input = 2 #
,
~ # Do something, because input = 3 #
# fall through to the next statement IF there is no "OUT" #
|
~ # Do something ELSE. #
)
)
Case statements may also be merged:
main:(
INT a1, a2, b1, b2, c99;
CASE 99 IN
a1,
a2
OUSE 99 IN
b1,
b2
OUT
c99
ESAC := 333
)
In this case a1, a2, b1 and b2 remain undefined, but c99 ends up being half-evil.
[edit] Ternary ( ~ | ~ | ~ )
Conditionals in C can also be done with the ternary operator, ?:. ALGOL 68 has the "brief" equivalents ( ~ | ~ | ~ )
main : ( INT input := 2; INT output := (input = 2 | 42 | 4711); # sets output to 42 # INT output2 := (input = 3 | 42 | 4711); # sets output2 to 4711 # PROC do_something = INT: random; PROC do_something_else = INT: random; input := (TRUE | do_something | do_something_else); # only calls do_something_else() # )
The "( ~ | ~ | ~ )" is directly interchangeable with the "IF ~ THEN ~ ELSE ~ FI" syntax. Hence the above could be written:
main : ( INT input := 2; INT output := IF input = 2 THEN 42 ELSE 4711 FI; # sets output to 42 # INT output2 := IF input = 3 THEN 42 ELSE 4711 FI; # sets output2 to 4711 # PROC do_something = INT: random; PROC do_something_else = INT: random; input := IF TRUE THEN do_something ELSE do_something_else FI; # only calls do_something_else() # )
A more complext example created by combining the above:
main: ( INT x,y,z; CASE 2 IN x,y OUT z ESAC := IF 1+2=2 THEN 333 ELSE 666 FI )
In the result y becomes half-evil, with x and z remaining undefined.
[edit] AppleScript
[edit] if-then-else
if myVar is "ok" then return true
set i to 0
if i is 0 then
return "zero"
else if i mod 2 is 0 then
return "even"
else
return "odd"
end if
[edit] BASIC
Works with: QuickBasic version 4.5
[edit] if-then-else
Ifs can work with equality
IF something = somethingElse THEN 'do stuff ELSE IF something =someOtherThing THEN 'do other stuff ELSE 'do more stuff END IF
Or like in #C
IF aNumber THEN 'the number is not 0 ELSE 'the number is 0 END IF
[edit] select case
SELECT CASE variable CASE value1 'do stuff CASE value2 'do other stuff CASE ELSE 'default case END SELECT
[edit] Befunge
Befunge only has one conditional structure which comes in two flavors: vertical IF ( | ) and horizontal IF ( _ ). Befunge only has two boolean commands, greater-than ( ` ) and not ( ! ). These snippets input a number and use the conditional operators to print a "0" if it is zero and an "X" otherwise.
v > "X",@ non-zero
> & |
> "0",@ zero
# is the skip command. It unconditionally skips one character, allowing a little flexibility in flow control.
& #v_ "0",@ zero > "X",@ non-zero
[edit] Brainf***
Brainf*** has two conditional jump instructions, [ and ]. the [ instruction jumps forward to the corresponding ] instruction if the value at the current memory cell is zero, while the ] instruction jumps back if the current memory cell is nonzero. Thus in the following sequence:
[.]
The . instruction will be skipped, while the following sequence
+[.]
will result in an infinite loop. Finally, in the following sequence
+[.-]
The . instruction will be executed once.
[edit] C
[edit] if-then-else
Works with: gcc version 4.1.2
int main (int argc, char ** argv) {
int input = 2;
if ( 3 == input ) {
/* Do something */
}
if ( 3 == input ) {
/* Do something */
} else {
/* Do something else */
}
}
One line predicates do not require curly braces (although their usage is always preferred for readability):
if(cond) expr;
if(cond) expr; else expr;
And these may be mixed:
if(cond)
expr;
else
{
/* multiple expressions */
}
if(cond)
{
/* multiple expressions */
}
else
expr;
[edit] Short-circuit conditionals
The short-circuit evaluation of the && and || boolean operators are also often used for control flow.
if (node != NULL && node->next != NULL && guarded_test(node)) node->next = node->next->next;
Neither the assignment nor guarded_test() will be called if the previous two conditions aren't met. Other languages such as Pascal don't make that guarantee.
