In the for all version, however, each calculation refers only to the original, unaltered A.įor i := 2 : N - 1 do A(i) := / 3 next i įor all i := 2 : N - 1 do A(i) := / 3 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. 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. 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. 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. Here is an example of the C-style traditional for-loop in Java.įor item in some_iterable_object : do_something () do_something_else () The loop is then repeated if the condition evaluates to true. The advancement to the next iteration part is performed exactly once every time the loop ends. If the condition is true, then the lines of code inside the loop are executed. The condition part checks a certain condition and exits the loop if false, even if the loop is never executed. If multiple variables are declared, they should all be of the same type. In the initialization part, any variables needed are declared (and usually assigned values). This type of "semicolon loops" came from B programming language and it was originally invented by Stephen Johnson. It requires 3 parts: the initialization ( loop variant), the condition, and the advancement to the next iteration. Some languages require a separate declaration of the control variable, some do not.Īnother form was popularized by the C programming language. 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. last do statementĭepending on the language, an explicit assignment sign may be used in place of the equal sign (and some languages require the word int even in the numerical case). 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:įor i = first to last do statement (* or just *) for i = first. Generally, for-loops fall into one of the following categories: Even ignoring minor differences in syntax there are many differences in how these statements work and the level of expressiveness they support. 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.įOR For loop illustration, from i=0 to i=2, resulting in data1=200Ī for-loop statement is available in most imperative programming languages. This is more explicit in ALGOL versions of the for statement where a list of possible values and increments can be specified. The loop body is executed "for" the given values of the loop variable. Rutishauser was involved in defining ALGOL 58 and ALGOL 60. It is the direct translation of the earlier German für and was used in Superplan (1949–1951) by Heinz Rutishauser. The term in English dates to ALGOL 58 and was popularized in ALGOL 60. For is used as the keyword in many programming languages to introduce a for-loop. The name for-loop comes from the word for. There are other possibilities, for example COBOL which uses PERFORM VARYING. Various keywords are used to indicate the usage of a for loop: descendants of ALGOL use " for", while descendants of Fortran use " do". For-loops can be thought of as shorthands for while-loops which increment and test a loop variable. For-loops are typically used when the number of iterations is known before entering the loop. This allows the body to know which iteration is being executed. The header often declares an explicit loop counter or loop variable. The header defines the iteration and the body is the code that is executed once per iteration. Specifically, a for loop functions by running a section of code repeatedly until a certain condition has been satisfied.įor-loops have two parts: a header and a body. In computer science a for-loop or for loop is a control flow statement for specifying iteration.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |