For loop
In computer science a for loop is a programming language statement which allows code to be repeatedly executed. A for loop is classified as an iteration statement. The keyword used to declare 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 developed an iterator will often use the same keyword to name an iterator, e.g. descendants of ALGOL will use "for", while descendants of Fortran will use "do." Some languages started from scratch, and didn't have a predefined term for an iterator to choose from, which is why COBOL 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 amount 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 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.[1]
for (INITIALIZATION; CONDITION; AFTERTHOUGHT) {
// Code for the for loop's body
// goes here.
}
The initialization declares (and perhaps assigns to) any variables required. 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 7.
for (int i = 0; i < 100; i++) {
/* Prints the numbers 0 to 99, each separated by a space. */
System.out.print(i);
System.out.print(' ');
}
System.out.println();
Iterator-based for loops
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 loop 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 warning in Visual C++.[2]
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.
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.
Syntax
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.
ActionScript 3
for (var counter:uint = 1; counter <= 5; counter++){
//statement;
}
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;
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 the loop.
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'".
BASIC
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.
C
for (initialization; condition; increment-decrement)
Statement
The ISO/IEC 9899:1999 publication (commonly known as C99) also allows initial declarations in for loops.
The Statement part is often a block-statement.
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
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
Java
for(int i = 0; i < 5; i++) {
//perform functions within the loop;
//can use the statement 'break;' to exit early;
}
For the extended for loop, see Foreach loop
JavaScript
JavaScript supports C-style "three-expression" loops. The break and continue statements are supported inside loops.
for (var i = 0; i < 5; i++) {
// ...
}
Lua
for i = start, stop, interval do
-- statements
end
So, this code will print
1 3 5
:
for i = 1, 5, 2 do
print(i)
end
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
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]
]
MATLAB
for i = 1:5
-- statements
end
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" */
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.
OCaml
See expression syntax.[3]
(* 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 ;;
Pascal
for Counter := 1 to 5 do
(*statement*);
The numeric-range for loop varies somewhat more. Pascal would use the above-displayed code, whereas Perl would use the following:
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 $counter (1..5) { # variable private to the loop
# statements;
}
(Note that "There's more than one way to do it" is a Perl programming paradigm.)
PHP
for ($i = 0; $i < 5; $i++) {
# statements;
}
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.
PostScript
PostScript supports a very simple kind of for loop.
The repeat loop, written as X { ... } repeat, repeats the body exactly X times.[4]
5 { STATEMENTS } repeat
Python
for counter in range(1, 6): # range(1, 6) gives values from 1 to 5 inclusive (but not 6)
# statements
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.
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.
Timeline of the for loop in various programming languages
1957: FORTRAN
Fortran's equivalent of the for loop is the DO loop. The syntax of Fortran's DO loop is:
DO label counter=start, stop, 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 SumSQ.
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.
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 ~ ODFurther, 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
ODSubsequent 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.
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
Implementation in Interpreted Programming Languages
In interpreted programming languages, for loops can be implemented in many ways. Often times, 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.[5] 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
References
- ^ "For loops in C++".
- ^ "Compiler Warning (level 4) C4127". Microsoft. Retrieved 29 June 2011.
- ^ OCaml expression syntax
- ^ "PostScript Tutorial - Loops".
- ^ "Computer Science 61B: Data Structures and Algorithms in Java 6 - For Loops".
External links
- For loop implementation in different languages at Wikia:Code