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{{Short description |Control flow statement for repeated execution}}
{{merge|target=Loop (statement)|While loop|Do while loop| discuss=Talk:While loop#Merge proposal|date=September 2025}}
[[File:For loop example.svg|thumb |right |Flow diagram of a for loop that prints five asterisks.]]
{{Loop constructs}}<!-- DO NOT remove. Discuss navigation concept at [[Talk:Do while loop#Helpbox experiment]] -->

In [[computer programming]], a '''for loop''' is a [[structured programming |structured]] [[control flow]] [[Statement (computer science)|statement]] that repeatedly runs a section of code until a condition is satisfied.

A for loop has two parts: a header and a body. The header defines how the loop iterates, and the body is the code executed once per [[iteration]]. The header often declares a loop [[Variable (computer science)|variable]] which can be used in the body to know which iteration of the loop is being executed. A relatively simple for loop iterates for a fixed number of times. For example, the following [[C (programming language)|C]] for loop declares a loop variable {{code |i}} and prints its value as it increments from 0 to 1 to 2:

<syntaxhighlight lang="c">
for (int i = 0; i < 3; ++i) {
    printf("%d", i);
}
</syntaxhighlight>

Depending on [[programming language]], various [[reserved word |keywords]] are used to denote a for loop. For example, descendants of [[ALGOL]] use {{code |for}},<ref>{{Cite book |last=Wirth |first=Niklaus |author-link=Niklaus Wirth |year=1973 |chapter=Preface |title=Systematic Programming: An Introduction |pages=xiii |publisher=Prentice-Hall |isbn=0138803692}}</ref> while descendants of [[Fortran]] use {{code |do}} and [[COBOL]] uses {{code |PERFORM VARYING}}.

==Variations==
[[File:For loop.svg|thumb|350px|For loop illustration, from i=0 to i=2, resulting in data1=200]]
Generally, a for loop falls into one of the following categories:

===Numeric===
A numeric for loop increments a control variable from a start value to and end value. In languages such as [[ALGOL]], [[Simula]], [[BASIC]], [[Pascal (programming language)|Pascal]], [[Modula]], [[Oberon (programming language)|Oberon]], [[Ada (programming language)|Ada]], [[MATLAB]], [[OCaml]], and [[F Sharp (programming language)|F#]] it looks like:

<syntaxhighlight Lang="pascal">
for i = first to last do statement
</syntaxhighlight>

Some languages support an optional step value (increment or decrement). Some languages require a separate declaration of the control variable.

===3 part===
A 3-part for loop, popularized by [[C (programming language)|C]], has the following parts separated by semi-colons: [[Declaration (computer programming)|initialization]] ([[loop variant]]), [[Boolean algebra |condition]], and advancement to the next value. Each part is optional and can be blank. This syntax came from [[B (programming language)|B programming language]] and was originally invented by [[Stephen C. Johnson]].<ref name="ken">{{cite AV media |last=Thompson |first=Ken |author-link=Ken Thompson |website=[[YouTube]] |url=https://www.youtube.com/watch?v=EY6q5dv_B-o&t=2330 |title=VCF East 2019 – Brian Kernighan interviews Ken Thompson |archive-url=https://ghostarchive.org/varchive/youtube/20211212/EY6q5dv_B-o |archive-date=2021-12-12 |url-status=live |quote=I saw Johnson's semicolon version of the for loop and I put that in [B], I stole it. |access-date=2020-11-16}}{{cbignore}}</ref> In the initialization part, any variables needed are declared and assigned values. The condition part checks a condition and exits the loop if false, even if the loop is never executed. If the condition is true, then the loop's block of  is executed. The advancement to the next iteration part is performed each time the block is executed. 

The following [[Java (programming language)|Java]] code is a C-style numeric loop that prints the numbers from 0 to 99.

<syntaxhighlight Lang="java">
for (int i = 0; i < 100; ++i) {
    System.out.printf("%d", i );
}
</syntaxhighlight>

This form is often used like the numeric for loop, but is more flexible and can be used for other patterns. For example, {{code |for (start(); done(); next())}} calls functions instead of using a control variable. The pattern of an infinite loop is implemented as {{code |for (;;)}}.

===Iterator-based===
A [[foreach loop]] enumerates the items of a sequence. It is usually uses an implicit or explicit [[iterator]], in which the loop variable represents each value of the sequence. The following [[Python (programming language)|Python]] code shows a foreach loop. {{code |items}} is either a data collection that supports implicit iteration, or may be an iterator itself. 

<syntaxhighlight Lang="python">
for item in items:
    do_something(item)
</syntaxhighlight>

===Vectorized===
Some languages offer a for loop that acts as if processing all iterations [[Automatic vectorization|in parallel]], such as the {{code|for all}} keyword in [[Fortran 95]] which has the interpretation that ''all'' [[Sides of an equation|right-hand-side]] expressions are evaluated before ''any'' assignments are made, as distinct from the explicit iteration form. For example, in the {{code |for}} statement in the following [[pseudocode]] fragment, when calculating the new value for {{code |A(i)}}, except for the first (with {{code|1=i = 2}}) the reference to {{code |A(i - 1)}} will obtain the new value that had been placed there in the previous step. In the {{code |for all}} version, however, each calculation refers only to the original, unaltered {{code |A}}.

