For loop

For loop flow diagram

In computer science a for-loop (or simply for loop) is a programming language control statement for specifying iteration, which allows code to be executed repeatedly. The syntax of a for-loop is based on the heritage of the language and the prior programming languages it borrowed from, so programming languages that are descendants of or offshoots of a language that originally provided an iterator will often use the same keyword to name an iterator, e.g., descendants of ALGOL use "for", while descendants of Fortran use "do." There are other possibilities, for example COBOL which uses "PERFORM VARYING".

Unlike many other kinds of loops, such as the while-loop, the for-loop is often distinguished by an explicit loop counter or loop variable. This allows the body of the for-loop (the code that is being repeatedly executed) to know about the sequencing of each iteration. For-loops are also typically used when the number of iterations is known before entering the loop. For-loops are the shorthand way to make loops when the number of iterations is known, as a for-loop can be written as a while-loop.

The name for-loop comes from the English word for, which is used as the keyword in most programming languages to introduce a for-loop. The term in English dates to ALGOL 58 and was popularized in the influential later ALGOL 60; it is the direct translation of the earlier German für, used in Superplan (1949–1951) by Heinz Rutishauser, who also was involved in defining ALGOL 58 and ALGOL 60. The loop body is executed "for" the given values of the loop variable, though this is more explicit in the ALGOL version of the statement, in which a list of possible values and/or increments can be specified.

In FORTRAN and PL/I though, the keyword DO is used and it is called a do-loop, but it is otherwise identical to the for-loop described here and is not to be confused with the do-while loop.

Kinds of for-loops

A for-loop statement is available in most imperative programming languages. Even ignoring minor differences in syntax there are many differences in how these statements work and the level of expressiveness they support. Generally, for-loops fall into one of the following categories:

Traditional for-loops

The traditional for-loop found in C/C++ requires 3 parts: the initialization, the condition, and the afterthought and all these three parts are optional.

for (INITIALIZATION; CONDITION; AFTERTHOUGHT) 
{
    // Code for the for-loop's body goes here.
}

The initialization declares (and perhaps assigns to) any variables required. The type of a variable should be same if you are using multiple variables in initialization part. The condition checks a condition, and quits the loop if false. The afterthought is performed exactly once every time the loop ends and then repeats.

Here is an example of the traditional for-loop in Java.

for (int i=0; i<100; i++)   // Prints the numbers 0 to 99 (and not 100), each separated by a space.
{
    System.out.print(i);
    System.out.print(' ');
}
System.out.println();

Iterator-based for-loops

Main article: Foreach loop

This type of for-loop is a falsification of the numeric range type of for-loop; as it allows for the enumeration of sets of items other than number sequences. It is usually characterized by the use of an implicit or explicit iterator, in which the loop variable takes on each of the values in a sequence or other order able data collection. A representative example in Python is:

for item in some_iterable_object:
    do Something
    do Something Else

Where some_iterable_object is either a data collection that supports implicit iteration (like a list of employee's names), or may in fact be an iterator itself. Some languages have this in addition to another for-loop syntax; notably, PHP has this type of loop under the name for each, as well as a three-expression for-loop (see below) under the name for.

Vectorised for-loops

Some languages offer a for-loop that acts as if processing all iterations in parallel, such as the for all keyword in FORTRAN 95 which has the interpretation that all right-hand-side expressions are evaluated before any assignments are made, as distinct from the explicit iteration form. For example, in the for statement in the following pseudocode fragment, when calculating the new value for A(i), except for the first (with i = 2) the reference to A(i - 1) will obtain the new value that had been placed there in the previous step. In the for all version, however, each calculation refers only to the original, unaltered 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 FORTRAN 95, PL/I) also offer array assignment statements, that enable many for-loops to be omitted. Thus pseudocode such as A := 0; would set all elements of array A to zero, no matter its size or dimensionality. The example loop could be rendered as

 A(2 : N - 1) := [A(1 : N - 2) + A(2 : N - 1) + A(3 : N)] / 3;

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.

Compound for-loops

Introduced with ALGOL 68 and followed by PL/I, this allows the iteration of a loop to be compounded with a test, as in

for i := 1 : N while A(i) > 0 do etc.

That is, a value is assigned to the loop variable i and only if the while expression is true will the loop body be executed. If the result were false the for-loop's execution stops short. Granted that the loop variable's value is defined after the termination of the loop, then the above statement will find the first non-positive element in array A (and if no such, its value will be N + 1), or, with suitable variations, the first non-blank character in a string, and so on.

