Foreach loop

File:For-Loop-Mint-Programming-Language-Type-2.gif

Template:Mono loops are almost always used to iterate over items in a sequence of elements.

In computer programming, foreach loop (or for-each loop) is a control flow statement for traversing items in a collection. Template:Mono is usually used in place of a standard Template:Mono loop statement. Unlike other Template:Mono loop constructs, however, Template:Mono loops<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> usually maintain no explicit counter: they essentially say "do this to everything in this set", rather than "do this Template:Mono times". This avoids potential off-by-one errors and makes code simpler to read. In object-oriented languages, an iterator, even if implicit, is often used as the means of traversal.

The Template:Mono statement in some languages has some defined order, processing each item in the collection from the first to the last. The Template:Mono statement in many other languages, especially array programming languages, does not have any particular order. This simplifies loop optimization in general and in particular allows vector processing of items in the collection concurrently.

SyntaxEdit

Syntax varies among languages. Most use the simple word for, although other use the more logical word foreach, roughly as follows:

foreach(key, value) in collection {
  # Do something to value #
}

Language supportEdit

Programming languages which support foreach loops include ABC, ActionScript, Ada, C++ (since C++11), C#, ColdFusion Markup Language (CFML), Cobra, D, Daplex (query language), Delphi, ECMAScript, Erlang, Java (since 1.5), JavaScript, Lua, Objective-C (since 2.0), ParaSail, Perl, PHP, Prolog,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Python, R, REALbasic, Rebol,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Red,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Ruby, Scala, Smalltalk, Swift, Tcl, tcsh, Unix shells, Visual Basic (.NET), and Windows PowerShell. Notable languages without foreach are C, and C++ pre-C++11.

ActionScript 3.0Edit

ActionScript supports the ECMAScript 4.0 Standard<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> for for each .. in<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> which pulls the value at each index.

<syntaxhighlight lang="actionscript3"> var foo:Object = { "apple":1, "orange":2 };

for each (var value:int in foo) { trace(value); }

// returns "1" then "2" </syntaxhighlight>

It also supports for .. in<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> which pulls the key at each index.

<syntaxhighlight lang="actionscript3"> for (var key:String in foo) { trace(key); }

// returns "apple" then "orange" </syntaxhighlight>

AdaEdit

Template:Sister project Ada supports foreach loops as part of the normal for loop. Say X is an array:

<syntaxhighlight lang="Ada"> for I in X'Range loop

  X (I) := Get_Next_Element;

end loop; </syntaxhighlight>

This syntax is used on mostly arrays, but will also work with other types when a full iteration is needed.

Ada 2012 has generalized loops to foreach loops on any kind of container (array, lists, maps...):

<syntaxhighlight lang="Ada"> for Obj of X loop

  -- Work on Obj

end loop; </syntaxhighlight>

CEdit

The C language does not have collections or a foreach construct. However, it has several standard data structures that can be used as collections, and foreach can be made easily with a macro.

However, two obvious problems occur:

The macro is unhygienic: it declares a new variable in the existing scope which remains after the loop.
One foreach macro cannot be defined that works with different collection types (e.g., array and linked list) or that is extensible to user types.

C string as a collection of char <syntaxhighlight lang="c" highlight="4-6" line>

include <stdio.h>

/* foreach macro viewing a string as a collection of char values */

define foreach(ptrvar, strvar) \

char* ptrvar; \ for (ptrvar = strvar; (*ptrvar) != '\0'; *ptrvar++)

int main(int argc, char** argv) {

char* s1 = "abcdefg";
char* s2 = "123456789";
foreach (p1, s1) {
 printf("loop 1: %c\n", *p1);
}
foreach (p2, s2) {
 printf("loop 2: %c\n", *p2);
}
return 0;