 '''for'''     i := 2 : N - 1 '''do''' A(i) := [A(i - 1) + A(i) + A(i + 1)] / 3; '''next''' i;
 '''for all''' i := 2 : N - 1 '''do''' A(i) := [A(i - 1) + A(i) + A(i + 1)] / 3;
The difference may be significant.

Some languages (such as PL/I, Fortran 90) also offer array assignment statements, that enable many for loops to be omitted. Thus pseudocode such as {{code|1=A:= 0;}} would set all elements of array A to zero, no matter its size or dimensionality. The example loop could be rendered as
<syntaxhighlight lang="Fortran">
 A(2 : N - 1) := [A(1 : N - 2) + A(2 : N - 1) + A(3 : N)] / 3;
</syntaxhighlight>
But whether that would be rendered in the style of the for loop or the for-all-loop or something else may not be clearly described in the compiler manual.

===Numeric & while===
Introduced with [[ALGOL 68]] and followed by PL/I, the compound numeric and while for loop iterates while the control value increments (as a numeric for loop) and while a condition is true. If the condition evaluates to false, then the loop exits before the loop variable reaches the last value. In the following code, {{code |i}} starts at 1 and the loops iterates until either {{code |i}} is {{code |N}} or {{code |Done}} is true.

 for i := 1 : N while not Done do Something()

==Loop counters==
In [[computer programming]], a ''' loop counter''' is a control variable that controls the iterations of a loop (a computer [[programming language]] construct). It is so named because most uses of this construct result in the variable taking on a range of integer values in some orderly sequences (for example., starting at 0 and ending at 10 in increments of 1)

Loop counters change with each iteration of a loop, providing a unique value for each iteration. The loop counter is used to decide when the loop should terminate and for the program flow to continue to the next [[instruction (computer science)|instruction]] after the loop.

A common [[identifier naming convention]] is for the loop counter to use the variable names '''i''', '''j''', and '''k''' (and so on if needed), where '''i''' would be the most outer loop, '''j''' the next inner loop, etc. The reverse order is also used by some programmers. This style is generally agreed to have originated from the early programming of Fortran{{Citation needed|date=August 2009}}, where these variable names beginning with these letters were implicitly declared as having an integer type, and so were obvious choices for loop counters that were only temporarily required. The practice dates back further to [[mathematical notation]] where [[index notation|indices]] for [[summation|sums]] and [[multiplication]]s are often '''i''', '''j''', etc. A variant convention is the use of duplicated letters for the index, '''ii''', '''jj''', and ''' kk''', as this allows easier searching and search-replacing than using a single letter.<ref>http://www.knosof.co.uk/vulnerabilities/loopcntrl.pdf Analysis of loop control variables in C</ref>

===Example===
An example of C code involving nested for loops, where the loop counter variables are '''i''' and '''j''':

<syntaxhighlight lang="c">
for (int i = 0; i < 100; i++) {
    for (int j = i; j < 10; j++) {
        someFunction(i, j);
    }
}
</syntaxhighlight>

Loops in C can also be used to print the reverse of a word. As:
<syntaxhighlight lang="c">
for (int i = 0; i < 6; i++) {
    scanf("%c", &a[i]);
}
for (int i = 4; i >= 0; i--) {
    printf("%c", a[i]);
}
</syntaxhighlight>
Here, if the input is {{code|apple}}, the output will be {{code|elppa}}.

==Additional semantics and constructs==
===Use as infinite loops===
{{Further|Infinite loop}}
A loop turns into an '''infinite loop''' when its condition never becomes false. This means the loop keeps running endlessly because it either doesn’t have a proper stopping condition, the termination condition is never met, or the loop is repeatedly instructed to restart from the beginning. <ref>{{Cite web |title=Loops in C: A Complete Guide |url=https://www.ccbp.in/blog/articles/loops-in-c |access-date=2025-11-17 |website=www.ccbp.in |language=en}}</ref>This C-style for loop is commonly the source of an [[infinite loop]] since the fundamental steps of iteration are completely in the control of the programmer. When infinite loops are intended, this type of for loop can be used (with empty expressions), such as:

<syntaxhighlight lang="c">
for (;;) {
    // loop body
}
</syntaxhighlight>

This style is used instead of infinite {{code|while (true)}} loops to avoid a [[type conversion]] warning in some C/C++ compilers.<ref>{{cite web |title=Compiler Warning (level 4) C4127 |url=http://msdn.microsoft.com/en-us/library/6t66728h.aspx |publisher=Microsoft |access-date=29 June 2011}}</ref> Some programmers prefer the more succinct {{code|for (;;)}} form over the semantically equivalent but more verbose {{code|while (true)}} form.