[edit] switch
Works with: gcc version 4.1.2
int main (int argc, char ** argv) {
int input = 42;
switch (input) {
case 0:
/* Do something, because input = 0 */
break;
case 1:
/* Do something, because input = 1 */
break;
case 2:
/* Do something, because input = 2 */
/* fall through to the next statement if there is no "break" */
default:
/* Do something else. */
break; /* Optional */
}
}
A little known fact is that the default branch need not be the last one:
int main(int argc, char* argv[])
{
switch (argc)
{
default:
/* this will be executed if argc is neither 2 nor 3 */
break;
case 2:
/* this will be executed if argc is 2 */
break;
case 3:
/* this will be executer if argc is 3 */
}
}
[edit] Ternary ?:
Works with: gcc version 4.0.2
Conditionals in C can also be done with the ternary operator, ?:. The arguments are expressions, and a?b:c is an expression as well. However, since many things in C are expressions (this especially includes assignments and function calls), ?: can be used for those, too. However, the if/else construct is usually more readable and therefore preferred.
int main(int argc, char ** argv)
{
int input = 2;
int output = (input == 2? 42 : 4711); /* sets output to 42 */
int output2 = (input == 3? 42 : 4711); /* sets output2 to 4711 */
int do_something();
int do_something_else();
input == 1? do_something() : do_something_else(); /* only calls do_something_else() */
}
[edit] Preprocessor Techniques (compile-time)
Conditional compile based on if a certain macro exists,
#ifdef FOO /* compile this only if macro FOO exist */ #endif
Conditional compile based on if a macro doesn't exist
#ifndef FOO /* only compiled if macro FOO does not exist */ #endif
Conditional compile based on if a certain macro exists, with else clause
#ifdef FOO /* compile this only if macro FOO exist */ #else /* compile this only if macro FOO does not exist */ #endif
Conditional compile based on expression
#if defined(FOO) && FOO == 1 /* only compiled if macro FOO is defined and expands to a constant expression evaluating to 1 */ #endif
Chain of conditionals
#if defined(FOO) /* only compiled if macro FOO is defined */ #elif defined(BAR) /* only compiled if macro FOO is not defined, but macro BAR is */ #else /* only compiled if neither FOO nor BAR is defined */ #endif
Typical usage: Include guards
#ifndef FOO_H_ALREADY_INCLUDED #define FOO_H_ALREADY_INCLUDED /* header content */ #endif
If the header is included the first time, the macro FOO_H_ALREADY_INCLUDED will not be defined, thus the code between #ifndef and #endif will be compiled. The first thing this code does is define that macro, so that the next time the header is included, the code will be ignored. This effectively avoids multiple inclusion.
[edit] C++
[edit] Run-Time Control Structures
- See C
[edit] Compile-Time Control Structures
[edit] Preprocessor Techniques
- See C
[edit] Template metaprogramming
Selecting a type depending on a compile time condition
template<bool Condition, typename ThenType, typename Elsetype> struct ifthenelse;
template<typename ThenType, typename ElseType> struct ifthenelse<true, ThenType, ElseType>
{
typedef ThenType type;
};
template<typename ThenType, typename ElseType> struct ifthenelse<false, ThenType, ElseType>
{
typedef ElseType type;
};
// example usage: select type based on size
ifthenelse<INT_MAX == 32767, // 16 bit int?
long int, // in that case, we'll need a long int
int> // otherwise an int will do
::type myvar; // define variable myvar with that type
[edit] Clean
[edit] if
There are no then or else keyword in Clean. The second argument of if is the then-part, the third argument is the else-part.