} </syntaxhighlight>

C int array as a collection of int (array size known at compile-time) <syntaxhighlight lang="c" highlight="4-6" line>

include <stdio.h>

/* foreach macro viewing an array of int values as a collection of int values */

define foreach(intpvar, intarr) \

int* intpvar; \ for (intpvar = intarr; intpvar < (intarr + (sizeof(intarr)/sizeof(intarr[0]))); ++intpvar)

int main(int argc, char** argv) {

int a1[] = {1, 1, 2, 3, 5, 8};
int a2[] = {3, 1, 4, 1, 5, 9};
foreach (p1, a1) {
 printf("loop 1: %d\n", *p1);
}
foreach (p2, a2) {
 printf("loop 2: %d\n", *p2);
}
return 0;

} </syntaxhighlight>

Most general: string or array as collection (collection size known at run-time)

<syntaxhighlight lang="text" class="" style="" inline="1">idxtype</syntaxhighlight> can be removed and typeof(col[0]) used in its place with GCC

include <stdio.h>
include <string.h>

/* foreach macro viewing an array of given type as a collection of values of given type */

define arraylen(arr) (sizeof(arr)/sizeof(arr[0]))
define foreach(idxtype, idxpvar, col, colsiz) \

idxtype* idxpvar; \ for (idxpvar = col; idxpvar < (col + colsiz); ++idxpvar)

int main(int argc, char** argv) {

char* c1 = "collection";
int c2[] = {3, 1, 4, 1, 5, 9};
double* c3;
int c3len = 4;
c3 = (double*)calloc(c3len, sizeof(double)); 
c3[0] = 1.2; c3[1] = 3.4; c3[2] = 5.6; c3[3] = 7.8;

foreach (char, p1, c1, strlen(c1)) {
 printf("loop 1: %c\n", *p1);
}
foreach (int, p2, c2, arraylen(c2)) {
 printf("loop 2: %d\n", *p2);
}
foreach (double, p3, c3, c3len) {
 printf("loop 3: %.1lf\n", *p3);
}
return 0;

} </syntaxhighlight>

C#Edit

In C#, assuming that myArray is an array of integers:

foreach (int x in myArray) { Console.WriteLine(x); }

</syntaxhighlight>

Language Integrated Query (LINQ) provides the following syntax, accepting a delegate or lambda expression:

myArray.ToList().ForEach(x => Console.WriteLine(x));

</syntaxhighlight>

C++Edit

C++11 provides a foreach loop. The syntax is similar to that of Java:

include <iostream>

int main() {

 int myint[] = {1, 2, 3, 4, 5};

 for (int i : myint)
 {
   std::cout << i << '\n';
 }

} </syntaxhighlight>

C++11 range-based for statements have been implemented in GNU Compiler Collection (GCC) (since version 4.6), Clang (since version 3.0) and Visual C++ 2012 (version 11 <ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>)

The range-based for is syntactic sugar equivalent to:

 for (auto __anon = begin(myint); __anon != end(myint); ++__anon)
 {
   auto i = *__anon;
   std::cout << i << '\n';
 }

</syntaxhighlight>

The compiler uses argument-dependent lookup to resolve the begin and end functions.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The C++ Standard Library also supports for_each,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> that applies each element to a function, which can be any predefined function or a lambda expression. While range-based for is only from the start to the end, the range or direction can be changed by altering the first two parameters.

include <iostream>
include <algorithm> // contains std::for_each
include <vector>

int main() {

 std::vector<int> v {1, 2, 3, 4, 5};

 std::for_each(v.begin(), v.end(), [](int i)
 {
   std::cout << i << '\n';
 });

 std::cout << "reversed but skip 2 elements:\n";

 std::for_each(v.rbegin()+2, v.rend(), [](int i)
 {
   std::cout << i << '\n';
 });

} </syntaxhighlight>

Qt, a C++ framework, offers a macro providing foreach loops<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> using the STL iterator interface:

include <QList>
include <QDebug>

int main() {

 QList<int> list;

 list << 1 << 2 << 3 << 4 << 5;

 foreach (int i, list)
 {
   qDebug() << i;
 }

}</syntaxhighlight>

Boost, a set of free peer-reviewed portable C++ libraries also provides foreach loops:<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

include <boost/foreach.hpp>
include <iostream>

int main() {

 int myint[] = {1, 2, 3, 4, 5};

 BOOST_FOREACH(int &i, myint)
 {
   std::cout << i << '\n';
 }

} </syntaxhighlight>

C++/CLIEdit

The C++/CLI language proposes a construct similar to C#.

