Loop structures: Difference between revisions
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Array.iteri</lang> |
Array.iteri</lang> |
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=={{header|Prolog}}== |
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There are three primitive methods of looping in Prolog: recursion, fail-driven loops, and repeat-driven loops. |
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<lang Prolog>% recursion as loop |
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print_each_element([E|T]) :- writeln(E), print_each_element(T). |
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print_each_element([]). |
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% fail-driven loop |
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fact(foo). |
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fact(bar). |
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fact(baz). |
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print_each_fact() :- |
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( fact(X), writeln(X), fail |
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; true ). |
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% equivalently |
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%print_each_fact :- fact(X), writeln(X), fail. |
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%print_each_fact. |
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% repeat-driven loop |
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print_each_fact_again :- |
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repeat, |
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fact(X), |
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writeln(X), |
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X = baz, |
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!. |
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go :- |
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print_each_element([foo, bar, baz]), |
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print_each_fact, |
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print_each_fact_again.</lang> |
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Of the three recursion is the favoured approach as it requires no non-logical predicates and is thus easy to read in its declarative form. |
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The fail-driven loop form is a(n ab)use of the built-in backtracking mechanism of Prolog's reasoning engine. In the specific example provided, fact(X) will first succeed, binding "foo" to X. It will then write "foo" to the output (as a side effect of the writeln/1 predicate). It then hits the call to fail/0 which is a non-logical predicate which always fails and thus always triggers backtracking. On backtracking, the runtime will try fact(X) again and will find that it is true when X is bound to "bar". This will then print and backtrack again. A third time binds to and prints "baz". A fourth time will fail because there is no more solution to the goal "fact(X)". This triggers a further backtrack and a try on the second branch of the disjunction. That second branch invokes the true/0 predicate which always succeeds. This exits the query with an overall success. |
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The repeat-driven loop uses similar (ab)use of the backtracking mechanism. Instead of employing a predicate that always fails, however, it employs one that will always succeed: repeat/0. Thus, in this sample, fact(X) works as before, as does writeln(X), but the attempt to unify X with "baz" will fail for the first two attempts, causing the system to backtrack until it hits repeat. Since repeat always succeeds it drives the engine forward again, testing each fact in succession. Once X is unified with "baz" (which is to say once X contains the value "baz") the predicate carries on. The cut operator !/0, guarantees that the predicate won't be re-entered later. |
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As with any language permitting higher-order invocations, using the looping primitives directly as above is often not a desirable thing. Instead higher-order features would be used. |
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<lang Prolog>% using maplist/2 to replace explicit recursion on a list |
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print_each_element(L) :- maplist(writeln, L). |
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% using forall/2 to replace an explicit fail-driven loop |
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fact(foo). |
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fact(bar). |
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fact(baz). |
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print_each_fact() :- forall(fact(X), writeln(X)).</lang> |
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There are a myriad of such predicates available in a useful Prolog implementation (SWI-Prolog provides, non-exhaustively: include/3, exclude/3, partition/4-5, maplist/2-5, foldl/4-7, scanl/4-6, aggregate/3-4, aggregate_all/3-4, forall/2, findall/3-4, findnsols/4-5, bagof/3, setof/3, … just as the more fundamental wrappings.) If the provided predicates do not permit the kinds of functionality desired for common patterns, it is trivial to make a new one. As an illustration, this is the source code for forall/2: |
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<lang Prolog>:- meta_predicate forall(0,0). |
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forall(A, B) :- \+ (call(A), \+ call(B)).</lang> |
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⚫ | |||
=== until === |
=== until === |
Revision as of 06:02, 11 June 2015
In this former task, we document loop structures offered by different languages.
What are loops?
Loops are control structures that allow sections of code to be executed repeatedly according to the controlling conditions of the loop.
There are two types of loops:
Repetition
Additionally, there is the overly simple repetitive loop: repetition. The simplistic construct executes a block of code, or a procedure, a given number of times, without explicitly exposing any state change to the looped procedure.
