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Putting these expressions together, we have a function definition that looks like this:
;;; First subtractive version. (defun triangle (number-of-rows) "Add up the number of pebbles in a triangle." (let ((total 0) (number-of-pebbles-in-row number-of-rows)) (while (> number-of-pebbles-in-row 0) (setq total (+ total number-of-pebbles-in-row)) (setq number-of-pebbles-in-row (1- number-of-pebbles-in-row))) total))
As written, this function works.
However, it turns out that one of the local variables,
number-of-pebbles-in-row
, is unneeded!
When the triangle
function is evaluated, the symbol
number-of-rows
will be bound to a number, giving it an initial
value. That number can be changed in the body of the function as if
it were a local variable, without any fear that such a change will
effect the value of the variable outside of the function. This is a
very useful characteristic of Lisp; it means that the variable
number-of-rows
can be used anywhere in the function where
number-of-pebbles-in-row
is used.
Here is a second version of the function written a bit more cleanly:
(defun triangle (number) ; Second version. "Return sum of numbers 1 through NUMBER inclusive." (let ((total 0)) (while (> number 0) (setq total (+ total number)) (setq number (1- number))) total))
In brief, a properly written while
loop will consist of three parts:
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