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An Example: print-elements-of-list

The print-elements-of-list function illustrates a while loop with a list.

The function requires several lines for its output. Since the echo area is only one line, we cannot illustrate how it works in the same way we have been illustrating functions in the past, by evaluating them inside Info. Instead, you need to copy the necessary expressions to your `*scratch*' buffer and evaluate them there. You can copy the expressions by marking the beginning of the region with C-SPC (set-mark-command), moving the cursor to the end of the region and then copying the region using M-w (copy-region-as-kill). In the `*scratch*' buffer, you can yank the expressions back by typing C-y (yank).

After you have copied the expressions to the `*scratch*' buffer, evaluate each expression in turn. Be sure to evaluate the last expression, (print-elements-of-list animals), by typing C-u C-x C-e, that is, by giving an argument to eval-last-sexp. This will cause the result of the evaluation to be printed in the `*scratch*' buffer instead of being printed in the echo area. (Otherwise you will see something like this in your echo area: ^Jgiraffe^J^Jgazelle^J^Jlion^J^Jtiger^Jnil, in which each `^J' stands for the newline that in the `*scratch*' buffer puts each word on its own line. You can evaluate these expressions right now in the Info buffer, if you like, to see this effect.)

(setq animals '(giraffe gazelle lion tiger))

(defun print-elements-of-list (list)
  "Print each element of LIST on a line of its own."
  (while list
    (print (car list))
    (setq list (cdr list))))

(print-elements-of-list animals)

When you evaluate the three expressions in sequence in the `*scratch*' buffer, this will be printed in the buffer:






Each element of the list is printed on a line of its own (that is what the function print does) and then the value returned by the function is printed. Since the last expression in the function is the while loop, and since while loops always return nil, a nil is printed after the last element of the list.

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