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 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 for loop declares a loop variable i and prints its value as it increments from 0, 1, and 2:
for(inti=0;i<3;++i){printf("%d",i);}
Depending on programming language, various keywords are used to denote a for loop. For example, descendants of ALGOL use for,[1] while descendants of Fortran use do and COBOL uses PERFORM VARYING.
Variations
For loop illustration, from i=0 to i=2, resulting in data1=200
Generally, a for loop falls into one of the following categories:
Some languages support an 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, has the following parts separated by semi-colons: initialization (loop variant), condition, and advancement to the next value. Each part is optional and can be blank. This syntax came from B programming language and was originally invented by Stephen C. Johnson.[2] 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 code is a C-style numeric loop that prints the numbers from 0 to 99.
for(inti=0;i<100;++i){System.out.printf("%d",i);}
This form is often used like the numeric for loop, but is more flexible and can be used for other patterns. For example, for (start(); done(); next()) calls functions instead of using a control variable. The pattern of an infinite loop is implemented as 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 code shows a foreach loop. items is either a data collection that supports implicit iteration, or may be an iterator itself.
foriteminitems:do_something(item)
Vectorized
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 allright-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 PL/I, Fortran 95) 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.
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, i starts at 1 and the loops iterates until either i is N or 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 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], 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 duplicated letters for the index, ii, jj, and kk, as this allows easier searching and search-replacing than using a single letter.[3]
Example
An example of C code involving nested for loops, where the loop counter variables are i and j:
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:
for(;;){// loop body}
This style is used instead of infinite while (true) loops to avoid a type conversion warning in some C/C++ compilers.[4] 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 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:
DO I=1,Nstatements!Executed for all values of "I", up to a disaster if any.IF(nogood)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".
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:
X1:DO I=1,NstatementsX2:DO J=1,MstatementsIF(trouble)CYCLE X1statementsEND DO X2statementsEND DO X1
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 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 unless the language supports procedure signatures and argument intents. Some examples in the style of pre-Fortran-90:
DO I=1,NI=7!Overt adjustment of the loop variable. Compiler should complainZ=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?
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 was 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
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:
doi=1,7,12to15;/*statements*/end;
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
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.
Fortran's equivalent of the for loop is the DO loop, using the keyword do instead of for, The syntax of Fortran's DO loop is:
DO labelcounter=first,last,stepstatementslabelstatement
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).
DO 9,ICOUNT=1,5,1WRITE(6,8)ICOUNT 8FORMAT(I2) 9CONTINUE
As of Fortran 90, block structured END DO was added to the language. With this, the end of loop label became optional:
do icounter=1,5write(*,'(i2)')icounterend do
The step part may be omitted if the step is one. Example:
In Fortran 90, the GO TO may be avoided by using an EXIT statement.
* DO loop example.program mainimplicit noneinteger::sumsqinteger::isumsq=0do i=1,9999999if(sumsq>1000)exitsumsq=sumsq+i**2end doprint*,sumsqend program
Alternatively, a DO - WHILE construct could be used:
program mainimplicit noneinteger::sumsqinteger::isumsq=0i=0do while(sumsq<=1000)i=i+1sumsq=sumsq+i**2end doprint*,sumsqend program
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.
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.
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.
In BASIC, a loop is sometimes named a for-next loop.
10REM THIS FOR LOOP PRINTS ODD NUMBERS FROM 1 TO 1520FORI=1TO15STEP230PRINTI40NEXTI
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 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.
The PL/I DO statement can act as either a for loop, a while loop, or a do until loop.
docounter=1to5by1;/* "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.
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 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 whilesyntactic 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 ALGOL 68 allowed the to syntactic element to be replaced with upto and downto to achieve a small optimization. The same compilers also incorporated:
The statement is often a block statement; an example of this would be:
//Using for loops to add numbers 1 - 5intsum=0;for(inti=1;i<=5;++i){sum+=i;}
The ISO/IEC 9899:1999 publication (commonly known as C99) also allows initial declarations in for loops. All three sections in the for loop are optional, with an empty condition equivalent to true.
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, using self as start value.
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:
forifromfbybtotwhilewdo# loop bodyod;
All parts except do and od are optional. The forI part, if present, must come first. The remaining parts (fromf, byb, tot, whilew) can appear in any order.
Iterating over a container is done using this form of loop:
foreincwhilewdo# loop bodyod;
The inc 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 od, end, or end do.
The for loop, written as [initial] [increment] [limit] { ... } for 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.[5]
116{STATEMENTS}for
There is also a simple repeat loop. The repeat-loop, written as X { ... } repeat, repeats the body exactly X times.[6]
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 $_; 1..5 creates a list of these 5 elements# statements;}statementfor1..5;# almost same (only 1 statement) with natural language orderformy$counter(1..5){# variable private to the loop# statements;}
# first formforiin12345do# must have at least one command in a loopecho$i# just print the value of idone
# second formfor((i=1;i<=5;i++))do# must have at least one command in a loopecho$i# just print the value of idone
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'".
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
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.
Python does not contain the classical for loop, rather a foreach loop is used to iterate over the output of the built-in range() function which returns an iterable sequence of integers.
foriinrange(1,6):# gives i values from 1 to 5 inclusive (but not 6)# statementsprint(i)# if we want 6 we must do the followingforiinrange(1,6+1):# gives i values from 1 to 6# statementsprint(i)
Using range(6) would run the loop from 0 to 5.
When the loop variable is not needed, it is common practice to use an underscore (_) as a placeholder. This convention signals to other developers that the variable will not be used inside the loop. For example:
for_inrange(5):print("Hello")
This will print “Hello” five times without using the loop variable.
A 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.
for(inti=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}
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