For loop


In computer science, a for-loop is a control flow statement for specifying iteration, which allows code to be executed repeatedly. Various keywords are used to specify this statement: descendants of ALGOL use "for", while descendants of Fortran use "do". There are other possibilities, for example COBOL which uses "PERFORM VARYING".
A for-loop has two parts: a header specifying the iteration, and a body which is executed once per iteration. The header often declares an explicit loop counter or loop variable, which allows the body to know which iteration is being executed. For-loops are typically used when the number of iterations is known before entering the loop. For-loops can be thought of as shorthands for while-loops which increment and test a loop variable.
The name for-loop comes from the English word, which is used as the keyword in many 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 , used in Superplan 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, the keyword DO is used for the same thing and it is called a do-loop; this is different from a do-while loop.

FOR

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 for-loop of languages like ALGOL, Simula, BASIC, Pascal, Modula, Oberon, Ada, Matlab, Ocaml, F#, and so on, requires a control variable with start- and end-values and looks something like this:

for i = first to last do statement
for i = first..last do statement

Depending on the language, an explicit assignment sign may be used in place of the equal sign. An optional step-value may also be included, although the exact syntaxes used for this differs a bit more between the languages. Some languages require a separate declaration of the control variable, some do not.
Another form was popularized by the C programming language. It requires 3 parts: the initialization, the condition, and the afterthought and all these three parts are optional.
The initialization declares 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.

// Prints the numbers from 0 to 99, each followed by a space.
for
System.out.println;

These loops are also sometimes called numeric for-loops when contrasted with foreach loops.

Iterator-based for-loops

This type of for-loop is a generalisation 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 data collection. A representative example in Python is:

for item in some_iterable_object:
do_something
do_something_else

Where is either a data collection that supports implicit iteration, 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, as well as a three-expression for-loop under the name.

Vectorised for-loops

Some languages offer a for-loop that acts as if processing all iterations in parallel, such as the 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 statement in the following pseudocode fragment, when calculating the new value for, except for the first the reference to will obtain the new value that had been placed there in the previous step. In the version, however, each calculation refers only to the original, unaltered.
for i := 2 : N - 1 do A := / 3; next i;
for all i := 2 : N - 1 do A := / 3;
The difference may be significant.
Some languages also offer array assignment statements, that enable many for-loops to be omitted. Thus pseudocode such as would set all elements of array A to zero, no matter its size or dimensionality. The example loop could be rendered as

A := / 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 > 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, or, with suitable variations, the first non-blank character in a string, and so on.

Loop counters

In computer programming a loop counter is the variable that controls the iterations of a loop. 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
Loop counters change with each iteration of a loop, providing a unique value for each individual iteration. The loop counter is used to decide when the loop should terminate and for the program flow to continue to the next instruction after the loop.
A common identifier naming convention is for the loop counter to use the variable names i, j, and k, 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, 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 indices for sums and multiplications are often i, j, etc. A variant convention is the use of reduplicated letters for the index, ii, jj, and kk, as this allows easier searching and search-replacing than using a single letter.

Example

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

for

It has been shown that a nested for loop, as in above example, performs more computations per unit time than a loop without it. This machine-independent optimisation means the nested for loop will finish faster, given the same number of computations to perform. This is an advantage nested for loop has over nested while loop, which behaves differently.
For loops in C can also be used to print the reverse of a word. As:

for
for

Here, if the input is, the output will be.

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, such as:

for
//loop body

This style is used instead of infinite loops to avoid a type conversion warning in some C/C++ compilers. Some programmers prefer the more succinct form over the semantically equivalent but more verbose 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.
A for statement also terminates when a break, goto, or return statement within the statement body is executed.
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 CYCLE !Skip this value of "I", continue with the next.
statements !Executed only where goodness prevails.
IF EXIT !Abandon the loop.
statements !While good and, no disaster.
END DO !Should align with the "DO".

Some languages offer further facilities such as naming the various loop statements so that with multiple nested loops there is no doubt as to which loop is involved. Fortran 95, for example:

X1:DO I = 1,N
statements
X2:DO J = 1,M
statements
IF CYCLE X1
statements
END DO X2
statements
END DO X1

Thus, when "trouble" is detected in the inner loop, the CYCLE X1 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 to memory. Actual behaviour may even vary according to the compiler's optimization settings, as with the Honywell Fortran66 compiler.
In some languages 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 !Function "ADJUST" might alter "I", to uncertain effect.
normal statements !Memory might fade that "I" is the loop variable.
PRINT, B !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 and with each iteration decrement this count while also adjusting the value of : double counting results. However, adjustments to the value of 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 on each iteration. Again, modifications of would not affect the control of the loop, but now a disjunction is possible: within the loop, references to the value of might be to the current value of or to the auxiliary variable and confusing results are guaranteed. For instance, within the loop a reference to element of an array would likely employ the auxiliary variable, but if is a parameter to some routine, it would likely be a reference to the proper variable 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 := A / A;
If the approach to compiling such a loop was to be the evaluation of first, last and step and the calculation of an iteration count via something like /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 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