Iterative loops
An iterative loop repeatedly executes a set of instructions as the iterator steps through a series of values. Types of iterative loops include forloops and foreach loops. An iterative loop is a repetition but with a variable dependent on the current iteration. This allows the looped procedure to vary slightly between iterations. For example, the same operation can be carried out on each iteration, but each time on a different object.
Conditional loops
A conditional loop tests for a condition around the loop, and repeatedly executes a block of instructions whilst the condition is true. Types of conditional loops include while loops and until loops.
Examples here should be migrated to an appropriate Iteration page and removed from here. If a page does not exist demonstrating a particular loop structure, discuss it here.
AmbientTalk
doTimes
<lang ambienttalk>
// print 1 2 3 ... 20
20.doTimes { |i| system.print(" "+i); } </lang>
each
Iterate over a collection:
<lang ambienttalk> [ "foo", "bar", "baz" ].each: { |e|
system.print(" "+e);
} // prints: foo bar baz </lang>
AppleScript
NOT COVERED IN LOOP PAGES
repeat-until
set i to 5 repeat until i is less than 0 set i to i - 1 end repeat
repeat --endless loop end repeat
repeat-with
repeat with i from 1 to 20 --do something end repeat
set array to {1,2,3,4,5} repeat with i in array display dialog i end repeat
Brainf***
NOT EXPLAINED THIS MUCH IN LOOP PAGES BF's only control flow construct of any kind is a loop. Two of the eight commands define the start and end of a conditional loop.
- [ - branch forward past matching ']' if the current cell is zero
- ] - branch back to the matching '[' if the current cell is non-zero
In practice this is equivalent to a "while not zero" loop in other languages.
[-] set current cell to zero
[->+>+<<]>>[-<<+>>] copy cell 0 to cell 1, using cell 2 as temporary storage
C++
Compile-Time Control Structures
Necessary? A compile-time for loop can be generated with template metaprogramming. Example:
// the loop template<int start, int finish, template<int n, typename T> class X> struct loop { typedef typename X<start, typename loop<start+1, finish, X>::type>::type type; }; // the termination of the loop template<int finish, template<int n, typename T> class X> struct loop<finish, finish, X> { typedef typename X<finish, void>::type type; };
// example usage: This implements just a very complicated way of building a multi-dimensional array // the loop body template<int n, typename T> struct build_array { typedef T type[n]; }; template<int n> struct build_array<n, void> { typedef double type; }; // the loop execution: This is equivalent to // typedef double array_type[2][3][4][5]; typedef loop<2,6,build_array>::type array_type;
Clojure
NOT COVERED IN LOOP PAGES
loop
<lang clojure>
;; loop/recur is the most general looping construct (loop [lst [1 3 5 7] ret []] (if lst (recur (rest lst) (conj ret (* (first lst) (first lst)))) ret)) ==> [1 9 25 49]
</lang>
Crack
For
<lang crack> for( i=0; i<9; i++)
cout ` $i\n`;
</lang>
Dao
For
<lang java>for( i=0; i<9; ++i) io.writeln( i ); for( i = 0 : 8 ) io.writeln( i );</lang>
For In
<lang java>items = { 1, 2, 3 } for( item in items ) io.writeln( item )</lang>
While
<lang java>i = 0 while( i < 5 ) { i += 1 }</lang>
Do While
<lang java>i = 0 do { i += 1 } while( i < 9 )</lang>
Déjà Vu
For
Déjà Vu has a for-loop protocol, so you can write your own iterators. The most commonly used iterators are in
and range
. The first iterates over a list, the second takes two arguments and goes from the first to the second, like a classic for-loop.
<lang dejavu>for i range 1 3:
!print i # prints 1, 2 and 3</lang>
While
<lang dejavu>while true:
!print "This is the song that never ends..."</lang>
Repeat
<lang dejavu>repeat 3:
!print "This sentence is printed three times."</lang>
Factor
NOT COVERED IN LOOP PAGES
Looping
Most looping is done with recursion. Tail recursion is properly optimized.