Assuming that myArray is an array of integers: <syntaxhighlight lang="c#"> for each (int x in myArray) {

   Console::WriteLine(x);

} </syntaxhighlight>

ColdFusion Markup Language (CFML)Edit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Script syntaxEdit

<syntaxhighlight lang="cfs"> // arrays arrayeach([1,2,3,4,5], function(v){

   writeOutput(v);

});

// or

for (v in [1,2,3,4,5]){

   writeOutput(v);

}

// or

// (Railo only; not supported in ColdFusion) letters = ["a","b","c","d","e"]; letters.each(function(v){

   writeOutput(v); // abcde

});

// structs for (k in collection){

   writeOutput(collection[k]);

}

// or

structEach(collection, function(k,v){

   writeOutput("key: #k#, value: #v#;");

});

// or // (Railo only; not supported in ColdFusion) collection.each(function(k,v){

   writeOutput("key: #k#, value: #v#;");

}); </syntaxhighlight>

Tag syntaxEdit

 <cfoutput>#v#</cfoutput>

</cfloop> </syntaxhighlight> CFML incorrectly identifies the value as "index" in this construct; the index variable does receive the actual value of the array element, not its index.

   <cfoutput>#collection[k]#</cfoutput>

</cfloop> </syntaxhighlight>

Common LispEdit

Common Lisp provides foreach ability either with the dolist macro: <syntaxhighlight lang="LISP"> (dolist (i '(1 3 5 6 8 10 14 17))

 (print i))

</syntaxhighlight> or the powerful loop macro to iterate on more data types <syntaxhighlight lang="LISP"> (loop for i in '(1 3 5 6 8 10 14 17)

     do (print i))

</syntaxhighlight>

and even with the mapcar function: <syntaxhighlight lang="LISP"> (mapcar #'print '(1 3 5 6 8 10 14 17)) </syntaxhighlight>

DEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="D"> foreach(item; set) {

 // do something to item

} </syntaxhighlight>

or

<syntaxhighlight lang="D"> foreach(argument) {

 // pass value

} </syntaxhighlight>

DartEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="Java"> for (final element in someCollection) {

 // do something with element

} </syntaxhighlight>

Object Pascal, DelphiEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Foreach support was added in Delphi 2005, and uses an enumerator variable that must be declared in the var section.

<syntaxhighlight lang="Delphi"> for enumerator in collection do begin

 //do something here

end; </syntaxhighlight>

EiffelEdit

The iteration (foreach) form of the Eiffel loop construct is introduced by the keyword across.

In this example, every element of the structure my_list is printed:

           across my_list as ic loop print (ic.item) end

</syntaxhighlight>

The local entity ic is an instance of the library class ITERATION_CURSOR. The cursor's feature item provides access to each structure element. Descendants of class ITERATION_CURSOR can be created to handle specialized iteration algorithms. The types of objects that can be iterated across (my_list in the example) are based on classes that inherit from the library class ITERABLE.

The iteration form of the Eiffel loop can also be used as a boolean expression when the keyword loop is replaced by either all (effecting universal quantification) or some (effecting existential quantification).

This iteration is a boolean expression which is true if all items in my_list have counts greater than three:

           across my_list as ic all ic.item.count > 3 end

</syntaxhighlight>

The following is true if at least one item has a count greater than three:

           across my_list as ic some ic.item.count > 3 end

</syntaxhighlight>

GoEdit

Go's foreach loop can be used to loop over an array, slice, string, map, or channel.

