Info Catalog eintr: Inc Example parts eintr: Incrementing Loop Details

eintr: Inc Example altogether

 
 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.
 

Info Catalog eintr: Inc Example parts eintr: Incrementing Loop Details