Editing Reduce (computer algebra system) (section)

==== Repetition statements: while ... do; repeat ... until ====
The two loop statements
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<code>while</code> ''boolean expression'' <code>do</code> ''statement''<br /><code>repeat</code> ''statement'' <code>until</code> ''boolean expression''
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are closely related to the conditional statement and execute ''statement'' repeatedly a number of times that need not be known in advance. Their difference is that <code>while</code> repetition stops when ''boolean expression'' becomes false whereas <code>repeat</code> repetition stops when ''boolean expression'' becomes true. Also, <code>repeat</code> always executes ''statement'' at least once and it can be used to initialize ''boolean expression'', whereas when using <code>while</code> ''boolean expression'' must be initialized before entering the loop.

The following <code>while</code> statement computes the value of <math>n!</math> as the value of the variable <code>fac</code>. Note that this code treats the assignment <code>n := n - 1</code> as an expression and uses its value.
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n := 5;
fac := n$ while n > 1 do fac := fac*(n := n - 1); fac;
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