Additional semantics and constructs

Use as infinite loops

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. In fact, when infinite loops are intended, this type of for-loop can be used (with empty expressions), such as:

for (;;)
   //loop body

This style is used instead of infinite while (1) loops to avoid a type conversion warning in some C/C++ compilers. Some programmers prefer the more succinct for (;;) form over the semantically equivalent but more verbose 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 and continue statements found in C and its derivatives. The break statement causes the inner-most 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. Other languages may have similar statements or otherwise provide means to alter the for-loop progress; for example in FORTRAN 95:

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", 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".

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 to memory. Actual behaviour may even vary according to the compiler's optimisation settings, as with the Honywell Fortran66 compiler.

In some languages (not C or C++) the loop variable is 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. Some examples in the style of Fortran:

DO I = 1, N
  I = 7                           !Overt adjustment of the loop variable. Compiler complaint likely.
  Z = ADJUST(I)                   !Function "ADJUST" might alter "I", to uncertain effect.
  normal statements               !Memory might fade that "I" is the loop variable.
  PRINT (A(I), B(I), I = 1, N, 2) !Implicit for-loop to print odd elements of arrays A and B, reusing "I"…
  PRINT I                         !What value will be presented?
END DO                            !How many times will the loop be executed?

A common approach is to calculate the iteration count at the start of a loop (with careful attention to overflow as in for i := 0 : 65535 do ... ; in sixteen-bit integer arithmetic) and with each iteration decrement this count while also adjusting the value of I: double counting results. However, adjustments to the value of 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 I on each iteration. Again, modifications of I would not affect the control of the loop, but now a disjunction is possible: within the loop, references to the value of I might be to the (possibly altered) current value of 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 I of an array would likely employ the auxiliary variable (especially if it were held in a machine register), but if 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 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 were to be the evaluation of first, last and step and the calculation of an iteration count via something like (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 A(last) changed.

List of value ranges

PL/I and Algol 68, allows 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:

do i = 1, 7, 12 to 15;
  /*statements*/
end;

Equivalence with while-loops

A for-loop can be converted into an equivalent while-loop by incrementing a counter variable directly. The following pseudocode illustrates this technique:

factorial = 1
 for counter from 1 to 5
     factorial = factorial * counter

is easily translated into the following while-loop:

factorial = 1
 counter = 1
 while counter <= 5
    factorial = factorial * counter
    counter = counter + 1

This translation is slightly complicated by languages which allow a statement to jump to the next iteration of the loop (such as the "continue" statement in C). These statements will typically implicitly increment the counter of a for-loop, but not the equivalent while-loop (since in the latter case the counter is not an integral part of the loop construct). Any translation will have to place all such statements within a block that increments the explicit counter before running the statement.

In practice

The formal equivalence applies only in so far as computer arithmetic also follows the axia of mathematics, in particular that x + 1 > x. Actual computer arithmetic suffers from the overflow of limited representations so that for example in sixteen-bit unsigned arithmetic, 65535 + 1 comes out as zero, because 65536 cannot be represented in unsigned sixteen-bit. Similar problems arise for other sizes, signed or unsigned. Compiler writers will handle the likes of for counter := 0 to 65535 do ... next counter, possibly by producing code that inspects the state of an "overflow" indicator, but unless there is some provision for the equivalent checking when calculating counter := counter + 1; the while-loop equivalence will fail because the counter will never exceed 65535 and so the loop will never end - unless some other mishap occurs.

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

While using the keyword do instead of for, this type of FORTRAN do-loop behaves similarly to the three argument for-loop in other languages. This example behaves the same as the others, initializing the counter variable to 1, incrementing by 1 each iteration of the loop and stopping at five (inclusive).

do counter = 1, 5, 1
  write(*, '(i2)') counter
end do

Fortran's equivalent of the for loop is the DO loop. The syntax of Fortran's DO loop is:

         DO label counter=initial, final, step
         statements
  label  statement

Where the step part may be omitted if the step is one. Example: (spaces are irrelevant in Fortran statements, thus SUM SQ is the same as SUMSQ)

! DO loop example
       PROGRAM MAIN
         SUM SQ = 0
         DO 101 I = 1, 9999999
           IF (SUM SQ.GT.1000) GO TO 109
           SUM SQ = SUM SQ + I**2
101      CONTINUE
109      CONTINUE
       END

1958: Algol

Algol was first formalised in the Algol58 report.