bool2int b = if b 1 0
[edit] case-of
case 6 * 7 of
42 -> "Correct"
_ -> "Wrong" // default, matches anything
[edit] function alternatives
answer 42 = True answer _ = False
[edit] guards
answer x
| x == 42 = True
| otherwise = False
case 6 * 7 of
n | n < 0 -> "Not even close"
42 -> "Correct"
// no default, could result in a run-time error
[edit] Cobol
[edit] if-then-else
if condition-1
imperative-statement-1
else
imperative-statement-2
end-if
if condition-1
if condition-a
imperative-statement-1a
else
imperative-statement-1
end-if
else
if condition-a
imperative-statement-2a
else
imperative-statement-2
end-if
end-if
[edit] evaluate
evaluate identifier-1
when 'good'
good-imperative-statement
when 'bad'
bad-imperative-statement
when 'ugly'
when 'awful'
ugly-or-awful-imperative-statement
when other
default-imperative-statement
end-evaluate
evaluate true
when condition-1
condition-1-imperative-statement
when condition-2
condition-2-imperative-statement
when condition-3
condition-3-imperative-statement
when other
default-condition-imperative-statement
end-evaluate
evaluate identifier-1 also identifier-2 when 10 also 20 one-is-10-and-two-is-20-imperative-statement when 11 also 30 one-is-11-and-two-is-30-imperative-statement when 20 also any one-is-20-and-two-is-anything-imperative-statement when other default-imperative-statement end-evaluate
[edit] ColdFusion
[edit] if-elseif-else
Compiler: ColdFusion any version
<cfif x eq 3> do something <cfelseif x eq 4> do something else <cfelse> do something else </cfif>
[edit] switch
Compiler: ColdFusion any version
<cfswitch expression="#x#"> <cfcase value="1"> do something </cfcase> <cfcase value="2"> do something </cfcase> <cfdefaultcase> do something </cfdefaultcase> </cfswitch>
[edit] C #
[edit] if-elseif-else
if (condition)
{
// Some Task
}
if (condition)
{
// Some Task
}
else if (condition2)
{
// Some Task
}
else
{
// Some Task
}
[edit] Ternary
// if condition is true var will be set to 1, else false. int var = (condition)? 1 : 2;
[edit] switch
switch (value)
{
case 1:
// Some task
break; // Breaks are required in C#.
case 2:
case 3:
// Some task
break;
default: // If no other case is matched.
// Some task
break;
}
If fall through algorithms are required use the goto keyword.
switch (value)
{
case 1:
// Some task
goto case 2; // will cause the code indicated in case 2 to be executed.
case 2:
// Some task
break;
case 3:
// Some task
break;
default: // If no other case is matched.
// Some task
break;
}
[edit] Delphi
- See Pascal
[edit] E
[edit] if-then-else
if (okay) {
println("okay")
} else if (!okay) {
println("not okay")
} else {
println("not my day")
}
The pick/2 message of booleans provides a value-based conditional:
println(okay.pick("okay", "not okay"))
It can therefore be used to construct a Smalltalk-style conditional:
okay.pick(fn {
println("okay")
}, fn {
println("not okay")
})()
All of the above conditionals are expressions and have a usable return value.
[edit] switch
E's "switch" allows pattern matching.
def expression := ["+", [1, 2]]
def value := switch (expression) {
match [`+`, [a, b]] { a + b }
match [`*`, [a, b]] { a * b }
match [op, _] { throw(`unknown operator: $op`) }
}
[edit] Forth
[edit] IF-ELSE
( condition ) IF ( true statements ) THEN ( condition ) IF ( true statements ) ELSE ( false statements ) THEN
example:
10 < IF ." Less than 10" ELSE ." Greater than or equal to 10" THEN
[edit] CASE-OF
( n -- ) CASE ( integer ) OF ( statements ) ENDOF ( integer ) OF ( statements ) ENDOF ( default instructions ) ENDCASE
example: a simple CASE selection
: test-case ( n -- )
CASE
0 OF ." Zero!" ENDOF
1 OF ." One!" ENDOF
." Some other number!"
ENDCASE ;
[edit] Execution vector
To obtain the efficiency of a C switch statement for enumerations, one needs to construct one's own execution vector.
: switch CREATE ( default-xt [count-xts] count -- ) DUP , 0 DO , LOOP , DOES> ( u -- ) TUCK @ MIN 1+ CELLS + @ EXECUTE ; :NONAME ." Out of range!" ; :NONAME ." nine" ; :NONAME ." eight" ; :NONAME ." seven" ; :NONAME ." six" ; :NONAME ." five" ; :NONAME ." four" ; :NONAME ." three" ; :NONAME ." two" ; :NONAME ." one" ; :NONAME ." zero" ; 10 switch digit 8 digit \ eight 34 digit \ Out of range!