===Early exit and continuation===
Some languages may also provide other supporting statements, which when present can alter how the for loop iteration proceeds. Common among these are the [[Break statement|break]] and [[Control flow#Continuation with next iteration|continue]] statements found in C and its derivatives. The break statement causes the innermost loop to be terminated immediately when executed. The continue statement will move at once to the next iteration without further progress through the loop body for the current iteration. A for statement also terminates when a break, goto, or [[return statement]] within the statement body is executed.[Wells] Other languages may have similar statements or otherwise provide means to alter the for loop progress; for example in Fortran 90:

<syntaxhighlight lang="Fortran">
DO I = 1, N
  statements!Executed for all values of "I", up to a disaster if any.
  IF (no good) CYCLE! Skip this value of "I", and continue with the next.
  Statements!Executed only where goodness prevails.
  IF (disaster) EXIT! Abandon the loop.
  Statements!While good and, no disaster.
END DO! Should align with the "DO".
</syntaxhighlight>

Some languages offer further facilities such as naming the various loop constructs so that with multiple nested loops there is no doubt as to which loop is involved. Fortran 90, for example:
<syntaxhighlight lang="Fortran">
X1:DO I = 1, N
     statements
  X2:DO J = 1, M
       statements
       IF (trouble) CYCLE X1
       statements
     END DO X2
     statements
   END DO X1
</syntaxhighlight>
Thus, when "trouble" is detected in the inner loop, the CYCLE X1 (not X2) means that the skip will be to the next iteration for I, ''not'' J. The compiler will also be checking that each END DO has the appropriate label for its position: this is not just a documentation aid. The programmer must still code the problem correctly, but some possible blunders will be blocked.

===Loop variable scope and semantics===
Different languages specify different rules for what value the loop variable will hold on termination of its loop, and indeed some hold that it "becomes undefined". This permits a [[compiler]] to generate code that leaves any value in the loop variable, or perhaps even leaves it unchanged because the loop value was held in a register and never stored in memory. Actual behavior may even vary according to the compiler's optimization settings, as with the Honeywell Fortran66 compiler.

In some languages (not [[C (programming language)|C]] or [[C++]]) the loop variable is [[Immutable object|immutable]] within the scope of the loop body, with any attempt to modify its value being regarded as a semantic error. Such modifications are sometimes a consequence of a programmer error, which can be very difficult to identify once made. However, only overt changes are likely to be detected by the compiler. Situations, where the address of the loop variable is passed as an argument to a [[subroutine]], make it very difficult to check because the routine's behavior is in general unknowable to the compiler unless the language supports procedure signatures and argument intents.  Some examples in the style of pre-Fortran-90:

<syntaxhighlight lang="Fortran">
DO I = 1, N
  I = 7 !Overt adjustment of the loop variable. Compiler should complain
  Z = ADJUST(I) !Function "ADJUST" might alter "I", to uncertain effect.
  PRINT *, (A(I), B(I), I = 1, N, 2) !Implicit for loop to print odd elements of arrays A and B, reusing "I"... Compiler should complain.
  PRINT *, I                         ! What value will be presented?
END DO! How many times will the loop be executed?
</syntaxhighlight>

A common approach is to calculate the iteration count at the start of a loop (with careful attention to overflow as in {{code|1=for i:= 0: 65535 do ... ;}} in sixteen-bit integer arithmetic) and with each iteration decrement this count while also adjusting the value of {{mono|I}}: double counting results. However, adjustments to the value of {{mono|I}} within the loop will not change the number of iterations executed.

Still, another possibility is that the code generated may employ an auxiliary variable as the loop variable, possibly held in a machine register, whose value may or may not be copied to {{mono|I}} on each iteration. Again, modifications of {{mono|I}} would not affect the control of the loop, but now a disjunction is possible: within the loop, references to the value of {{mono|I}} might be to the (possibly altered) current value of {{mono|I}} or to the auxiliary variable (held safe from improper modification) and confusing results are guaranteed. For instance, within the loop a reference to element {{mono|I}} of an array would likely employ the auxiliary variable (especially if it were held in a machine register), but if {{mono|I}} is a parameter to some routine (for instance, a ''print''-statement to reveal its value), it would likely be a reference to the proper variable {{mono|I}} instead. It is best to avoid such possibilities.

====Adjustment of bounds====
Just as the index variable might be modified within a for loop, so also may its bounds and direction. But to uncertain effect. A compiler may prevent such attempts, they may have no effect, or they might even work properly - though many would declare that to do so would be wrong. Consider a statement such as
 '''for''' i := first : last : step '''do'''
   A(i) := A(i) / A(last);
If the approach to compiling such a loop was to be the evaluation of {{mono|first}}, {{mono|last}} and {{mono|step}} and the calculation of an iteration count via something like {{code|(last - first)/step}} once only at the start, then if those items were simple variables and their values were somehow adjusted during the iterations, this would have no effect on the iteration count even if the element selected for division by {{code|A(last)}} changed.