1960: COBOL

COBOL was formalised in late 1959 and has had many elaborations. It uses the PERFORM verb which has many options, with the later addition of "structured" statements such as END-PERFORM. Ignoring the need for declaring and initialising variables, the equivalent of a for-loop would be

      PERFORM VARYING I FROM 1 BY 1 UNTIL I > 1000
             ADD I**2 TO SUM-SQ.
      END-PERFORM

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

Loops in BASIC are sometimes called for-next loops.

For I = 1 to 5;
 Print I;
Next I

Notice that the end-loop marker specifies the name of the index variable, which must correspond to the name of the index variable in the start of the for-loop. Some languages (PL/I, FORTRAN 95 and later) allow a statement label on 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 EXIT and 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

do counter = 1 to 5 by 1; /* "by 1" is the default if not specified */
  /*statements*/;
  end;

The LEAVE statement may be used to exit the loop. Loops can be labeled, and leave may leave a specific labeled loop in a group of nested loops. Some PL/I dialects include the ITERATE statement to terminate the current loop iteration and begin the next.

1968: Algol 68

Algol68 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 do ~ od portion was compulsory, in which case the loop will iterate indefinitely.
• thus the clause to 100 do ~ od, will iterate exactly 100 times.
• the while syntactic element allowed a programmer to break from a for loop early, as in:

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 Algol68 allowed the to syntactic element to be replaced with upto and downto to achieve a small optimization. The same compilers also incorporated:

until

for late loop termination.

foreach

for working on arrays in parallel.

1970: Pascal

for Counter := 1 to 5 do
  (*statement*);

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

for Counter := 5 downto 1 do
  (*statement*);

The numeric-range for-loop varies somewhat more.

1972: C/C++

Further information: C syntax § Iteration statements

for (initialization; condition; increment/decrement)
    statement

The statement is often a block statement; an example of this would be:

//Using for-loops to add numbers 1 - 5
int sum = 0;
for (int i = 1; i < 6; ++i) {
    sum += i;
}

The ISO/IEC 9899:1999 publication (commonly known as C99) also allows initial declarations in for loops.

1972: Smalltalk

1 to: 5 do: [ :counter | "statements" ]

Contrary to other languages, in Smalltalk a for-loop is not a language construct but defined in the class Number as a method with two parameters, the end value and a closure, using self as start value.

1980: Ada

for Counter in 1 .. 5 loop
   -- statements
end loop;

The exit statement may be used to exit the loop. Loops can be labeled, and exit may leave a specific labeled loop in a group of nested loops:

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;

1980: Maple

Maple has two forms of for-loop, one for iterating of 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 do and od are optional. The for i part, if present, must come first. The remaining parts (from f, by b, to t, while w) 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 in c clause specifies the container, which may be a list, set, sum, product, unevaluated function, array, or an object implementing an iterator.

A for-loop may be terminated by od, end, or end do.

1982: Maxima CAS

In Maxima CAS one can use also non integer values :

for x:0.5 step 0.1 thru 0.9 do
    /* "Do something with x" */

1982: PostScript

The for-loop, written as [initial] [increment] [limit] { ... } for initialises an internal variable, executes the body as long as the internal variable is not more than limit (or not less, if 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.

1 1 6 {STATEMENTS} for

There is also a simple repeat-loop. The repeat-loop, written as X { ... } repeat, repeats the body exactly X times.

5 { STATEMENTS } repeat

1983: Ada 83 and above

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;

1984: MATLAB

for i = 1:5 
     -- statements
end

1987: Perl

for ($counter = 1; $counter <= 5; $counter++) { # implictly or predefined variable
  # statements;
}
for (my $counter = 1; $counter <= 5; $counter++) { # variable private to the loop
  # statements;
}
for (1..5) { # variable impicitly 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;
}

(Note that "there's more than one way to do it" is a Perl programming motto.)