[edit] Fortran
In ISO Fortran 90 and later, there are three conditional structures. There are also a number of other *unstructured* conditional statements, all of which are old and many of which are marked as "deprecated" in modern Fortran standards. These examples will, as requested, only cover conditional *structures*:
[edit] IF-THEN-ELSE
ANSI FORTRAN 77 or later has an IF-THEN-ELSE structure:
if ( a .gt. 20.0 ) then
q = q + a**2
else if ( a .ge. 0.0 ) then
q = q + 2*a**3
else
q = q - a
end if
[edit] SELECT-CASE
ISO Fortran 90 or later has a SELECT-CASE structure:
select (i)
case (21:) ! matches all integers greater than 20
q = q + i**2
case (0:20) ! matches all integers between 0 and 20 (inclusive)
q = q + 2*i**3
case default ! matches all other integers (negative in this particular case)
q = q - I
end select
[edit] WHERE-ELSEWHERE
ISO Fortran 90 and later has a concurrent, array-expression-based WHERE-ELSEWHERE structure. The logical expressions in WHERE and ELSEWHERE clauses must be array-values. All statements inside the structure blocks must be array-valued. Furthermore, all array-valued expressions and statements must have the same "shape". That is, they must have the same number of dimensions, and each expression/statement must have the same sizes in corresponding dimensions as each other expression/statement. For each block, wherever the logical expression is true, the corresponding elements of the array expressions/statements are evaluated/executed.
! diffusion grid time step
where (edge_type(1:n,1:m) == center)
anew(1:n,1:m) = (a(1:n,1:m) + a(0:n-1,1:m) + a(2:n+1,1:m) + a(1:n,0:m-1) + a(1:n,2:m+1)) / 5
elsewhere (edge_type(1:n,1:m) == left)
anew(1:n,1:m) = (a(1:n,1:m) + 2*a(2:n+1,1:m) + a(1:n,0:m-1) + a(1:n,2:m+1)) / 5
elsewhere (edge_type(1:n,1:m) == right)
anew(1:n,1:m) = (a(1:n,1:m) + 2*a(0:n-1,1:m) + a(1:n,0:m-1) + a(1:n,2:m+1)) / 5
elsewhere (edge_type(1:n,1:m) == top)
anew(1:n,1:m) = (a(1:n,1:m) + a(0:n-1,1:m) + a(2:n+1,1:m) + 2*a(1:n,2:m+1)) / 5
elsewhere (edge_type(1:n,1:m) == bottom)
anew(1:n,1:m) = (a(1:n,1:m) + a(0:n-1,1:m) + a(2:n+1,1:m) + 2*a(1:n,0:m-1)) / 5
elsewhere (edge_type(1:n,1:m) == left_top)
anew(1:n,1:m) = (a(1:n,1:m) + 2*a(2:n+1,1:m) + 2*a(1:n,2:m+1)) / 5
elsewhere (edge_type(1:n,1:m) == right_top)
anew(1:n,1:m) = (a(1:n,1:m) + 2*a(0:n-1,1:m) + 2*a(1:n,2:m+1)) / 5
elsewhere (edge_type(1:n,1:m) == left_bottom)
anew(1:n,1:m) = (a(1:n,1:m) + 2*a(2:n+1,1:m) + 2*a(1:n,0:m-1)) / 5
elsewhere (edge_type(1:n,1:m) == right_bottom)
anew(1:n,1:m) = (a(1:n,1:m) + 2*a(0:n-1,1:m) + 2*a(1:n,0:m-1)) / 5
elsewhere ! sink/source, does not change
anew(1:n,1:m) = a(1:n,1:m)
end where
[edit] Haskell
[edit] if-then-else
fac x = if x==0 then
1
else x * fac (x - 1)
[edit] Guards
fac x | x==0 = 1
| x>0 = x * fac (x-1)
[edit] Pattern matching
fac 0 = 1 fac x = x * fac (x-1)
[edit] case statement
fac x = case x of 0 -> 1
_ -> x * fac (x-1)
[edit] IDL
[edit] if-else
Basic if/then:
if a eq 5 then print, "a equals five" [else print, "a is something else"]
Any one statement (like these print statements) can always be expanded into a {begin ... end} pair with any amount of code in between. Thus the above will expand like this:
if a eq 5 then begin ... some code here ... endif [else begin ... some other code here ... endelse]
[edit] case
case <expression> of (choice-1): <command-1> [(choice-2): <command-2> [...]] [else: <command-else>] endcase
(Or replace any of the commands with {begin..end} pairs)
[edit] switch
switch <expression> of (choice-1): <command-1> [(choice-2): <command-2> [...]] [else: <command-else>] endswitch
The switch will execute all commands starting with the matching result, while the case will only execute the matching one.
[edit] on_error
on_error label
Will resume execution at label when an error is encountered. on_ioerror is similar but for IO errors.
[edit] Java
[edit] if-then-else
if(s.equals("Hello World"))
{
foo();
}
else if(s.equals("Bye World"))
{
bar();
}
else
{
deusEx();
}
Java also supports short-circuit evaluation. So in a conditional like this:
if(obj != null && obj.foo()){
aMethod();
}
obj.foo() will not be executed if obj != null returns false.
[edit] ternary
s.equals("Hello World") ? foo : bar
[edit] switch
This structure will only work if the code being switched on evaluates to an integer or character. There is no switching on Objects or floating-point types in Java.
switch(c) {
case 'a':
foo();
break;
case 'b':
bar();
default:
foobar();
}
This particular example can show the "fallthrough" behavior of a switch statement. If c is the character b, then bar() and foobar() will both be called. If c is the character a, only foo() will be called because of the break statement at the end of that case.
Also, the switch statement can be easily translated into an if-else if-else statement. The example above is equivalent to:
if(c == 'a'){
foo();
}else if(c == 'b'){
bar();
foobar();
}else{
foobar();
}
Cases without breaks at the end require duplication of code for all cases underneath them until a break is found (like the else if block shown here).
[edit] JavaScript
[edit] if-then-else
if( s == "Hello World" ) {
foo();
}
else if( s == "Bye World" ) {
bar();
}
else {
deusEx();
}
[edit] switch
switch(object) {
case 1:
one();
break;
case 2:
case 3:
case 4:
twoThreeOrFour();
break;
case 5:
five();
break;
default:
everythingElse();
}
[edit] conditional operator (?:)
var num = window.obj ? obj.getNumber() : null;
[edit] Lisaac
Works with: Lisaac version 0.13.1
[edit] if-then-else
+ n:INTEGER;
n := 3;
(n = 2).if {
IO.put_string "n is 2\n";
}.elseif {n = 3} then {
IO.put_string "n is 3\n";
}.elseif {n = 4} then {
IO.put_string "n is 4\n";
} else {
IO.put_string "n is none of the above\n";
}
[edit] Logo
if :x < 0 [make "x 0 - :x]
ifelse emptyp :list [print [empty]] [print :list]
UCBLogo and its descendants have also case:
make "letter "e show case :letter [ [[a e i o u] "true] [else "false] ] make "letter "x show case :letter [ [[a e i o u] "true] [else "false] ]
Output is:
true false
Logo also provides TEST which is local to a procedure:
to mytest :arg1 :arg2 test :arg1 = :arg2 iftrue [print [Arguments are equal]] iffalse [print [Arguments are not equal]] end
[edit] LSE64
The simple conditionals take single words rather than blocks of statements, as in most other languages.
t : " true" ,t f : " false" ,t true if t false ifnot f true ifelse t f
Cascading conditionals are constructed using duplicate definitions and "then", yielding a syntax reminiscent of functional language Pattern Matching.
onetwo : drop " Neither one nor two" ,t # default declared first onetwo : dup 2 = then " Two" ,t onetwo : dup 1 = then " One" ,t
Short-circuit operators "&&" and "||" are used for complex conditionals.
dup 0 = || ,t # avoid printing a null string
[edit] MAXScript
[edit] if
if x == 1 then
(
print "one"
)
else if x == 2 then
(
print "two"
)
else
(
print "Neither one or two"
)
[edit] case
Form one
case x of
(
1: (print "one")
2: (print "two")
default: (print "Neither one or two")
)
Form two
case of
(
(x == 1): (print "one")
(x == 2): (print "two")
default: (print "Neither one or two")
)
[edit] newLISP
[edit] if
Interpreter: newLISP v.9.0
(set 'x 1) (if (= x 1) (println "is 1"))
A third expression can be used as an else.
(set 'x 0) (if (= x 1) (println "is 1") (println "not 1"))
[edit] Object Pascal
- See Pascal
[edit] Objective-C
- See also C
One difference: the preprocessor has been extended with an #import directive which does the same thing as #include with "include guards".