===List of value ranges===
ALGOL 60, PL/I, and ALGOL 68, allow loops in which the loop variable is iterated over a list of ranges of values instead of a single range. The following PL/I example will execute the loop with six values of i: 1, 7, 12, 13, 14, 15:
<syntaxhighlight lang="rexx">
do i = 1, 7, 12 to 15;
  /*statements*/
end;
</syntaxhighlight>

==Equivalence with while-loops==
A for loop is generally equivalent to a while-loop:

 factorial := 1
  for counter from 2 to 5
      factorial := factorial * counter
 counter:= counter - 1
 print counter + "! equals " + factorial

Is equivalent to:

 factorial := 1
 counter := 1
  while counter < 5
     counter := counter + 1
     factorial := factorial * counter
 print counter + "! equals " + factorial

As demonstrated by the output of the variables.

==Timeline of the ''for loop'' syntax in various programming languages==
Given an action that must be repeated, for instance, five times, different languages' for loops will be written differently. The syntax for a three-expression for loop is nearly identical in all languages that have it, after accounting for different styles of block termination and so on.

===1957: FORTRAN===
{{Further|Fortran}}
Fortran's equivalent of the {{mono|for}} loop is the {{mono|DO}} loop,
using the keyword do instead of for,
The syntax of Fortran's {{mono|DO}} loop is:
<syntaxhighlight lang="fortranfixed">
        DO label counter = first, last, step
          statements
label     statement
</syntaxhighlight>
The following two examples behave equivalently to the three argument for loop in other languages,
initializing the counter variable to 1, incrementing by 1 each iteration of the loop, and stopping at five (inclusive).
<syntaxhighlight lang="fortranfixed">
        DO 9, ICOUNT = 1, 5, 1
          WRITE (6,8) ICOUNT
    8     FORMAT( I2 )
    9   CONTINUE
</syntaxhighlight>
As of Fortran 90, block structured {{mono|END DO}} was added to the language.  With this, the end of loop label became optional:
<syntaxhighlight lang="Fortran">
do icounter = 1, 5
  write(*, '(i2)') icounter
end do
</syntaxhighlight>
The step part may be omitted if the step is one. Example:
<syntaxhighlight lang="fortranfixed">
* DO loop example.
       PROGRAM MAIN
         INTEGER SUMSQ
         SUMSQ = 0
         DO 199 I = 1, 9999999
           IF (SUMSQ.GT.1000) GO TO 200
199        SUMSQ = SUMSQ + I**2
200      PRINT 206, SUMSQ
206      FORMAT( I2 )
       END
</syntaxhighlight>

In Fortran 90, the {{mono|GO TO}} may be avoided by using an {{mono|EXIT}} statement.
<syntaxhighlight lang="fortranfixed">
* DO loop example.
       program main
         implicit none

         integer:: sumsq
         integer:: i

         sumsq = 0
         do i = 1, 9999999
           if (sumsq > 1000) exit
           sumsq = sumsq + i**2
         end do
         print *, sumsq

       end program
</syntaxhighlight>
Alternatively, a {{mono|DO - WHILE}} construct could be used:
<syntaxhighlight lang="fortranfixed">
       program main
         implicit none

         integer:: sumsq
         integer:: i

         sumsq = 0
         i = 0
         do while (sumsq <= 1000)
           i = i+1
           sumsq = sumsq + i**2
         end do
         print *, sumsq

       end program
</syntaxhighlight>

===1958: ALGOL===
{{Further|ALGOL 58}}
ALGOL 58 introduced the {{code|for}} statement, using the form as Superplan:

  FOR ''Identifier'' = ''Base'' (''Difference'') ''Limit''

For example to print 0 to 10 incremented by 1:
<syntaxhighlight lang="pascal">
FOR x = 0 (1) 10 BEGIN
PRINT (FL) = x END
</syntaxhighlight>

In [[ALGOL 60]] the looping construct was changed.  A series of 'for list elements', separated by commas, can be specified.  An element can either be an arithmetic expression element, a 'step-until' element, or a 'while' element.

To print from 0 to 10, then the number 42:
<syntaxhighlight lang="pascal">
  for x := 0 step 1 until 10, 42 do
    begin
      outinteger (1, x);
      outstring (1, "\n")
    end;
</syntaxhighlight>

A mostly equivalent loop using a 'while' element could be written:
<syntaxhighlight lang="pascal">
  for x := 0, x while x <= 10, 42 do
    begin
      outinteger (1, x);
      outstring (1, "\n");
      x := x + 1
    end;
</syntaxhighlight>

Note the value of x after the first loop has completed will be 42, whereas in the second loop it will be 43.

===1960: COBOL===
{{Further|COBOL}}
[[COBOL]] was formalized in late 1959 and has had many elaborations. It uses the PERFORM verb which has many options. Originally all loops had to be out-of-line with the iterated code occupying a separate paragraph. Ignoring the need for declaring and initializing variables, the COBOL equivalent of a for loop would be.

<syntaxhighlight lang="cobol">
      PERFORM SQ-ROUTINE VARYING I FROM 1 BY 1 UNTIL I > 1000

      SQ-ROUTINE
             ADD I**2 TO SUM-SQ.
</syntaxhighlight>

In the 1980s, the addition of in-line loops and ''[[structured programming]]'' statements such as END-PERFORM resulted in a for loop with a more familiar structure.