: forever ( quot -- ) dup slip forever ; inline [ "A hungry raptor stalks you..." print flush 2000 random sleep ] forever
Iteration
Most indices are implicit or not present at all.
3 [ "pint" drink ] times { "high" "level" "language" } [ print ] each high level language 10 [ sq ] map { 0 1 4 9 16 25 36 49 64 81 } { 1 2 3 } { 4 5 6 } [ * ] 2map . { 4 10 18 } 10 [ even? ] subset . V{ 0 2 4 6 8 } 0 10 3 <range> >array . { 0 3 6 9 } 10 1 -2 <range> >array . { 10 8 6 4 2 } 2222 [ dup 0 > ] [ 2/ dup ] [ ] unfold nip . { 1111 555 277 138 69 34 17 8 4 2 1 0 }
Iterating with an index:
: indexed-alphabet. ( -- ) "abcdefghijklmnopqrstuvwxyz" [ [ 1string ] [ number>string ] bi* ": " glue print ] each-index ;
Forth
DO-LOOP
( limit start ) DO ( iterated statements ) LOOP ( limit start ) DO ( iterated statements ) ( increment ) +LOOP LEAVE \ exits a DO loop UNLOOP EXIT \ cleans up loop counters from return stack before returning from the current word
example: Two standard iterations
10 0 DO I . LOOP \ Prints the numbers from 0 to 9 10 0 DO I . 2 +LOOP \ Prints the even numbers from 0 to 8
BEGIN-UNTIL
BEGIN ( iterated statements ) ( conditional ) UNTIL
example: Counts down from a given number to zero
: COUNTDOWN ( n -- ) BEGIN DUP CR . 1- DUP 0< UNTIL DROP ;
BEGIN-AGAIN
BEGIN ( iterated statements ) AGAIN
example: echo user's input
: FOREVER ( -- ) BEGIN KEY EMIT AGAIN ;
BEGIN-WHILE-REPEAT
BEGIN ( unconditional iterated statements ) ( conditional ) WHILE ( conditional iterated statements ) REPEAT
example: counts down from a given number to one
: COUNTDOWN ( n -- ) BEGIN DUP WHILE CR DUP . 1- REPEAT DROP ;
Additional WHILE clauses may be added to a loop, but each extra WHILE requires a matching THEN after the REPEAT.
Mixed Structures
Because Forth's compiler is laid bare to the programmer, it is quite easy to both define your own looping structures or combine existing structures in interesting ways. The rules for such combining are somewhat involved, though discussions can be found in the gforth user's manual, among other places. These more complex looping constructs can make up for Forth's lack of a "break" word, and can allow expressing complex loops without resorting to boolean variables. A practical example is also found in the Binary search task.
A good example of a useful combination is this complex loop:
BEGIN ( condition 1 ) WHILE ( condition 2 ) UNTIL ( condition 2 succeeded ) ELSE ( condition 1 failed ) THEN
An example of using this idiom in practice might be this pseudo-Forth
BEGIN read-next-record WHILE found-record UNTIL process-record ELSE error" Ran out of records looking for the right one!" THEN
Frink
In all of the loops below, the curly braces can be omitted if the body is a single statement.
For Loop
A for
loop is really a foreach
loop that can work with range operators or iterate through various data structures. The to
operator creates an enumerating expression that lazily steps through its range.
<lang frink>
for i = 1 to 1000000
{
println[i]
} </lang>
The to
operator can be combined with a step
statement:
<lang frink>
for i = 1 to 1000000 step 3
println[i]
</lang>
As a foreach
statement. The for
construct can iterate over the elements of an array, set, dictionary, or enumerating expression.
