eintr: Looping with while
Looping with ‘while’
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So long as the true-or-false-test of the ‘while’ expression returns a
true value when it is evaluated, the body is repeatedly evaluated. This
process is called a loop since the Lisp interpreter repeats the same
thing again and again, like an airplane doing a loop. When the result
of evaluating the true-or-false-test is false, the Lisp interpreter does
not evaluate the rest of the ‘while’ expression and exits the loop.
Clearly, if the value returned by evaluating the first argument to
‘while’ is always true, the body following will be evaluated again and
again ... and again ... forever. Conversely, if the value returned is
never true, the expressions in the body will never be evaluated. The
craft of writing a ‘while’ loop consists of choosing a mechanism such
that the true-or-false-test returns true just the number of times that
you want the subsequent expressions to be evaluated, and then have the
test return false.
The value returned by evaluating a ‘while’ is the value of the
true-or-false-test. An interesting consequence of this is that a
‘while’ loop that evaluates without error will return ‘nil’ or false
regardless of whether it has looped 1 or 100 times or none at all. A
‘while’ expression that evaluates successfully never returns a true
value! What this means is that ‘while’ is always evaluated for its side
effects, which is to say, the consequences of evaluating the expressions
within the body of the ‘while’ loop. This makes sense. It is not the
mere act of looping that is desired, but the consequences of what
happens when the expressions in the loop are repeatedly evaluated.
Info Catalog
eintr: while
eintr: Loop Example