<syntaxhighlight lang="cobol">
      PERFORM VARYING I FROM 1 BY 1 UNTIL I > 1000
             ADD I**2 TO SUM-SQ.
      END-PERFORM
</syntaxhighlight>
If the PERFORM verb has the optional clause TEST AFTER, the resulting loop is slightly different: the loop body is executed at least once, before any test.

===1964: BASIC===
{{Further|BASIC}}
In [[BASIC]], a loop is sometimes named a ''for-next loop''.

<syntaxhighlight lang="basic">
10 REM THIS FOR LOOP PRINTS ODD NUMBERS FROM 1 TO 15
20 FOR I = 1 TO 15 STEP 2
30 PRINT I
40 NEXT I
</syntaxhighlight>
The end-loop marker specifies the name of the index variable, which must correspond to the name of the index variable at the start of the for loop. Some languages (PL/I, Fortran 95, and later) allow a statement label at the start of a for loop that can be matched by the compiler against the same text on the corresponding end-loop statement. Fortran also allows the {{code|EXIT}} and {{code|CYCLE}} statements to name this text; in a nest of loops, this makes clear which loop is intended. However, in these languages, the labels must be unique, so successive loops involving the same index variable cannot use the same text nor can a label be the same as the name of a variable, such as the index variable for the loop.

===1964: PL/I===
{{Further|PL/I}}
The PL/I <code>DO</code> statement can act as either a for loop, a [[while loop]], or a do until loop.
<syntaxhighlight lang="rexx">
do counter = 1 to 5 by 1; /* "by 1" is the default if not specified */
  /*statements*/;
  end;
</syntaxhighlight>

The {{mono|LEAVE}} statement may be used to exit the loop. Loops can be [[Label (computer science)|labeled]], and ''leave'' may leave a specific labeled loop in a group of nested loops. Some PL/I dialects include the {{mono|ITERATE}} statement to terminate the current loop iteration and begin the next.

===1968: ALGOL 68===
{{Further|ALGOL 68}}
ALGOL 68 has what was considered ''the'' universal loop, the full syntax is:

 FOR i FROM 1 BY 2 TO 3 WHILE i≠4 DO ~ OD

Further, the single iteration range could be replaced by a list of such ranges. There are several unusual aspects of the construct
* only the {{code|do ~ od}} portion was compulsory, in which case the loop will iterate indefinitely.
* thus the clause {{code|to 100 do ~ od}}, will iterate exactly 100 times.
* The {{code|while}} ''syntactic element'' allowed a programmer to break from a {{code|for}} loop early, as in:
{{pre|1=
INT sum sq := 0;
FOR i
 WHILE
  print(("So far:", i, new line)); # Interposed for tracing purposes. #
  sum sq ≠ 70↑2                    # This is the test for the WHILE   #
DO
  sum sq +:= i↑2
OD
}}

Subsequent ''extensions'' to the standard ALGOL 68 allowed the {{code|to}} syntactic element to be replaced with {{code|{{sic|hide=y|up|to}}}} and {{code|downto}} to achieve a small optimization. The same compilers also incorporated:
;{{code|until}}: for late loop termination.
;{{code|foreach}}: for working on arrays in [[parallel computing|parallel]].

===1970: Pascal===
{{Further|Pascal (programming language)}}
<syntaxhighlight lang="pascal">
for Counter:= 1 to 5 do
  (*statement*);
</syntaxhighlight>

Decrementing (counting backwards) is using {{code|downto}} keyword instead of {{code|to}}, as in:

<syntaxhighlight lang="pascal">
for Counter:= 5 downto 1 do
  (*statement*);
</syntaxhighlight>

The numeric range for loop varies somewhat more.

===1972: C, C++===
{{Further|C (programming language)|C++|C syntax#Iteration statements}}
<syntaxhighlight lang="c">
for (initialization; condition; increment/decrement)
    statement
</syntaxhighlight>
The {{mono|statement}} is often a block statement; an example of this would be:
<syntaxhighlight lang="c">
//Using for loops to add numbers 1 - 5
int sum = 0;
for (int i = 1; i <= 5; ++i) {
    sum += i;
}
</syntaxhighlight>
The ISO/IEC 9899:1999 publication (commonly known as [[C99]]) also allows initial declarations in {{code|for}} loops. All three sections in the for loop are optional, with an empty condition equivalent to true.

===1972: Smalltalk===
{{Further|Smalltalk}}
<syntaxhighlight lang="smalltalk">1 to: 5 do: [ :counter | "statements" ]</syntaxhighlight>
Contrary to other languages, in [[Smalltalk]] a for loop is not a [[language construct]] but is defined in the class Number as a method with two parameters, the end value and a [[Closure (computer science)|closure]], using self as start value.