<lang frink>
for i = [2,3,7,9]
println[i]
</lang>
Do...While Loop
<lang frink> i=0 do {
i = i+1
} while i<1000 </lang>
Groovy
While Loops
while (true) { println 'groovy' }
For Loops
// iterate over a range x = 0 for (i in 1..3) { x += i } assert x == 6 // iterate over a list x = 0 for (i in [1, 2, 3]) { x += i } assert x == 6 // iterate over an array x = 0 for (i in (1..3).toArray()) { x += i } assert x == 6 // iterate over a map's key/value pairs x = 0 for (i in map) { x += i.value } assert x = 6 // iterate over a map's values x = 0 for (i in map.values()) { x += i } assert x == 6 // iterate over the characters in a string text = 'abc' list = [] for (c in text) { list.add(c) } assert list == ['a', 'b', 'c']
Each
def stringList = [ "java", "perl", "python", "ruby" ]; def stringMap = [ "Su" : "Sunday", "Mo" : "Monday", "Tu" : "Tuesday", "We" : "Wednesday", "Th" : "Thursday", "Fr" : "Friday", "Sa" : "Saturday" ]; stringList.each() { print " ${it}" }; println ""; // java perl python ruby stringMap.each() { key, value -> println "${key} == ${value}" }; // Su == Sunday // We == Wednesday // Mo == Monday // Sa == Saturday // Th == Thursday // Tu == Tuesday // Fr == Friday stringList.eachWithIndex() { obj, i -> println " ${i}: ${obj}" }; // 0: java // 1: perl // 2: python // 3: ruby stringMap.eachWithIndex() { obj, i -> println " ${i}: ${obj}" }; // 0: Su=Sunday // 1: We=Wednesday // 2: Mo=Monday // 3: Sa=Saturday // 4: Th=Thursday // 5: Tu=Tuesday // 6: Fr=Friday
Haskell
Most of the usual applications for loops are realized in Haskell by operations on (lazy) lists, like map, fold or filter. It's unusual to use loops in an imperative style. However, if one insists on it, it's easy to make your own implementation of any loop variant.
Here are a few examples:
Pre-checked loop (while)
whileM :: Monad m => m Bool -> m a -> m () whileM cond body = cond >>= \b -> if b then body >> untilM cond body else return ()
Post-checked loop (repeat-until)
untilM :: Monad m => m Bool -> m a -> m () untilM cond body = body >> cond >>= \b -> if b then return () else untilM cond body
For-style loop
Simplest done by iterating over a list:
forM :: Monad m => [a] -> (a -> m ()) -> m () forM [] f = return () forM (x:xs) f = f x >> forM xs f
IDL
It should be noted that IDL programmers tend to avoid loops -- most of the time loops are used to access the elements of arrays or vectors, and since IDL is an array language the same purpose can almost always be served in a faster, more elegant and more readable way though any of the array operations.
for
for i=0,50,2 do print,i
prints out every second number starting at 0 stopping at 50. Wherever a single command can go in IDL, there can always go a begin...end pair with arbitrary amount of code in between. Thus the above can also read
for variable = start, stop [,increment] do begin ;some code here ;some more code endfor
It is allowed but not required to use the appropriate "type of end" - i.e. it would be allowed to just say "end" instead of "endfor". However "endfor" (and "endwhile", "endif" etc) will throw an error if the wrong one is encountered at compile time and thus it is recommended to always use the more descriptive form since it makes debugging a lot easier.
while
while condition do command
Same extensions as above:
while running do begin ; various snippets of code that might change the variable ; "running" from something true to something false end[while]
repeat
repeat command until condition
etc
goto
'Goto' exists and can in principle be forced to make a loop:
label: ;some code [if condition then $] goto label
break
The break statement will immediately terminate the current innermost for, while, repeat, if, case or switch without having to resort to a goto.