Using the two-value form gets the index/key (first element) and the value (second element): <syntaxhighlight lang="go"> for index, value := range someCollection { // Do something to index and value } </syntaxhighlight>

Using the one-value form gets the index/key (first element): <syntaxhighlight lang="go"> for index := range someCollection { // Do something to index } </syntaxhighlight><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

GroovyEdit

Groovy supports for loops over collections like arrays, lists and ranges:

<syntaxhighlight lang="Groovy"> def x = [1,2,3,4] for (v in x) // loop over the 4-element array x {

   println v

}

for (v in [1,2,3,4]) // loop over 4-element literal list {

   println v

}

for (v in 1..4) // loop over the range 1..4 {

   println v

} </syntaxhighlight>

Groovy also supports a C-style for loop with an array index:

<syntaxhighlight lang="Groovy"> for (i = 0; i < x.size(); i++) {

   println x[i]

} </syntaxhighlight>

Collections in Groovy can also be iterated over using the each keyword and a closure. By default, the loop dummy is named it

<syntaxhighlight lang="Groovy"> x.each{ println it } // print every element of the x array x.each{i-> println i} // equivalent to line above, only loop dummy explicitly named "i" </syntaxhighlight>

HaskellEdit

Haskell allows looping over lists with monadic actions using mapM_ and forM_ (mapM_ with its arguments flipped) from Control.Monad:

code	prints
<syntaxhighlight lang=Haskell> mapM_ print [1..4] </syntaxhighlight>	1 2 3 4
<syntaxhighlight lang=Haskell> forM_ "test" $ \char -> do putChar char putChar char </syntaxhighlight>	tteesstt

It's also possible to generalize those functions to work on applicative functors rather than monads and any data structure that is traversable using traverse (for with its arguments flipped) and mapM (forM with its arguments flipped) from Data.Traversable.

HaxeEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="actionscript"> for (value in iterable) {

   trace(value);

}

Lambda.iter(iterable, function(value) trace(value)); </syntaxhighlight>

JavaEdit

In Java, a foreach-construct was introduced in Java Development Kit (JDK) 1.5.0.<ref name="jdk5release"> "Enhanced for Loop - This new language construct[...]" {{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Official sources use several names for the construct. It is referred to as the "Enhanced for Loop",<ref name="jdk5release"/> the "For-Each Loop",<ref> "The For-Each Loop" {{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and the "foreach statement".<ref> "Implementing this interface allows an object to be the target of the "foreach" statement." {{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=Bloch>Template:Cite book</ref>Template:Rp

<syntaxhighlight lang="Java"> for (Type item : iterableCollection) {

   // Do something to item

} </syntaxhighlight>

Java also provides the stream api since java 8:<ref name=Bloch>Template:Cite book</ref>Template:Rp <syntaxhighlight lang="Java">

  List<Integer> intList = List.of(1, 2, 3, 4);
  intList.stream().forEach(i -> System.out.println(i));

</syntaxhighlight>

JavaScriptEdit

In ECMAScript 5, a callback-based forEach() method was added to the array prototype:<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><syntaxhighlight lang="javascript"> myArray.forEach(function (item, index) {

   // Do stuff with item and index 
   // The index variable can be omitted from the parameter list if not needed

}); </syntaxhighlight>The ECMAScript 6 standard introduced a more conventional for..of syntax that works on all iterables rather than operating on only array instances. However, no index variable is available with the syntax. <syntaxhighlight lang="javascript"> for (const item of myArray) {

   // Do stuff with item

} </syntaxhighlight>

For unordered iteration over the keys in an object, JavaScript features the for..in loop:

<syntaxhighlight lang="javascript"> for (const key in myObject) {

   // Do stuff with myObject[key]

} </syntaxhighlight>

To limit the iteration to the object's own properties, excluding those inherited through the prototype chain, it's often useful to add a hasOwnProperty() test (or a hasOwn() test if supported).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

<syntaxhighlight lang="javascript">for (const key in myObject) {

   // Available in older browsers
   if (myObject.hasOwnProperty(key)) {
       // Do stuff with object[key]
   }
   // Preferred in modern browsers
   if (Object.hasOwn(myObject, key)) {
       // Do stuff with object[key]
   }

}</syntaxhighlight>

Alternatively, the Object.keys() method combined with the for..of loop can be used for a less verbose way to iterate over the keys of an object.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> <syntaxhighlight lang="javascript"> const book = {

   name: "A Christmas Carol", 
   author: "Charles Dickens"

}; for (const key of Object.keys(book)) {

   console.log(`Key: ${key}, Value: ${book[key]}`);

} </syntaxhighlight>

LuaEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Source:<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Iterate only through numerical index values:<syntaxhighlight lang="lua"> for index, value in ipairs(array) do -- do something end </syntaxhighlight>Iterate through all index values:<syntaxhighlight lang="lua"> for index, value in pairs(array) do -- do something end </syntaxhighlight>

MathematicaEdit

In Mathematica, Do will simply evaluate an expression for each element of a list, without returning any value.