===1980: Ada===
{{Further|Ada (programming language)}}
<syntaxhighlight lang="ada">
for Counter in 1 .. 5 loop
   -- statements
end loop;
</syntaxhighlight>

The ''exit'' statement may be used to exit the loop. Loops can be labeled, and ''exit'' may leave a specifically labeled loop in a group of nested loops:
<syntaxhighlight lang="ada">
Counting:
    For Counter in 1 .. 5 loop
   Triangle:
       for Secondary_Index in 2 .. Counter loop
          -- statements
          exit Counting;
          -- statements
       end loop Triangle;
    end loop Counting;
</syntaxhighlight>

===1980: Maple===
{{Further|Maple (software)}}
Maple has two forms of for loop, one for iterating over a range of values, and the other for iterating over the contents of a container. The value range form is as follows:

 '''for''' ''i'' '''from''' ''f'' '''by''' ''b'' '''to''' ''t'' '''while''' ''w'' '''do'''
     ''# loop body''
 '''od''';

All parts except <code>'''do'''</code> and <code>'''od'''</code> are optional. The <code>''' for''' ''I''</code> part, if present, must come first. The remaining parts (<code>'''from''' ''f''</code>, <code>'''by''' ''b''</code>, <code>'''to''' ''t''</code>, <code>'''while''' ''w''</code>) can appear in any order.

Iterating over a container is done using this form of loop:

 '''for''' ''e'' '''in''' ''c'' '''while''' ''w'' '''do'''
     ''# loop body''
 '''od''';

The <code>''' in''' ''c''</code> clause specifies the container, which may be a list, set, sum, product, unevaluated function, array, or object implementing an iterator.

A for loop may be terminated by <code>'''od'''</code>, <code>'''end'''</code>, or <code>'''end do'''</code>.

===1982: Maxima CAS===
{{Further|Maxima (software)}}
In Maxima CAS, one can use also integer values:
<syntaxhighlight lang="maxima">
for x:0.5 step 0.1 thru 0.9 do
    /* "Do something with x" */
</syntaxhighlight>

===1982: PostScript===
{{Further|PostScript}}
The for loop, written as {{code|[initial] [increment] [limit] { ... } for|postscript}} initializes an internal variable, and executes the body as long as the internal variable is not more than the limit (or not less, if the increment is negative) and, at the end of each iteration, increments the internal variable. Before each iteration, the value of the internal variable is pushed onto the stack.<ref>{{cite book |year=1999 |title=PostScript Language Reference |publisher=Addison-Wesley Publishing Company |page=596 |isbn=0-201-37922-8}}</ref>
<syntaxhighlight lang="postscript">
1 1 6 {STATEMENTS} for
</syntaxhighlight>
There is also a simple repeat loop.
The repeat-loop, written as {{code|X { ... } repeat|postscript}}, repeats the body exactly X times.<ref>{{cite web|url=http://pscript.dubmun.com/tutorial4.html|title=PostScript Tutorial - Loops}}</ref>
<syntaxhighlight lang="postscript">
5 { STATEMENTS } repeat
</syntaxhighlight>

===1983: Ada 83 and above===
{{Further|Ada (programming language)}}
<syntaxhighlight lang="ada">
procedure Main is
  Sum_Sq : Integer := 0;
begin
  for I in 1 .. 9999999 loop
    if Sum_Sq <= 1000 then
      Sum_Sq := Sum_Sq + I**2
    end if;
  end loop;
end;
</syntaxhighlight>

===1984: MATLAB===
{{Further|MATLAB}}
<syntaxhighlight lang="Matlab">
for n = 1:5
     -- statements
end</syntaxhighlight>
After the loop, {{code|n}} would be 5 in this example.

As {{code|i}} is used for the [[Imaginary unit]], its use as a loop variable is discouraged.

===1987: Perl===
{{Further|Perl}}
<syntaxhighlight lang="perl">
for ($counter = 1; $counter <= 5; $counter++) { # implicitly or predefined variable
    # statements;
}
for (my $counter = 1; $counter <= 5; $counter++) { # variable private to the loop
    # statements;
}
for (1..5) { # variable implicitly called $_; 1..5 creates a list of these 5 elements
    # statements;
}
statement for 1..5; # almost same (only 1 statement) with natural language order
for my $counter (1..5) { # variable private to the loop
    # statements;
}
</syntaxhighlight>

"[[There's more than one way to do it]]" is a Perl programming motto.

===1988: Mathematica===
{{Further|Wolfram Mathematica|Wolfram Language}}
The construct corresponding to most other languages' for loop is named ''Do'' in Mathematica.

<syntaxhighlight lang="mathematica">
Do[f[x], {x, 0, 1, 0.1}]
</syntaxhighlight>

Mathematica also has a For construct that mimics the for loop of C-like languages.

<syntaxhighlight lang="mathematica">
For[x= 0 , x <= 1, x += 0.1,
    f[x]
]
</syntaxhighlight>

===1989: Bash===
{{Further|Bash (Unix shell)}}
<syntaxhighlight lang="bash">
# first form
for i in 1 2 3 4 5
do
    # must have at least one command in a loop
    echo $i  # just print the value of i
done
</syntaxhighlight>

<syntaxhighlight lang="bash">
# second form
for (( i = 1; i <= 5; i++ ))
do
    # must have at least one command in a loop
    echo $i  # just print the value of i
done
</syntaxhighlight>

An empty loop (i.e., one with no commands between {{code|do}} and {{code|done}}) is a syntax error. If the above loops contained only comments, execution would result in the message "syntax error near unexpected token 'done'".

