Английская Википедия:For loop

Flow diagram of the following for loop code:

for (i = 0; i < 5; i++)
  printf("*");

The loop will cause five asterisks to be printed.

Шаблон:Loop constructs

In computer science a for-loop or for loop is a control flow statement for specifying iteration. Specifically, a for loop functions by running a section of code repeatedly until a certain condition has been satisfied.

For-loops have two parts: a header and a body. The header defines the iteration and the body is the code that is executed once per iteration. The header often declares an explicit loop counter or loop variable. This 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.

Various keywords are used to indicate the usage of a for loop: descendants of ALGOL use "Шаблон:Mono", while descendants of Fortran use "Шаблон:Mono". There are other possibilities, for example COBOL which uses Шаблон:Nowrap.

The name for-loop comes from the word for. For is used as the reserved word (or keyword) in many programming languages to introduce a for-loop. The term in English dates to ALGOL 58 and was popularized in ALGOL 60. It is the direct translation of the earlier German Шаблон:Wikt-lang and was used in Superplan (1949–1951) by Heinz Rutishauser. Rutishauser was involved in defining ALGOL 58 and ALGOL 60.^[1] The loop body is executed "for" the given values of the loop variable. This is more explicit in ALGOL versions of the for statement where a list of possible values and increments can be specified.

In Fortran and PL/I, the keyword Шаблон:Mono is used for the same thing and it is named a do-loop; this is different from a do-while loop.

FOR

Файл:For loop.svg

For loop illustration, from i=0 to i=2, resulting in data1=200

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 four 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, which looks something like this:

for i = first to last do statement
(* or just *)
for i = first..last do statement

Depending on the language, an explicit assignment sign may be used in place of the equal sign (and some languages require the word Шаблон:Code even in the numerical case). An optional step-value (an increment or decrement ≠ 1) 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 language. It requires 3 parts: the initialization (loop variant), the condition, and the advancement to the next iteration. All these three parts are optional.^[2] This type of "semicolon loops" came from B programming language and it was originally invented by Stephen Johnson.^[3]

In the initialization part, any variables needed are declared (and usually assigned values). If multiple variables are declared, they should all be of the same type. The condition part checks a certain condition and exits the loop if false, even if the loop is never executed. If the condition is true, then the lines of code inside the loop are executed. The advancement to the next iteration part is performed exactly once every time the loop ends. The loop is then repeated if the condition evaluates to true.

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

// Prints the numbers from 0 to 99 (and not 100), each followed by a space.

for (int i=0; i<100; i++)
{
    System.out.print(i);
    System.out.print(' ');
}
System.out.println();

These loops are also sometimes named numeric for-loops when contrasted with foreach loops (see below).

Iterator-based for-loops

Шаблон:Further 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 Шаблон:Code 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 Шаблон:Code, as well as a three-expression for-loop (see below) under the name Шаблон:Code.

Vectorised for-loops

Some languages offer a for-loop that acts as if processing all iterations in parallel, such as the Шаблон:Code 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 Шаблон:Code statement in the following pseudocode fragment, when calculating the new value for Шаблон:Code, except for the first (with Шаблон:Code) the reference to Шаблон:Code will obtain the new value that had been placed there in the previous step. In the Шаблон:Code version, however, each calculation refers only to the original, unaltered Шаблон:Code.

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 Шаблон:Code 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.

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 (example., starting at 0 and end at 10 in increments of 1)

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 (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 FortranШаблон: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.^[4]

Example

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

for (i = 0; i < 100; i++) {
    for (j = i; j < 10; j++) {
        some_function(i, j);
    }
}

For loops in C can also be used to print the reverse of a word. As:

for (i = 0; i < 6; i++) {
    scanf("%c", &a[i]);
}
for (i = 4; i >= 0; i--) {
    printf("%c", a[i]);
}

Here, if the input is Шаблон:Code, the output will be Шаблон:Code.

Additional semantics and constructs

Use as infinite loops

Шаблон:Further 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 Шаблон:Code loops to avoid a type conversion warning in some C/C++ compilers.^[5] Some programmers prefer the more succinct Шаблон:Code form over the semantically equivalent but more verbose Шаблон:Code 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.[Wells] 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".