<syntaxhighlight lang="Mathematica"> In[]:= Do[doSomethingWithItem, {item, list}] </syntaxhighlight>

It is more common to use Table, which returns the result of each evaluation in a new list.

<syntaxhighlight lang="Mathematica"> In[]:= list = {3, 4, 5};

In[]:= Table[item^2, {item, list}] Out[]= {9, 16, 25} </syntaxhighlight>

MATLABEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="MATLAB"> for item = array %do something end </syntaxhighlight>

MintEdit

For each loops are supported in Mint, possessing the following syntax: <syntaxhighlight lang="Ruby"> for each element of list

   /* 'Do something.' */

end </syntaxhighlight>

The for (;;) or while (true) infinite loop in Mint can be written using a for each loop and an infinitely long list.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

<syntaxhighlight lang="Ruby"> import type /* 'This function is mapped to'

* 'each index number i of the'
* 'infinitely long list.'
*/

sub identity(x)

   return x

end /* 'The following creates the list'

* '[0, 1, 2, 3, 4, 5, ..., infinity]'
*/

infiniteList = list(identity) for each element of infiniteList

   /* 'Do something forever.' */

end </syntaxhighlight>

Objective-CEdit

Foreach loops, called Fast enumeration, are supported starting in Objective-C 2.0. They can be used to iterate over any object that implements the NSFastEnumeration protocol, including NSArray, NSDictionary (iterates over keys), NSSet, etc.

<syntaxhighlight lang="ObjC"> NSArray *a = [NSArray new]; // Any container class can be substituted

for(id obj in a) { // Dynamic typing is used. The type of object stored

                                 // in 'a' can be unknown. The array can hold many different 
                                 // types of object.

   printf("%s\n", [[obj description] UTF8String]);  // Must use UTF8String with %s
   NSLog(@"%@", obj);                               // Leave as an object

} </syntaxhighlight>

NSArrays can also broadcast a message to their members:

<syntaxhighlight lang="ObjC"> NSArray *a = [NSArray new];

[a makeObjectsPerformSelector:@selector(printDescription)]; </syntaxhighlight>

Where blocks are available, an NSArray can automatically perform a block on every contained item:

<syntaxhighlight lang="ObjC"> [myArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) { NSLog(@"obj %@", obj); if ([obj shouldStopIterationNow]) *stop = YES; }]; </syntaxhighlight>

The type of collection being iterated will dictate the item returned with each iteration. For example:

<syntaxhighlight lang="ObjC"> NSDictionary *d = [NSDictionary new];

for(id key in d) {

   NSObject *obj = [d objectForKey:key];      // We use the (unique) key to access the (possibly nonunique) object.
   NSLog(@"%@", obj);

} </syntaxhighlight>

OCamlEdit

OCaml is a functional programming language. Thus, the equivalent of a foreach loop can be achieved as a library function over lists and arrays.

For lists:

<syntaxhighlight lang="OCaml"> List.iter (fun x -> print_int x) [1;2;3;4];; </syntaxhighlight>

or in short way:

<syntaxhighlight lang="OCaml"> List.iter print_int [1;2;3;4];; </syntaxhighlight>

For arrays:

<syntaxhighlight lang="OCaml"> Array.iter (fun x -> print_int x) [|1;2;3;4|];; </syntaxhighlight>

or in short way:

<syntaxhighlight lang="OCaml"> Array.iter print_int [|1;2;3;4|];; </syntaxhighlight>

ParaSailEdit

The ParaSail parallel programming language supports several kinds of iterators, including a general "for each" iterator over a container:

<syntaxhighlight lang="parasail"> var Con : Container<Element_Type> := ... // ... for each Elem of Con concurrent loop // loop may also be "forward" or "reverse" or unordered (the default)

