In computing, iteration is a technique that marks out of a block of statements within a
computer program for a defined number of repetitions. That block of statements is said to be
iterated. A computer programmer might also refer to that block of statements as
an iteration.
Implementations Loops constitute the most common language constructs for performing iterations. The following
pseudocode "iterates" three times the line of code between begin & end through a
for loop, and uses the values of
i as increments. a := 0 for i := 1 to 3 do { loop three times } begin a := a + i; { add the current value of i to a } end; print(a); { the number 6 is printed (0 + 1; 1 + 2; 3 + 3) } It is permissible, and often necessary, to use values from other parts of the program outside the bracketed block of statements, to perform the desired function.
Iterators constitute alternative language constructs to loops, which ensure consistent iterations over specific data structures. They can eventually save time and effort in later coding attempts. In particular, an iterator allows one to repeat the same kind of operation at each node of such a data structure, often in some pre-defined order.
Iteratees are purely functional language constructs, which accept or reject data during the iterations.
Relation with recursion Recursions and iterations have different algorithmic definitions, even though they can generate identical results. The primary difference is that recursion can be a solution without prior knowledge as to how many times the action must repeat, while a successful iteration requires that foreknowledge. Some types of programming languages, known as
functional programming languages, are designed such that they do not set up a block of statements for explicit repetition, as with the
for loop. Instead, those programming languages exclusively use
recursion. Rather than call out a block of code to repeate a pre-defined number of times, the executing code block instead "divides" the work into a number of separate pieces, after which the code block executes itself on each individual piece. Each piece of work is divided repeatedly until the "amount" of work is as small as possible, at which point the algorithm does that work very quickly. The algorithm then "reverses" and reassembles the pieces into a complete whole. The classic example of recursion is in list-sorting algorithms, such as
merge sort. The merge sort recursive algorithm first repeatedly divides the list into consecutive pairs. Each pair is then ordered, then each consecutive pair of pairs, and so forth until the elements of the list are in the desired order. The code below is an example of a recursive algorithm in the
Scheme programming language that outputs the same result as the pseudocode under the previous heading. (let iterate ((i 1) (a 0)) (if ( == Education ==