[edit] OCaml
[edit] if-then-else
Compiler: OCaml 3.09
let condition = true if condition then 1 (* evaluate something *) else 2 (* evaluate something *)
If-then-else has higher precedence than ; (the semicolon), so if you want to have multiple statements with side effects inside an "if", you have to enclose it with begin...end or with parentheses:
if condition then begin (); (* evaluate things for side effects *) 5 end else begin (); (* evaluate things for side effects *) 42 end
[edit] match-with
match expression with | 0 -> () (* evaluate something *) | 1 -> () (* evaluate something *) | n when n mod 2 = 0 -> () (* evaluate something *) | _ -> () (* evaluate something *)
The first | is optional, and usually omitted.
Match is especially useful for Pattern Matching on various types of data structures.
Nested match's need to be surrounded by begin-end or parentheses, or else it won't know where it ends.
[edit] Pascal
[edit] if-then-else
Compiler: Turbo Pascal 7.0
IF condition1 THEN
procedure1
ELSE
procedure3;
IF condition1 THEN
BEGIN
procedure1;
procedure2;
END
ELSE
procedure3;
IF condition 1 THEN
BEGIN
procedure1;
procedure2;
END
ELSE
BEGIN
procedure3;
procedure4;
END;
[edit] case
Works with: Turbo Pascal version 7.0
In Pascal there's no fall-through, that is, if execution reaches the end of any case, execution continues after the end of the case statement, not in the code for the next case. Pascal implements the ELSE operator for all cases that do not match any of the given cases. Case selectors must be an ordinal type. This might seem to be a restriction, but with a little thought, just about anything can be resolved to an ordinal type with a bit of imagination. Additionally, each item in the case may consist of more then one item.
case i of
1,4,9: { executed if i is 1, 4 or 9 }
DoSomething;
11, 13 .. 17: { executed if i is 11, 13, 14, 15, 16 or 17 }
DoSomethingElse;
42: { executed only if i is 42 }
DoSomeOtherThing;
else
DoYetAnotherThing;
end;
Given the variable "X" as a char the following is valid:
Case X of
'A' : statement ;
'B' : statement ;
in ['C'..'W'] : statement ;
else
Statement ;
end;
[edit] Perl
Works with: Perl version 5
[edit] if/else
if ($expression) {
do_something;
};
# postfix conditional
do_something if $expression;
if ($expression) {
do_something;
} else {
do_fallback;
};
if ($expression1) {
do_something;
} elsif ($expression2) {
do_something_different;
} else {
do_fallback;
};
[edit] unless
unless behaves like if, only logically negated. You can use it wherever you can use if.
[edit] ternary operator
This is the same as the if/else example, only less readable.
$expression ? do_something : do_fallback;
[edit] switch
Switches are usually implemented with if chains as shown above. The for topicaliser, however, is great if you're going to do a lot of matching. Use next to avoid fall-through.
for ($input) {
$_ == 0 and print 'zero';
$_ eq 'coffee' and print 'C8H10N4O2';
print 'small' if 1 <= $_ and $_ <= 9 ;
/rats/ and print 'eek!';
do_fallback;
};
The Switch module is a funky source filter and rewrites the syntax on the fly, which has got a bad reputation.
use Switch;
switch ($input) {
case 0 {print 'input == 0'};
case 'coffee' {print 'input equal coffee'};
case [1..9] {print 'input between 1 and 9'};
case /rats/ {print 'input matches rats'};
else {
do_fallback;
};
};
[edit] PHP
[edit] if
Interpreter: PHP 3.x & 4.x & 5.x
<?php $foo = 3; if ($foo == 2) //do something if ($foo == 3) //do something else //do something else
if ($foo != 0)
{
//do something
}
else
{
//do another thing
}
?>
[edit] switch
Interpreter: PHP 3.x & 4.x & 5.x
<?php
switch ($i)
{
case "apple":
echo "i is apple";
break;
case "bar":
echo "i is bar";
break;
case "cake":
echo "i is cake";
break;
}
?>
[edit] See Also
[edit] Pop11
The simplest conditional is:
if condition then ;;; Action endif;
Two way conditional looks like:
if condition then ;;; Action1 else ;;; Alternative action endif;
One can do multiway choice using elseif clause
if condition1 then ;;; Action1 elseif condition2 then ;;; Action1 elseif condition2 then ;;; Action2 elseif condition3 then ;;; Action3 else ;;; Alternative action endif;
Instead of if keyword one can use unless keyword.
unless condition then /* Action */ endunless;
has the same meaning as
if not(condition) then /* Action */ endif;
One can also use elseunless keword.
if condition1 then ;;; Action1 elseunless condition2 then ;;; Action2 endif; ;;; Action2 endif;
has the same meaning as
if condition1 then ;;; Action1 elseif not(condition2) then ;;; Action2 endif;
Note that conditional must end in matching keyword, if must be finished by endif, unless must be finished by endunless (in the middle one can mix elseif with elseunless.