Logo
forever [right random 360 forward 10]
repeat 5 [right 180-36 forward 100]
Repeat and forever also have access to a loop counter, starting at 1.
repeat 10 [print repcount]
while [:x > 0] [make "x :x / 2] do.while [make "x :x / 2] [:x > 0]
until [:x = 0] [make "x :x / 2] do.until [make "x :x / 2] [:x = 0]
LSE64
LSE's loop words all work via tail recursion, like Scheme, by re-executing the current word. If used interactively, "repeat" works upon the current line. Exiting words, like "&&", "||", and "exit" can be used to exit an otherwise infinite loop (see other tasks).
infinite : " again and " ,t repeat limited : continue? &repeat
Counted loops execute a specified word n times. Within that word, "count" accesses a loop counter, counted down to zero.
body : count , sp 10 body iterate # 10 9 8 7 6 5 4 3 2 1 body? : count , sp count 5 > 10 body? &iterate # 10 9 8 7 6 5
Make
Make does looping through recursion. SUCC=`expr $* + 1` MAX=10 all: 0-n; %-n: %-echo @-make -f loop.mk $(SUCC)-n MAX=$(MAX) %-echo: @echo $* $(MAX)-n: $(MAX)-echo;
Invoking it
|make -f loop.mk MAX=2 0 1 2
newLISP
dotimes
(dotimes (x 10) (println (+ x 1)))
do-until
(set 'x 1) (do-until (= x 10) (inc 'x) (println x))
do-while
(set 'x 1) (do-while (< x 10) (inc 'x) (println x))
for
(for (x 1 10) (println x))
OCaml
let rec
The for and while loops are imperative features of OCaml, and most often it is rather recommended to prefer using functional loop designed with recursive functions, or better when iterating over a list or an array using a built-in iterator from the standard library or the extLib library. <lang ocaml>let rec loop i =
Printf.printf "%d\n" i; if i <= 4 then loop (i + 1)
in loop 0</lang>
Built-in Iterators
<lang ocaml>List.iter List.fold_left Array.iter Array.iteri</lang>
Prolog
There are three primitive methods of looping in Prolog: recursion, fail-driven loops, and repeat-driven loops.
<lang Prolog>% recursion as loop print_each_element([E|T]) :- writeln(E), print_each_element(T). print_each_element([]).
% fail-driven loop fact(foo). fact(bar). fact(baz). print_each_fact() :-
( fact(X), writeln(X), fail ; true ).
% equivalently %print_each_fact :- fact(X), writeln(X), fail. %print_each_fact.
% repeat-driven loop print_each_fact_again :-
repeat, fact(X), writeln(X), X = baz, !.
go :-
print_each_element([foo, bar, baz]), print_each_fact, print_each_fact_again.</lang>
Of the three recursion is the favoured approach as it requires no non-logical predicates and is thus easy to read in its declarative form.
The fail-driven loop form is a(n ab)use of the built-in backtracking mechanism of Prolog's reasoning engine. In the specific example provided, fact(X) will first succeed, binding "foo" to X. It will then write "foo" to the output (as a side effect of the writeln/1 predicate). It then hits the call to fail/0 which is a non-logical predicate which always fails and thus always triggers backtracking. On backtracking, the runtime will try fact(X) again and will find that it is true when X is bound to "bar". This will then print and backtrack again. A third time binds to and prints "baz". A fourth time will fail because there is no more solution to the goal "fact(X)". This triggers a further backtrack and a try on the second branch of the disjunction. That second branch invokes the true/0 predicate which always succeeds. This exits the query with an overall success.
The repeat-driven loop uses similar (ab)use of the backtracking mechanism. Instead of employing a predicate that always fails, however, it employs one that will always succeed: repeat/0. Thus, in this sample, fact(X) works as before, as does writeln(X), but the attempt to unify X with "baz" will fail for the first two attempts, causing the system to backtrack until it hits repeat. Since repeat always succeeds it drives the engine forward again, testing each fact in succession. Once X is unified with "baz" (which is to say once X contains the value "baz") the predicate carries on. The cut operator !/0, guarantees that the predicate won't be re-entered later.
As with any language permitting higher-order invocations, using the looping primitives directly as above is often not a desirable thing. Instead higher-order features would be used.
<lang Prolog>% using maplist/2 to replace explicit recursion on a list print_each_element(L) :- maplist(writeln, L).
