Foreach

From Wikipedia, the free encyclopedia

For each (or foreach) is a computer language idiom for traversing items in a collection. Foreach is usually used in place of a standard for statement. Unlike other for loop constructs, however, foreach loops (usually) maintain no explicit counter: they essentially say "do this to everything in this set", rather than "do this x times". This can potentially avoid off-by-one errors and make code simpler to read. In object-oriented languages an iterator, even if implicit, is often used as the means of traversal.

The Python programming language is notable in having no kind of for loop other than one with the foreach semantics.

1 Syntax
2 Language support
- 2.1 C++
3 See also

[edit] Syntax

Syntax varies between languages. Most use the simple word for, roughly as follows:

for item in set:
  do something to item

[edit] Ada

Wikibooks' Ada_Programming has more about this subject:

Control

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

for I in X'Range loop
   X (I) := Get_Next_Element;
end loop;

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

[edit] Curly bracket programming language

In curly bracket programming languages, the syntax is similar to this:

[edit] C#

foreach (type item in set) {
  // do something to item
}

[edit] D

foreach(item; set) {
  // do something to item
}

[edit] Java

for (type item : set) {
  // do something to item
}

[edit] JavaScript

for (var strProperty in objObject) {
  /*
    do something to:
    objObject [strProperty]
  */
}

[edit] PHP

PHP has an idiosyncratic syntax:

foreach($set as $item)
  // do something to $item;

You can also extract the key from the set using this syntax:

$set = array (
   "name" => "John",
   "address" => "123 Main Street"
);
 
foreach ($set as $key => $value) {
  echo "{$key} has a value of {$value}";
}

The result would be:

name has a value of John address has a value of 123 Main Street

[edit] Python

for item in set:
  # do something to item

[edit] Smalltalk

Contrary to other languages, in Smalltalk the foreach loop is not a language construct but defined in the class Collection as a method with one parameter, which is the body as a closure. Also, Smalltalk defines collect, select, reject, etc. as known from OCL, however Smalltalk predates OCL by about twenty years.

coll := Array with: 'foo' with: 'bar' with: 'qux'.
coll do: [ :each | Transcript print: each ].

[edit] Visual Basic .NET

For Each item As type In set
 ' do something to item
Next item

[edit] Windows PowerShell

foreach ($item in $set) {
  # do something to $item
}

[edit] Language support

Some of the languages with support for foreach loops include ABC, Ada, C#, Cobra, D, ECMAScript, Java, Javascript, Perl, PHP, Python, REALbasic, Ruby, Smalltalk, Tcl, tcsh, Daplex (a query language), Unix shells, Visual Basic .NET and Windows PowerShell. Notable languages without foreach are C and C++.

[edit] C++

C++ does not have foreach, but its standard library includes a for_each function (in <algorithm>) which applies a function to all items between two iterators, and the Qt toolkit provides a foreach pseudo-keyword for its container classes, implemented as a macro. [1] There is also a similar macro in boost which performs within a few percent of the equivalent hand-coded loop. [2]

Foreach

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