In theory

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. These statements will typically implicitly increment the counter of a for-loop, but not the equivalent while-loop. 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, and for saturated arithmetic where 65535 + 1 would produce 65535. Compiler writers will handle the likes of, possibly by producing code that inspects the state of a hardware "overflow" indicator, but unless there is some provision for the equivalent checking after calculating 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

Fortran's equivalent of the loop is the loop,
using the keyword do instead of for,
The syntax of Fortran's loop is:

DO label counter = first, last, step
statements
label statement
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.

DO 9, COUNTER = 1, 5, 1
WRITE COUNTER
8 FORMAT
9 CONTINUE

In Fortran 77, this may also be written as:

do counter = 1, 5
write counter
end do

The step part may be omitted if the step is one. Example:

  • DO loop example.
PROGRAM MAIN
SUM SQ = 0
DO 199 I = 1, 9999999
IF GO TO 200
199 SUM SQ = SUM SQ + I**2
200 PRINT 206, SUMSQ
206 FORMAT
END

Spaces are irrelevant in fixed-form Fortran statements, thus is the same as. In the modern free-form Fortran style, blanks are significant.
In Fortran 90, the may be avoided by using an statement.

  • DO loop example.
program main
implicit none
integer :: sumsq
integer :: i
sumsq = 0
do i = 1, 9999999
if exit
sumsq = sumsq + i**2
end do
print *, sumsq
end program

1958: Algol

was first formalised in the Algol58 report.

1960: 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 initialising variables, the COBOL equivalent of a for-loop would be.

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

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

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.

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

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 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 and 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 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 statement to terminate the current loop iteration and begin the next.

1968: 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

INT sum sq := 0;
FOR i
 WHILE
 print); # 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 syntactic element to be replaced with and to achieve a small optimization. The same compilers also incorporated:
;: for late loop termination.
;: for working on arrays in parallel.

1970: Pascal


for Counter := 1 to 5 do
;

Decrementing is using keyword instead of, as in:

for Counter := 5 downto 1 do
;

The numeric-range for-loop varies somewhat more.

1972: C/C++


for
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

The ISO/IEC 9899:1999 publication also allows initial declarations in loops. All the three sections in the for loop are optional.

1972: Smalltalk

1 to: 5 do:
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 specifically 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 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 initialises an internal variable, executes the body as long as the internal variable is not more than limit 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 for

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

5 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 n = 1:5
-- statements
end
After the loop, would be 5 in this example.
As is used for the Imaginary unit, its use as a loop variable is discouraged.

1987: Perl


for
for
for
statement for 1..5; # almost same with natural language order
for my $counter

1988: Mathematica

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

Do, ]

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

For

1989: Bash


  1. 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


  1. second form
for )
do
# must have at least one command in loop
echo $i # just print value of i
done

Note that an empty loop 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_ $ \indx -> do statements

or get each iteration result as a list in

statements_result_list <- forM $ \indx -> do statements

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

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

and used as:

forLoopM_ $ \indx -> do -- whatever with the index

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


FOR Counter := 1 TO 5 DO

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

Python does not contain the classical for loop, rather a foreach loop is used to iterate over the output of the builtin range function which returns an iterable list of integers.
for i in range: # gives i values from 1 to 5 inclusive
# statements
print
  1. if we want 6 we must do the following
for i in range: # gives i values from 1 to 6
# statements
print
Using range would run the loop from 0 to 5.

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
repeat with i in x
log i
end repeat

You may also use 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
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

Using an array:

for

Using a list of string values:

loop index="i" list="1;2,3;4,5" delimiters=",;"

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

Tag syntax

Simple index loop:




Using an array:




Using a "list" of string values:



1995: Java


for

For the extended for-loop, see Foreach loop

1995: JavaScript

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

for

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

for

1995: PHP

This prints out a triangle of *

for

1995: Ruby


for counter in 1..5
# statements
end
5.times do |counter| # counter iterates from 0 to 4
# statements
end
1.upto do |counter|
# statements
end

Ruby has several possible syntaxes, including the above samples.

1996: OCaml

See expression syntax.


for i = 1 to 5 do

done ;;
for j = 5 downto 0 do

done ;;

1998: ActionScript 3


for

2008: Small Basic


For i = 1 To 10
' Statements
EndFor

2010: Rust


for i in 0..10

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
print item
SYSTEM_VAR_0000++
end