1988: Mathematica

The construct corresponding to most other languages' for-loop is called Do in Mathematica

Do[f[x], {x, 0, 1, 0.1}]

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

For[x= 0 , x <= 1, x += 0.1,
    f[x]
]

1989: Bash

# first form
for i in 1 2 3 4 5
do
    # must have at least one command in loop
    echo $i  # just print value of i
done

# second form
for (( i = 1; i <= 5; i++ ))
do
    # must have at least one command in loop
    echo $i  # just print value of i
done

Note that an empty loop (i.e., one with no commands between do and 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

The built-in imperative forM_ maps a monadic expression into a list, as

forM_ [1..5] $ \indx -> do statements

or get each iteration result as a list in

statements_result_list <- forM [1..5] $ \indx -> do statements

But, if you want to save the space of the [1..5] list, a more authentic monadic forLoop_ construction can be defined as

import Control.Monad as M

forLoopM_ :: Monad m => a -> (a -> Bool) -> (a -> a) -> (a -> m ()) -> m ()
forLoopM_ indx prop incr f = do
        f indx
        M.when (prop next) $ forLoopM_ next prop incr f
  where      
    next = incr indx

and used as:

  forLoopM_ (0::Int) (< len) (+1) $ \indx -> do -- whatever with the index

1991: Oberon-2, Oberon-07, or Component Pascal

FOR Counter := 1 TO 5 DO
  (* statement sequence *)
END

Note that 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

for counter in range(1, 6):  # range(1, 6) gives values from 1 to 5 inclusive (but not 6)
  # statements

1993: AppleScript

repeat with i from 1 to 5
	-- statements
	log i
end repeat

You can also iterate through a list of items, similar to what you can do with arrays in other languages:

set x to {1, "waffles", "bacon", 5.1, false}
repeat with i in x
	log i
end repeat

You may also use exit repeat to exit a loop at any time. Unlike other languages, AppleScript does not currently have any command to continue to the next iteration of a loop.

1993: Lua

for i = start, stop, interval do
     -- statements
end

So, this code

for i = 1, 5, 2 do
     print(i)
end

will print:

1 3 5

For-loops can also loop through a table using

ipairs()

to iterate numerically through arrays and

pairs()

to iterate randomly through dictionaries.

Generic for-loop making use of closures:

for name, phone, address in contacts() do
     -- contacts() must be an iterator function
end

1995: CFML

Script syntax

Simple index loop:

for (i = 1; i <= 5; i++) {
	// statements
}

Using an array:

for (i in [1,2,3,4,5]) {
	// statements
}

Using a "list" of string values:

loop index="i" list="1;2,3;4,5" delimiters=",;" {
	// statements
}

The above "list" example is only available in the dialect of CFML used by Lucee and Railo.

Tag syntax

Simple index loop:

<cfloop index="i" from="1" to="5">
	<!--- statements --->
</cfloop>

Using an array:

<cfloop index="i" array="#[1,2,3,4,5]#">
	<!--- statements --->
</cfloop>

Using a "list" of string values:

<cfloop index="i" list="1;2,3;4,5" delimiters=",;">
	<!--- statements --->
</cfloop>

1995: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
}

For the extended for-loop, see Foreach loop

1995:JavaScript

JavaScript supports C-style "three-expression" loops. The break and continue statements are supported inside loops.

for (var i = 0; i < 5; i++) {
    // ...
}

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

for (var key in array) {  // also works for assoc. arrays
  // use array[key]
  ...
}

1995: PHP

for ($i = 0; $i <= 5; $i++)
{
  for ($j = 0; $j <= $i; $j++)
  {
    echo "*";
  }
  echo "<br>";
}

1995: Ruby

for 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

Ruby has several possible syntaxes, including the above samples.

1996: OCaml

See expression syntax.

 (* for_statement := "for" ident '='  expr  ( "to" ∣  "downto" ) expr "do" expr "done" *)

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

for j = 5 downto 0 do
    (* statements *)
  done ;;

1998: ActionScript 3

for (var counter:uint = 1; counter <= 5; counter++){
  //statement;
}

Implementation in interpreted programming languages

In interpreted programming languages, for-loops can be implemented in many ways. Oftentimes, the for-loops are directly translated to assembly-like compare instructions and conditional jump instructions. However, this is not always so. In some interpreted programming languages, for-loops are simply translated to while-loops. For instance, take the following Mint/Horchata code:

for i = 0; i < 100; i++
    print i
end

for each item of sequence
    print item
end

/* 'Translated traditional for-loop' */
i = 0
while i < 100
    print i
    i++
end

/* 'Translated for each loop' */
SYSTEM_VAR_0000 = 0
while SYSTEM_VAR_0000 < sequence.length()
    item = sequence[SYSTEM_VAR_0000]
    print item
    SYSTEM_VAR_0000++
end

See also

• Do while loop
• Foreach
• Loop counter
• While loop