 // ... do something with Elem

end loop </syntaxhighlight>

ParaSail also supports filters on iterators, and the ability to refer to both the key and the value of a map. Here is a forward iteration over the elements of "My_Map" selecting only elements where the keys are in "My_Set": <syntaxhighlight lang="parasail"> var My_Map : Map<Key_Type => Univ_String, Value_Type => Tree<Integer>> := ... const My_Set : Set<Univ_String> := ["abc", "def", "ghi"];

for each [Str => Tr] of My_Map {Str in My_Set} forward loop

  // ... do something with Str or Tr

end loop </syntaxhighlight>

PascalEdit

In Pascal, ISO standard 10206:1990 introduced iteration over set types, thus:

<syntaxhighlight lang="Pascal"> var

 elt: ElementType;
 eltset: set of ElementType;

{...}

for elt in eltset do

 { ... do something with elt }

</syntaxhighlight>

PerlEdit

In Perl, foreach (which is equivalent to the shorter for) can be used to traverse elements of a list. The expression which denotes the collection to loop over is evaluated in list-context and each item of the resulting list is, in turn, aliased to the loop variable.

List literal example:

<syntaxhighlight lang="Perl"> foreach (1, 2, 3, 4) {

   print $_;

} </syntaxhighlight>

Array examples:

<syntaxhighlight lang="Perl"> foreach (@arr) {

   print $_;

} </syntaxhighlight>

<syntaxhighlight lang="Perl"> foreach $x (@arr) { #$x is the element in @arr

   print $x;

} </syntaxhighlight>

Hash example:

<syntaxhighlight lang="Perl"> foreach $x (keys %hash) {

   print $x . " = " . $hash{$x}; # $x is a key in %hash and $hash{$x} is its value

} </syntaxhighlight>

Direct modification of collection members:

<syntaxhighlight lang="Perl"> @arr = ( 'remove-foo', 'remove-bar' ); foreach $x (@arr){

   $x =~ s/remove-//;

}

Now @arr = ('foo', 'bar');

</syntaxhighlight>

PHPEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="php"> foreach ($set as $value) {

   // Do something to $value;

} </syntaxhighlight>

It is also possible to extract both keys and values using the alternate syntax:

<syntaxhighlight lang="php"> foreach ($set as $key => $value) {

   echo "{$key} has a value of {$value}";

} </syntaxhighlight>

Direct modification of collection members:

<syntaxhighlight lang="php"> $arr = array(1, 2, 3); foreach ($arr as &$value) { // The &, $value is a reference to the original value inside $arr

   $value++;

} // Now $arr = array(2, 3, 4);

// also works with the full syntax foreach ($arr as $key => &$value) {

   $value++;

} </syntaxhighlight>

More information

PythonEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="python"> for item in iterable_collection:

   # Do something with item

</syntaxhighlight>

Python's tuple assignment, fully available in its foreach loop, also makes it trivial to iterate on (key, value) pairs in dictionaries:

<syntaxhighlight lang="python"> for key, value in some_dict.items(): # Direct iteration on a dict iterates on its keys

   # Do stuff

</syntaxhighlight>

As for ... in is the only kind of for loop in Python, the equivalent to the "counter" loop found in other languages is...

<syntaxhighlight lang="python"> for i in range(len(seq)):

   # Do something to seq[i]

</syntaxhighlight>

... although using the enumerate function is considered more "Pythonic":

<syntaxhighlight lang="python"> for i, item in enumerate(seq):

   # Do stuff with item
   # Possibly assign it back to seq[i]

</syntaxhighlight>

REdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="R"> for (item in object) {

   # Do something with item

} </syntaxhighlight>

As for ... in is the only kind of for loop in R, the equivalent to the "counter" loop found in other languages is...

<syntaxhighlight lang="R"> for (i in seq_along(object)) {

   # Do something with objecti

} </syntaxhighlight>

RacketEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="racket"> (for ([item set])

 (do-something-with item))

</syntaxhighlight>

or using the conventional Scheme for-each function:

<syntaxhighlight lang="racket"> (for-each do-something-with a-list) </syntaxhighlight> do-something-with is a one-argument function.

