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11.1.1 A while Loop and a List

A common way to control a while loop is to test whether a list has any elements. If it does, the loop is repeated; but if it does not, the repetition is ended. Since this is an important technique, we will create a short example to illustrate it.

A simple way to test whether a list has elements is to evaluate the list: if it has no elements, it is an empty list and will return the empty list, (), which is a synonym for nil or false. On the other hand, a list with elements will return those elements when it is evaluated. Since Emacs Lisp considers as true any value that is not nil, a list that returns elements will test true in a while loop.

For example, you can set the variable empty-list to nil by evaluating the following setq expression:

     (setq empty-list ())

After evaluating the setq expression, you can evaluate the variable empty-list in the usual way, by placing the cursor after the symbol and typing C-x C-e; nil will appear in your echo area:

     empty-list

On the other hand, if you set a variable to be a list with elements, the list will appear when you evaluate the variable, as you can see by evaluating the following two expressions:

     (setq animals '(gazelle giraffe lion tiger))
     
     animals

Thus, to create a while loop that tests whether there are any items in the list animals, the first part of the loop will be written like this:

     (while animals
            ...

When the while tests its first argument, the variable animals is evaluated. It returns a list. So long as the list has elements, the while considers the results of the test to be true; but when the list is empty, it considers the results of the test to be false.

To prevent the while loop from running forever, some mechanism needs to be provided to empty the list eventually. An oft-used technique is to have one of the subsequent forms in the while expression set the value of the list to be the cdr of the list. Each time the cdr function is evaluated, the list will be made shorter, until eventually only the empty list will be left. At this point, the test of the while loop will return false, and the arguments to the while will no longer be evaluated.

For example, the list of animals bound to the variable animals can be set to be the cdr of the original list with the following expression:

     (setq animals (cdr animals))

If you have evaluated the previous expressions and then evaluate this expression, you will see (giraffe lion tiger) appear in the echo area. If you evaluate the expression again, (lion tiger) will appear in the echo area. If you evaluate it again and yet again, (tiger) appears and then the empty list, shown by nil.

A template for a while loop that uses the cdr function repeatedly to cause the true-or-false-test eventually to test false looks like this:

     (while test-whether-list-is-empty
       body...
       set-list-to-cdr-of-list)

This test and use of cdr can be put together in a function that goes through a list and prints each element of the list on a line of its own.