===1990: Haskell===
{{Further|Haskell}}

In Haskell98, the function ''mapM_'' maps a [[monad (functional programming)|monadic]] function over a list, as
<syntaxhighlight lang="haskell">
mapM_ print [4, 3 .. 1]
-- prints
-- 4
-- 3
-- 2
-- 1
</syntaxhighlight>

The function ''mapM'' collects each iteration result in a list:
<syntaxhighlight lang="Haskell">
result_list <- mapM (\ indx -> do{ print indx; return (indx - 1) }) [1..4]
-- prints
-- 1
-- 2
-- 3
-- 4
-- result_list is [0,1,2,3,4]
</syntaxhighlight>

Haskell2010 adds functions ''forM_'' and ''forM'', which are equivalent to ''mapM_'' and ''mapM'', but with their arguments flipped:
<syntaxhighlight lang="Haskell">
forM_ [0..3] $ \ indx -> do
    print indx
-- prints
-- 0
-- 1
-- 2
-- 3

result_list <- forM ['a'..'d'] $ \ indx -> do
    print indx
    return indx
-- prints
-- 'a'
-- 'b'
-- 'c'
-- 'd'
-- result_list is ['a','b','c','d']
</syntaxhighlight>

When compiled with optimization, none of the expressions above will create lists. But, to save the space of the [1..5] list if optimization is turned off, a ''forLoop_'' function could be defined as

<syntaxhighlight lang="Haskell">
import Control.Monad as M

forLoop_ :: Monad m => a -> (a -> Bool) -> (a -> a) -> (a -> m ()) -> m ()
forLoop_ startIndx test next f = theLoop startIndx
  where
    theLoop indx = M.when (test indx) $ do
        f indx
        theLoop (next indx)
</syntaxhighlight>
and used as
<syntaxhighlight lang="Haskell">
forLoopM_ (0::Int) (< len) (+1) $ \indx -> do
    -- statements
</syntaxhighlight>

===1991: Oberon-2, Oberon-07, Component Pascal===
{{Further|Oberon (programming language)|Oberon-2|Component Pascal}}
<syntaxhighlight lang="modula2">
FOR Counter:= 1 TO 5 DO
  (* statement sequence *)
END
</syntaxhighlight>
In the original Oberon language, the for loop was omitted in favor of the more general Oberon loop construct. The for loop was reintroduced in Oberon-2.

===1991: Python===
{{Further|Python (programming language)}}
Python does not contain the classical for loop, rather a <code>foreach</code> loop is used to iterate over the output of the built-in <code>range()</code> function which returns an iterable sequence of integers.<syntaxhighlight lang="python">
for i in range(1, 6):  # gives i values from 1 to 5 inclusive (but not 6)
    # statements
    print(i)
# if we want 6 we must do the following
for i in range(1, 6 + 1):  # gives i values from 1 to 6
    # statements
    print(i)
</syntaxhighlight>Using <code>range(6)</code> would run the loop from 0 to 5.

When the loop variable is not needed, it is common practice to use an underscore (<code>_</code>) as a placeholder. This convention signals to other developers that the variable will not be used inside the loop. For example:
<syntaxhighlight lang="python">
for _ in range(5):
    print("Hello")
</syntaxhighlight>
This will print “Hello” five times without using the loop variable.

===1993: AppleScript===
{{Further|AppleScript}}
<syntaxhighlight lang="applescript">
repeat with i from 1 to 5
	-- statements
	log i
end repeat
</syntaxhighlight>

It can also iterate through a list of items, similar to what can be done with arrays in other languages:
<syntaxhighlight lang="applescript">
set x to {1, "waffles", "bacon", 5.1, false}
repeat with i in x
	log i
end repeat
</syntaxhighlight>

A {{code|exit repeat}} may also be used to exit a loop at any time. Unlike other languages, AppleScript currently has no command to continue to the next iteration of a loop.

===1993: Crystal===
{{Further|Crystal (programming language)}}
<syntaxhighlight lang="lua">
for i = start, stop, interval do
  -- statements
end</syntaxhighlight>

So, this code <syntaxhighlight lang="lua">
for i = 1, 5, 2 do
  print(i)
end</syntaxhighlight> will print:
<syntaxhighlight lang="lua">1 3 5</syntaxhighlight>

A for loop can also loop through a table using <syntaxhighlight lang="lua">ipairs()</syntaxhighlight> to iterate numerically through arrays and <syntaxhighlight lang="lua">pairs()</syntaxhighlight> to iterate randomly through dictionaries.
Generic for loop making use of closures:

<syntaxhighlight lang="lua">
for name, phone, and address in contacts() do
  -- contacts() must be an iterator function
end</syntaxhighlight>

===1995: ColdFusion Markup Language (CFML)===
{{Further|ColdFusion Markup Language}}

====Script syntax====
Simple index loop:
<syntaxhighlight lang="cfs">
for (i = 1; i <= 5; i++) {
	// statements
}
</syntaxhighlight>

Using an array:
<syntaxhighlight lang="cfs">
for (i in [1,2,3,4,5]) {
	// statements
}
</syntaxhighlight>

Using a list of string values:
<syntaxhighlight lang="cfs">
loop index="i" list="1;2,3;4,5" delimiters=",;" {
	// statements
}
</syntaxhighlight>

The above {{code|list}} example is only available in the dialect of CFML used by [[Lucee]] and [[Railo]].