RakuEdit

In Raku, a sister language to Perl, for must be used to traverse elements of a list (foreach is not allowed). The expression which denotes the collection to loop over is evaluated in list-context, but not flattened by default, and each item of the resulting list is, in turn, aliased to the loop variable(s).

List literal example:

<syntaxhighlight lang="Perl6"> for 1..4 {

   .say;

} </syntaxhighlight>

Array examples:

<syntaxhighlight lang="Perl6"> for @arr {

   .say;

} </syntaxhighlight>

The for loop in its statement modifier form:

<syntaxhighlight lang="Perl6"> for @arr -> $x {

   say $x;

} </syntaxhighlight>

<syntaxhighlight lang="Perl6"> for @arr -> $x, $y { # more than one item at a time

   say "$x, $y";

} </syntaxhighlight>

Hash example:

<syntaxhighlight lang="Perl6"> for keys %hash -> $key {

   say "$key: $hash{$key}";

} </syntaxhighlight>

or

<syntaxhighlight lang="Perl6"> for %hash.kv -> $key, $value {

   say "$key: $value";

} </syntaxhighlight>

or

<syntaxhighlight lang="Perl6"> for %hash -> $x {

   say "$x.key(): $x.value()";    # Parentheses needed to inline in double quoted string

} </syntaxhighlight>

Direct modification of collection members with a doubly pointy block, <->:

<syntaxhighlight lang="Perl6"> my @arr = 1,2,3; for @arr <-> $x {

   $x *= 2;

}

Now @arr = 2,4,6;

</syntaxhighlight>

RubyEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="Ruby"> set.each do |item|

 # do something to item

end </syntaxhighlight>

or

<syntaxhighlight lang="Ruby"> for item in set

 # do something to item

end </syntaxhighlight>

This can also be used with a hash.

<syntaxhighlight lang="Ruby"> set.each do |key,value|

 # do something to key
 # do something to value

end </syntaxhighlight>

RustEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} The for loop has the structure <syntaxhighlight lang="rust" inline>for <pattern> in <expression> { /* optional statements */ }</syntaxhighlight>. It implicitly calls the IntoIterator::into_iter method on the expression, and uses the resulting value, which must implement the Iterator trait. If the expression is itself an iterator, it is used directly by the for loop through an implementation of IntoIterator for all Iterators that returns the iterator unchanged. The loop calls the Iterator::next method on the iterator before executing the loop body. If Iterator::next returns Some(_), the value inside is assigned to the pattern and the loop body is executed; if it returns None, the loop is terminated.

<syntaxhighlight lang="Rust"> let mut numbers = vec![1, 2, 3];

// Immutable reference: for number in &numbers { // calls IntoIterator::into_iter(&numbers)

   println!("{}", number);

}

for square in numbers.iter().map(|x| x * x) { // numbers.iter().map(|x| x * x) implements Iterator

   println!("{}", square);

}

// Mutable reference: for number in &mut numbers { // calls IntoIterator::into_iter(&mut numbers)

   *number *= 2;

}

// prints "[2, 4, 6]": println!("{:?}", numbers);

// Consumes the Vec and creates an Iterator: for number in numbers { // calls IntoIterator::into_iter(numbers)

   // ...

}

// Errors with "borrow of moved value": // println!("{:?}", numbers); </syntaxhighlight>

ScalaEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="Scala"> // return list of modified elements items map { x => doSomething(x) } items map multiplyByTwo

for {x <- items} yield doSomething(x) for {x <- items} yield multiplyByTwo(x)

// return nothing, just perform action items foreach { x => doSomething(x) } items foreach println

for {x <- items} doSomething(x) for {x <- items} println(x)

// pattern matching example in for-comprehension for ((key, value) <- someMap) println(s"$key -> $value") </syntaxhighlight>

SchemeEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="Scheme"> (for-each do-something-with a-list) </syntaxhighlight> do-something-with is a one-argument function.

SmalltalkEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="Smalltalk"> collection do: [:item| "do something to item" ] </syntaxhighlight>

SwiftEdit

Swift uses the for…in construct to iterate over members of a collection.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

<syntaxhighlight lang="objc"> for thing in someCollection {

   // do something with thing

} </syntaxhighlight>

The for…in loop is often used with the closed and half-open range constructs to iterate over the loop body a certain number of times.

<syntaxhighlight lang="objc"> for i in 0..<10 {

   // 0..<10 constructs a half-open range, so the loop body
   // is repeated for i = 0, i = 1, …, i = 9.

}

for i in 0...10 {

   // 0...10 constructs a closed range, so the loop body
   // is repeated for i = 0, i = 1, …, i = 9, i = 10.

} </syntaxhighlight>

SystemVerilogEdit

SystemVerilog supports iteration over any vector or array type of any dimensionality using the foreach keyword.

A trivial example iterates over an array of integers:

code	prints
<syntaxhighlight lang="systemverilog"> int array_1d[] = '{ 3, 2, 1, 0 }; foreach array_1d[index] $display("array_1d[%0d]: %0d", index, array_1d[index]); </syntaxhighlight>	array_1d[0]: 3 array_1d[1]: 2 array_1d[2]: 1 array_1d[3]: 0

A more complex example iterates over an associative array of arrays of integers:

code	prints
<syntaxhighlight lang="systemverilog"> int array_2d[string][] = '{ "tens": '{ 10, 11 }, "twenties": '{ 20, 21 } }; foreach array_2d[key,index] $display("array_2d[%s,%0d]: %0d", key, index, array_2d[key,index]); </syntaxhighlight>	array_2d[tens,0]: 10 array_2d[tens,1]: 11 array_2d[twenties,0]: 20 array_2d[twenties,1]: 21

TclEdit

Tcl uses foreach to iterate over lists. It is possible to specify more than one iterator variable, in which case they are assigned sequential values from the list.

code	prints
<syntaxhighlight lang=Tcl> foreach {i j} {1 2 3 4 5 6} { puts "$i $j" } </syntaxhighlight>	1 2 3 4 5 6

It is also possible to iterate over more than one list simultaneously. In the following i assumes sequential values of the first list, j sequential values of the second list:

code	prints
<syntaxhighlight lang=Tcl> foreach i {1 2 3} j {a b c} { puts "$i $j" } </syntaxhighlight>	1 a 2 b 3 c

Visual Basic (.NET)Edit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="VBNet"> For Each item In enumerable

   ' Do something with item.

Next </syntaxhighlight>

or without type inference

<syntaxhighlight lang="VBNet"> For Each item As type In enumerable

   ' Do something with item.

Next </syntaxhighlight>

WindowsEdit

Conventional command processorEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Invoke a hypothetical frob command three times, giving it a color name each time. <syntaxhighlight lang="doscon"> C:\>FOR %%a IN ( red green blue ) DO frob %%a </syntaxhighlight>

Windows PowerShellEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="PowerShell"> foreach ($item in $set) {

   # Do something to $item

} </syntaxhighlight>

From a pipeline

<syntaxhighlight lang="PowerShell"> $list | ForEach-Object {Write-Host $_}

or using the aliases

$list | foreach {write $_} $list | % {write $_} </syntaxhighlight>

XSLTEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} <syntaxhighlight lang="xml">

<xsl:for-each select="set">
</xsl:for-each>

</syntaxhighlight><ref name="xslforeach">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

ReferencesEdit

Template:Reflist

ru:Цикл просмотра

Foreach loop

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HaxeEdit

JavaEdit

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MintEdit

Objective-CEdit

OCamlEdit

ParaSailEdit

PascalEdit

PerlEdit

PHPEdit

PythonEdit

REdit

RacketEdit

RakuEdit

RubyEdit

RustEdit

ScalaEdit

SchemeEdit

SmalltalkEdit

SwiftEdit

SystemVerilogEdit

TclEdit

Visual Basic (.NET)Edit

WindowsEdit

Conventional command processorEdit

Windows PowerShellEdit

XSLTEdit

See alsoEdit

ReferencesEdit