Pop11 conditional is an expression:
if x > 0 then 1 elseif x < 0 then -1 else 0 endif -> sign_x ;
assigns sign of x to sign_x.
Instead of multiway if one can use switchon construct (which is equivalent to a special case of if, but may be shorter).
switchon(x)
case .isstring then printf('A1');
notcase .isinteger then printf('A2');
case = 2 orcase = 3 then printf('A3');
case > 4 andcase < 15 then printf('A4');
else printf('A5');
endswitchon;
There is also multiway goto statement and conditional control transfers, we explain them together with other control transfers and loops (in case of loop exit/continue statements).
Pop11 also has preprocessor allowing conditional compilation:
#_IF condition1 /* Variant 1 */ #_ELSEIF condition2 /* Variant 2 */ #_ELSE /* Variant 3 */ #_ENDIF
condition1 and condition2 are arbitrary Pop11 expressions (they have access to all previously compiled code).
Also note that Pop11 syntax is user extensible, so users may create their own conditional constructs.
[edit] PostScript
The "if" operator uses two items form the stack, a procedure and a boolean. It will execute the procedure if the boolean is true. It will not leave anything on the stack (but the procedure might):
9 10 lt {(9 is less than 10) show} if
The "ifelse" operator expects two procedures and executes the one or the other depending on the value of the boolean. I.e. this:
/a 5 lt {(yeah)} {(nope)} ifelse show
will render either the string "yeah" or "nope" depending on whether a is less than 5 or not.
[edit] Python
[edit] if-then-else
if x == 0:
foo()
elif x == 1:
bar()
elif x == 2:
baz()
else:
boz()
[edit] ternary
Interpreter: Python 2.5
true_value if condition else false_value
Example:
secret='foo' print 'got it' if secret=='foo' else 'try again' 'got it'
[edit] Function dispatch dictionary
In some cases it's useful to associate functions with keys in a dictionary; and simply use this in lieu of long sequences of "if...elif...elif..." statements.
dispatcher = dict()
dispatcher[0]=foo # Not foo(): we bind the dictionary entry to the function's object,
# NOT to the results returned by an invocation of the function
dispatcher[1]=bar
dispatcher[2]=baz
dispatcher['default']=boz
# Then later
results = dispatcher.get(x, 'default')() # binding results to a name is optional
# or with no "default" case:
if x in dispatcher:
results=dispatcher[x]()
This can be particularly handy when using currying techniques, or when lampda expressions or meta-function generators (factories) can be used in place of normal named functions.
[edit] Rhope
Works with: Rhope version alpha 1
[edit] if-then-else
If[cond]
|:
Do Something[]
:||:
Do Something Else[]
:|
[edit] Ruby
[edit] if-then-else
if s == 'Hello World' foo elsif s == 'Bye World' bar else deus_ex end
[edit] case-when-else
case cartoon_character when 'Tom' chase when 'Jerry' flee else nil end
[edit] ternary
s == 'Hello World' ? foo : bar
[edit] Smalltalk
The pattern for handling a multi-option switch is to create classes for the various options, and let Polymorphism take care of the decisions.
[edit] ifTrue/ifFalse
"Conditionals in Smalltalk are really messages sent to Boolean objects"
balance > 0
ifTrue: [Transcript cr; show: 'still sitting pretty!'.]
ifFalse: [Transcript cr; show: 'No money till payday!'.].
You can also use them as the ternary operator
abs := x > 0 ifTrue: [ x ] ifFalse: [ x negated ]
[edit] SNUSP
$==?\==zero=====!/==#
\==non-zero==/
? is the only conditional operator. It skips one character if the current cell is zero.
! is an unconditional skip. !/ is the idiom for joining two lines of execution. ?! inverts the test.
\ and / redirect the flow of control. All the other characters besides $ and # are commentary.
