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(emacs-lisp-intro.info)Inc Example altogether

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Putting the function definition together
........................................

   We have created the parts for the function definition; now we need to
put them together.

   First, the contents of the `while' expression:

     (while (<= row-number number-of-rows)   ; true-or-false-test
       (setq total (+ total row-number))
       (setq row-number (1+ row-number)))    ; incrementer

   Along with the `let' expression varlist, this very nearly completes
the body of the function definition.  However, it requires one final
element, the need for which is somewhat subtle.

   The final touch is to place the variable `total' on a line by itself
after the `while' expression.  Otherwise, the value returned by the
whole function is the value of the last expression that is evaluated in
the body of the `let', and this is the value returned by the `while',
which is always `nil'.

   This may not be evident at first sight.  It almost looks as if the
incrementing expression is the last expression of the whole function.
But that expression is part of the body of the `while'; it is the last
element of the list that starts with the symbol `while'.  Moreover, the
whole of the `while' loop is a list within the body of the `let'.

   In outline, the function will look like this:

     (defun NAME-OF-FUNCTION (ARGUMENT-LIST)
       "DOCUMENTATION..."
       (let (VARLIST)
         (while (TRUE-OR-FALSE-TEST)
           BODY-OF-WHILE... )
         ... )                     ; Need final expression here.

   The result of evaluating the `let' is what is going to be returned
by the `defun' since the `let' is not embedded within any containing
list, except for the `defun' as a whole.  However, if the `while' is
the last element of the `let' expression, the function will always
return `nil'.  This is not what we want!  Instead, what we want is the
value of the variable `total'.  This is returned by simply placing the
symbol as the last element of the list starting with `let'.  It gets
evaluated after the preceding elements of the list are evaluated, which
means it gets evaluated after it has been assigned the correct value
for the total.

   It may be easier to see this by printing the list starting with
`let' all on one line.  This format makes it evident that the VARLIST
and `while' expressions are the second and third elements of the list
starting with `let', and the `total' is the last element:

     (let (VARLIST) (while (TRUE-OR-FALSE-TEST) BODY-OF-WHILE... ) total)

   Putting everything together, the `triangle' function definition
looks like this:

     (defun triangle (number-of-rows)    ; Version with
                                         ;   incrementing counter.
       "Add up the number of pebbles in a triangle.
     The first row has one pebble, the second row two pebbles,
     the third row three pebbles, and so on.
     The argument is NUMBER-OF-ROWS."
       (let ((total 0)
             (row-number 1))
         (while (<= row-number number-of-rows)
           (setq total (+ total row-number))
           (setq row-number (1+ row-number)))
         total))

   After you have installed `triangle' by evaluating the function, you
can try it out.  Here are two examples:

     (triangle 4)
     
     (triangle 7)

The sum of the first four numbers is 10 and the sum of the first seven
numbers is 28.