% using forall/2 to replace an explicit fail-driven loop fact(foo). fact(bar). fact(baz). print_each_fact() :- forall(fact(X), writeln(X)).</lang>
There are a myriad of such predicates available in a useful Prolog implementation (SWI-Prolog provides, non-exhaustively: include/3, exclude/3, partition/4-5, maplist/2-5, foldl/4-7, scanl/4-6, aggregate/3-4, aggregate_all/3-4, forall/2, findall/3-4, findnsols/4-5, bagof/3, setof/3, … just as the more fundamental wrappings.) If the provided predicates do not permit the kinds of functionality desired for common patterns, it is trivial to make a new one. As an illustration, this is the source code for forall/2:
<lang Prolog>:- meta_predicate forall(0,0). forall(A, B) :- \+ (call(A), \+ call(B)).</lang>
Pop11
until
Variant of while is until loop:
until condition do /* Action */ enduntil;
is equivalent to:
while not(condition) do /* Action */ endwhile;
for
One can process all elements of a list:
for x in [a b c] do x => endfor;
It is possible to simultaneously process multiple lists:
for x y in [a b c], [1 2 3] do [^x ^y] => endfor;
in first iteration sets x to "a" and y to 1, in the second x is "b" and y is 2, in the third (last) iteration x is "c" and y is 3. The iteration ends when the shortest list is exhausted.
Sometimes one wants to process tails of the list, to do this use on keyword instead of in keyword:
for x on [a b c] do x => endfor;
in first iteration sets x to [a b c], in the second to [b c], etc...
There is also "counting" version of for loop:
for x from 2 by 2 to 7 do x => endfor;
goes trough 2, 4 and 6. Ommited by frase means by 1.
There is alse a C-like for loop:
for action1 step action2 till condition do /* Actions */ endfor;
is equivalent to
action1 while not(condition) do /* Actions */ action2 endwhile;
There are more specialized kinds of loops, but we skip them here.
quitloop quitif quitunless
Inside loops one can use control transfers to prematurely exit the loop or end current iteration and start the next one:
while true do n - 1 -> n; quitif(n=0); endwhile;
quits loop when n=0. quitloop unconditionally quits loop, quitunless(x) is equivalent to quitif(not(x)).
nextloop nextif nextunless
Similarely to quitloop nextloop unconditionally ends current iteration and starts the new one, nextif(x) ends current iteration when x is true, nextunless(x) is equivalent to nextif(not(x)). The loop control transfers can be also used inside for (and until) loops.
Finally, it is frequently possible to avoid explicit iteration using higher order map functions (like appdata and mapdata).
PostScript
The "for" operator expects three numbers and a procedure on the stack. It will consume all four arguments then it will push the first number on the stack, execute the procedure, increase the first number by the second number, push it on the stack, execute the procedure etc until the third number is reached. For example
10 12 200 {dup moveto 100 0 lineto} for stroke
will add lines to the currentpath that start at coordinates {10,10}; {22,22}; {34,34} ... and all end at {100,0}. The "stroke" operator then renders these lines on the current output device (usually a screen or a piece of paper).
Python
for
Frequently one wants to both iterate over a list and increment a counter:
<lang python>
mylist = ["foo", "bar", "baz"] for i, x in enumerate(mylist): print "Element no.", i, " is", x
</lang>
Iterating over more than one list + incrementing a counter:
<lang python>
for counter, [x, y, z] in enumerate(zip(lst1, lst2, lst3)): print counter, x, y, z
</lang>
list comprehension expressions
Typically used when you want to create a list and there is little logic involved. Faster than for loop:
positives = [n for n in numbers if n > 0]
A list comprehension is an expression rather than a statement. This allows them to be used as arguments to a function:
<lang python>
def square_each(n): results = [] for each in n: results.append(each * each) return results squares_3x5 = square_each([x for x in range(100) if (x%3)==0 and (x%5)==0]) # Return a list of all the squares of numbers from 1 up to 100 those numbers are # multiples of both 3 and 5.