====Tag syntax====
{{Further|Tag (programming)}}
Simple index loop:
<syntaxhighlight lang="cfm">
<cfloop index="i" from="1" to="5">
	<!--- statements --->
</cfloop>
</syntaxhighlight>

Using an array:
<syntaxhighlight lang="cfm">
<cfloop index="i" array="#[1,2,3,4,5]#">
	<!--- statements --->
</cfloop>
</syntaxhighlight>

Using a "list" of string values:
<syntaxhighlight lang="cfm">
<cfloop index="i" list="1;2,3;4,5" delimiters=",;">
	<!--- statements --->
</cfloop>
</syntaxhighlight>

===1995: Java===
{{Further|Java (programming language)}}
<syntaxhighlight lang="java">
for (int i = 0; i < 5; i++) {
    //perform functions within the loop;
    //can use the statement 'break;' to exit early;
    //can use the statement 'continue;' to skip the current iteration
}
</syntaxhighlight>

For the extended for loop, see {{slink|Foreach loop#Java}}.

===1995: JavaScript===
{{Further|JavaScript}}
JavaScript supports C-style "three-expression" loops. The {{code|break}} and {{code|continue}} statements are supported inside loops.

<syntaxhighlight lang="javascript">
for (var i = 0; i < 5; i++) {
    // ...
}
</syntaxhighlight>

Alternatively, it is possible to iterate over all keys of an array.

<syntaxhighlight lang="javascript">
for (var key in array) {  // also works for assoc. arrays
    // use array[key]
    ...
}
</syntaxhighlight>

===1995: PHP===
{{Further|PHP}}
This prints out a triangle of *
<syntaxhighlight lang="php">
for ($i = 0; $i <= 5; $i++) {
    for ($j = 0; $j <= $i; $j++) {
        echo "*";
    }
    echo "<br />\n";
}
</syntaxhighlight>

===1995: Ruby===
{{Further|Ruby (programming language)}}
<syntaxhighlight lang="ruby">
for the counter in 1..5
  # statements
end

5.times do |counter|  # counter iterates from 0 to 4
  # statements
end

1.upto(5) do |counter|
  # statements
end
</syntaxhighlight>
[[Ruby programming language|Ruby]] has several possible syntaxes, including the above samples.

===1996: OCaml===
{{Further|OCaml}}
See expression syntax.<ref>{{Cite web |url=http://caml.inria.fr/pub/docs/manual-ocaml-4.00/expr.html |title=OCaml expression syntax |access-date=2013-03-19 |archive-date=2013-04-12 |archive-url=https://archive.today/20130412180254/http://caml.inria.fr/pub/docs/manual-ocaml-4.00/expr.html |url-status=dead }}</ref>
<syntaxhighlight lang="ocaml">
 (* for_statement:= "for" ident '='  expr  ( "to" ∣  "down to" ) expr "do" expr "done" *)

for i = 1 to 5 do
    (* statements *)
  done ;;

for j = 5 down to 0 do
    (* statements *)
  done ;;
</syntaxhighlight>

===1998: ActionScript 3===
{{Further|ActionScript}}
<syntaxhighlight lang="actionscript3">
for (var counter:uint = 1; counter <= 5; counter++){
    //statement;
}
</syntaxhighlight>

===2008: Small Basic===
{{Further|Microsoft Small Basic}}
<syntaxhighlight lang="vbnet">
For i = 1 To 10
    ' Statements
EndFor
</syntaxhighlight>

===2008: Nim===
{{Further|Nim (programming language)}}
[[Nim (programming language)|Nim]] has a <code>foreach</code>-type loop and various operations for creating iterators.<ref>https://nim-lang.org/docs/system.html#...i%2CT%2CT  ".. iterator"</ref>
<syntaxhighlight lang="nim">
for i in 5 .. 10:
  # statements
</syntaxhighlight>

===2009: Go===
{{Further|Go (programming language)}}
<syntaxhighlight lang="go">
for i := 0; i <= 10; i++ {
    // statements
}
</syntaxhighlight>

===2010: Rust===
{{Further|Rust (programming language)}}
<syntaxhighlight lang="rust">
for i in 0..10 {
    // statements
}
</syntaxhighlight>

===2011: Kotlin===
{{Further|Kotlin (programming language)}}
<syntaxhighlight lang="kotlin">
for (i in 1..10) {
    // statements
}
</syntaxhighlight>


===2012: Julia===
{{Further|Julia (programming language)}}
<syntaxhighlight lang="Julia">
for j = 1:10
    # statements
end
</syntaxhighlight>

==See also==
* [[Do while loop]]
* [[Foreach]]
* [[While loop]]
* [[Primitive recursive function]]
* [[General recursive function]]

==References==
{{Reflist}}

[[Category:Control flow]]
[[Category:Iteration in programming]]
[[Category:Programming language comparisons]]
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