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 (trouble) CYCLE X1
       statements
     END DO X2
     statements
   END 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 to memory. Actual behaviour may even vary according to the compiler's optimization 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 Шаблон:Code in sixteen-bit integer arithmetic) and with each iteration decrement this count while also adjusting the value of Шаблон:Mono: double counting results. However, adjustments to the value of Шаблон:Mono 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 Шаблон:Mono on each iteration. Again, modifications of Шаблон:Mono would not affect the control of the loop, but now a disjunction is possible: within the loop, references to the value of Шаблон:Mono might be to the (possibly altered) current value of Шаблон:Mono or to the auxiliary variable (held safe from improper modification) and confusing results are guaranteed. For instance, within the loop a reference to element Шаблон:Mono of an array would likely employ the auxiliary variable (especially if it were held in a machine register), but if Шаблон:Mono 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 Шаблон:Mono 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 Шаблон:Mono, Шаблон:Mono and Шаблон:Mono and the calculation of an iteration count via something like Шаблон:Code 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 Шаблон:Code 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 is generally equivalent to a while-loop:

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

is equivalent to:

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

as demonstrated by the output of the variables.

Timeline of the for-loop syntax in various programming languages

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

1957: FORTRAN

Шаблон:Further Fortran's equivalent of the Шаблон:Mono loop is the Шаблон:Mono loop, using the keyword do instead of for, The syntax of Fortran's Шаблон:Mono 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 (inclusive).

        DO 9, COUNTER = 1, 5, 1
          WRITE (6,8) COUNTER
    8     FORMAT( I2 )
    9   CONTINUE

In Fortran 77 (or later), this may also be written as:

do counter = 1, 5
  write(*, '(i2)') 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 (SUM SQ.GT.1000) GO TO 200
199        SUM SQ = SUM SQ + I**2
200      PRINT 206, SUMSQ
206      FORMAT( I2 )
       END

Spaces are irrelevant in fixed-form Fortran statements, thus Шаблон:Mono is the same as Шаблон:Mono. In the modern free-form Fortran style, blanks are significant.

In Fortran 90, the Шаблон:Mono may be avoided by using an Шаблон:Mono statement.

* DO loop example.
       program main
         implicit none

         integer :: sumsq
         integer :: i

         sumsq = 0
         do i = 1, 9999999
           if (sumsq > 1000.0) exit
           sumsq = sumsq + i**2
          end do
         print *, sumsq

       end program

1958: ALGOL

Шаблон:Further ALGOL 58 introduced the Шаблон:Code statement, using the form as Superplan:

 FOR Identifier = Base (Difference) Limit

For example to print 0 to 10 incremented by 1:

FOR x = 0 (1) 10 BEGIN
PRINT (FL) = x END

1960: COBOL

Шаблон:Further 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 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.

      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

Шаблон:Further In BASIC, a loop is sometimes named a for-next loop.

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

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 Шаблон:Code and Шаблон:Code 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.

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Английская Википедия:For loop

FOR

Traditional for-loops

Iterator-based for-loops

Vectorised for-loops

Compound for-loops

Loop counters

Example

Additional semantics and constructs

Use as infinite loops

Early exit and continuation

Loop variable scope and semantics

Adjustment of bounds

List of value ranges

Equivalence with while-loops

Timeline of the for-loop syntax in various programming languages

1957: FORTRAN

1958: ALGOL

1960: COBOL

1964: BASIC

1964: PL/I

1968: ALGOL 68

1970: Pascal

1972: C, C++

1972: Smalltalk

1980: Ada

1980: Maple

1982: Maxima CAS

1982: PostScript

1983: Ada 83 and above

1984: MATLAB

1987: Perl

1988: Mathematica

1989: Bash

1990: Haskell

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

1991: Python

1993: AppleScript

1993: Crystal

1995: ColdFusion Markup Language (CFML)

Script syntax

Tag syntax

1995: Java

1995: JavaScript

1995: PHP

1995: Ruby

1996: OCaml

1998: ActionScript 3

2008: Small Basic

2008: Nim

2009: Go

2010: Rust

2012: Julia

See also

References

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