</lang>
while
Typical use:
<lang python>
while True: # Do stuff... if condition: break
</lang>
You can add optional else, which is executed only if the expression tested was false. Typically used for searches.
<lang python>
while True: # Do stuff... if found: results = ... break else: print 'Not found'
</lang>
Since Python has no "bottom-tested" loop construct (such as "do ... until") ... it's common Python practice to either rethink the design in terms of iteration or to use an while 1 (infinite loop) and break out of it as appropriate.
Raven
each
10 each as i # counted loop "index is %(i)d\n" print 'hello world' each as c # string characters "%(c)c" print [ 'a' 'b' 'c' ] each as item # list items "%(item)s" print { 'a' 1 'b' 2 } each pair as k, v # hash key/val pairs "%(k)s => %(v)d\n" print 'SELECT * FROM table' mysql query each as row # iterable resource row print
repeat while/until
repeat <some_condition> while <some_process> repeat <come_condition> until <some_process>
Seed7
In Seed7 new statements can be declared easily. This feature is not explained here. Here are examples of the predefined loops:
while
while TRUE do foo; end while;
repeat
repeat foo; until FALSE;
for
for i range 0 to 4 do foo; end for;
for i range 4 downto 0 do foo; end for;
for stri range []("foo", "bar", "baz") do writeln(stri); end for;
SIMPOL
In SIMPOL there is only one looping construct. It is very flexible and can be used as a while, a repeat, or a combination of both.
while
<lang simpol>while [condition]
// Actions taken here
end while [condition]</lang>
Either condition or both can be present. Not setting either condition will result in an endless loop. The first condition will continue the loop if the result is true, the second will exit the loop if it is true. In SIMPOL code it is common to see both present, with the initial condition being the standard method of exiting the loop, and the end while condition used for exiting because of an error.
In this abbreviated example, the code will read records from a database table in a loop, placing each in record object in an array element, until it reaches the end of the table.
<lang simpol>function readrecs(sbme1table table)
sbme1record r array recs integer e, i
e = 0 i = 0 recs =@ array.new() r =@ table.select(error=e) while r !@= .nul i = i + 1 recs[i] =@ r r =@ r.select(error=e) end while e != 0
end function recs</lang>
The final condition should best be read as: "end the while, if this is true".
Another typical use of the loop is the traversing of rings that are commonly used in SIMPOL.
<lang simpol>function getfieldnames(sbme1table table)
sbme1field field integer i array fieldnames
fieldnames =@ array.new() i = 0 field =@ table.firstfield while field !@= .nul i = i + 1 fieldnames[i] = field.name field =@ field.next end while field =@= table.firstfield
end function fieldnames</lang>
SNUSP
==in==!/==body==?\==out== \==ydob===/
The basic loop structure can be modified in many ways. You can have a pre-loop test by including ? before the loop.
A unique feature of SNUSP is the ability to write bi-directional loops, designed for different effect depending on which direction the flow of control is running around the loop. The two entry points into this subroutine will move a value either up or down one cell, determined by the direction the loop is circulating.
# # up1====?!/->+<\ ? ? down1==?!\<+>-/ # #
Sparkling
While loop (test-first loop):
<lang sparkling>var n = -1; print("Enter 0 to exit");
while n != 0 {
n = toint(getline()); print(n);
}</lang>
For loop (alternate form of test-first loop, idiomatic approach to counting):
<lang sparkling>var i; for i = 0; i < 10; i++ {
print(i);
}</lang>
Do-while loop (test-last loop):
<lang sparkling>print("enter empty line to exit");
var s; do {
s = getline(); print(s);
} while s != "";</lang>
Toka
countedLoop
Counts up or down until the boundaries are met.
10 0 [ i . ] countedLoop 0 10 [ i . ] countedLoop
whileTrue
Repeatedly executes a quote until the quote returns true.
100 [ dup . 1 - dup 0 <> ] whileTrue
whileFalse
Repeatedly executes a quote until the quote returns true.
0 [ dup . 1 + dup 101 = ] whileFalse