[edit] Tcl
[edit] if-then-else
if {$foo == 3} {
puts "foo is three"
} elseif {$foo == 4} {
puts "foo is four"
} else {
puts "foo is neither three nor four"
}
[edit] switch
switch $foo {
3 {puts "foo is three"}
4 {puts "foo is four"}
default {puts "foo is something else"}
}
[edit] SQL
Works with: MS SQL version 2005
[edit] Conditional Expression
case when a then b else c end
declare @n int set @n=124 print case when @n=123 then 'equal' else 'not equal' end --If/ElseIf expression set @n=5 print case when @n=3 then 'Three' when @n=4 then 'Four' else 'Other' end
[edit] If/Else
declare @n int
set @n=123
if @n=123
BEGIN --begin/end needed if more than one statement inside
print 'one two three'
END
ELSE
if @n=124 print 'one two four'
else print 'other'
[edit] Toka
[edit] ifTrue
( condition ) ( quote ) ifTrue
100 100 = [ ." True\n" ] ifTrue 100 200 = [ ." True\n" ] ifTrue
[edit] ifFalse
( condition ) ( quote ) ifFalse
100 100 = [ ." True\n" ] ifFalse 100 200 = [ ." True\n" ] ifFalse
[edit] ifTrueFalse
( condition ) ( true quote ) ( false quote ) ifTrueFalse
100 100 = [ ." Equal\n" ] [ ." Not Equal\n" ] ifTrueFalse 100 200 = [ ." Equal\n" ] [ ." Not Equal\n" ] ifTrueFalse
[edit] V
[edit] ifThenElse
[true] ['is true' puts] ['is false' puts] ifte
=is true
[edit] ifThen
[true] ['is true' puts] if =is true
[edit] When
3 [ [1 =] [1 *] [2 =] [10 *] [3 =] [100 *] [4 =] [1000 *] ] when
=300
[edit] Choice
true 1 2 choice
=1
false 1 2 choice
=2
[edit] Visual Basic .NET
[edit] if-then-else
Basic
Dim result As String, a As String = "pants", b As String = "glasses" If a = b Then result = "passed" Else result = "failed" End If
Condensed
Dim result As String, a As String = "pants", b As String = "glasses" If a = b Then result = "passed" Else result = "failed" If a = b Then result = "passed" Else : result = "failed" End If If a = b Then : result = "passed" Else result = "failed" End If
[edit] if-then-elseif
Dim result As String, a As String = "pants", b As String = "glasses" If a = b Then result = "passed" ElseIf a <> b Then result = "failed" Else result = "impossible" End If
[edit] select-case-else
Dim result As String, a As String = "pants", b As String = "glasses"
Select Case a
Case b
result = "match"
Case a : result = "duh"
Case Else
result = "impossible"
End Select
[edit] inline-conditional
Imports Microsoft.VisualBasic
...
Dim result As String = CType(IIf("pants" = "glasses", "passed", "failed"), String) 'VB 1-8
Dim result As String = If("pants" = "glasses", "passed", "failed") 'VB 9
[edit] generic-inline-conditional
Works with: Microsoft (R) Visual Basic Compiler version 8.0
Imports Microsoft.VisualBasic
...
Function IIf2(Of T)(ByVal condition As Boolean, ByVal truepart As T, ByVal falsepart As T) As T
If condition Then Return truepart Else Return falsepart
End Function
...
Dim result As String = IIf2("pants" = "glasses", "passed", "failed") ' type is inferred
[edit] generic-inline-conditional
Language Version: 9.0+
Dim result As String = If("pants" = "glasses", "passed", "failed") ' type is inferred
[edit] VBScript
[edit] if-then-else
if (condition) then result = "met" else Result = "not met" end if
[edit] XSLT
The <xsl:if> element allows simple conditional processing.
<xsl:if test="condition"> <!-- executed if XPath expression evaluates to true --> </xsl:if>
The <xsl:choose>, <xsl:when>, and <xsl:otherwise> elements allow more general conditional processing.
<xsl:choose>
<xsl:when test="condition1">
<!-- executed if condition1 evaluates to true -->
</xsl:when>
<xsl:when test="condition2">
<!-- executed if condition2 evaluates to true -->
</xsl:when>
<-- etc. -->
<xsl:otherwise>
<!-- optional catch-all processing, like C's else or default -->
</xsl:otherwise>
</xsl:choose>
The XPath expressions allowed for the "test" attribute are those which return a boolean value:
@attrib = 'false' position() != last() not( false() ) boolean( $param ) contains( node, "stuff" ) and (position() > first())
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