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(emacs-lisp-intro.info)graph-body-print

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The `graph-body-print' Function
===============================

   After our preparation in the preceding section, the
`graph-body-print' function is straightforward.  The function will
print column after column of asterisks and blanks, using the elements
of a numbers' list to specify the number of asterisks in each column.
This is a repetitive act, which means we can use a decrementing `while'
loop or recursive function for the job.  In this section, we will write
the definition using a `while' loop.

   The `column-of-graph' function requires the height of the graph as
an argument, so we should determine and record that as a local variable.

   This leads us to the following template for the `while' loop version
of this function:

     (defun graph-body-print (numbers-list)
       "DOCUMENTATION..."
       (let ((height  ...
              ...))
     
         (while numbers-list
           INSERT-COLUMNS-AND-REPOSITION-POINT
           (setq numbers-list (cdr numbers-list)))))

We need to fill in the slots of the template.

   Clearly, we can use the `(apply 'max numbers-list)' expression to
determine the height of the graph.

   The `while' loop will cycle through the `numbers-list' one element
at a time.  As it is shortened by the `(setq numbers-list (cdr
numbers-list))' expression, the CAR of each instance of the list is the
value of the argument for `column-of-graph'.

   At each cycle of the `while' loop, the `insert-rectangle' function
inserts the list returned by `column-of-graph'.  Since the
`insert-rectangle' function moves point to the lower right of the
inserted rectangle, we need to save the location of point at the time
the rectangle is inserted, move back to that position after the
rectangle is inserted, and then move horizontally to the next place
from which `insert-rectangle' is called.

   If the inserted columns are one character wide, as they will be if
single blanks and asterisks are used, the repositioning command is
simply `(forward-char 1)'; however, the width of a column may be
greater than one.  This means that the repositioning command should be
written `(forward-char symbol-width)'.  The `symbol-width' itself is
the length of a `graph-blank' and can be found using the expression
`(length graph-blank)'.  The best place to bind the `symbol-width'
variable to the value of the width of graph column is in the varlist of
the `let' expression.

   These considerations lead to the following function definition:

     (defun graph-body-print (numbers-list)
       "Print a bar graph of the NUMBERS-LIST.
     The numbers-list consists of the Y-axis values."
     
       (let ((height (apply 'max numbers-list))
             (symbol-width (length graph-blank))
             from-position)
     
         (while numbers-list
           (setq from-position (point))
           (insert-rectangle
            (column-of-graph height (car numbers-list)))
           (goto-char from-position)
           (forward-char symbol-width)
           ;; Draw graph column by column.
           (sit-for 0)
           (setq numbers-list (cdr numbers-list)))
         ;; Place point for X axis labels.
         (forward-line height)
         (insert "\n")
     ))

The one unexpected expression in this function is the `(sit-for 0)'
expression in the `while' loop.  This expression makes the graph
printing operation more interesting to watch than it would be
otherwise.  The expression causes Emacs to `sit' or do nothing for a
zero length of time and then redraw the screen.  Placed here, it causes
Emacs to redraw the screen column by column.  Without it, Emacs would
not redraw the screen until the function exits.

   We can test `graph-body-print' with a short list of numbers.

  1. Install `graph-symbol', `graph-blank', `column-of-graph', which
     are in Note: Columns of a graph, and `graph-body-print'.

  2. Copy the following expression:

          (graph-body-print '(1 2 3 4 6 4 3 5 7 6 5 2 3))

  3. Switch to the `*scratch*' buffer and place the cursor where you
     want the graph to start.

  4. Type `M-:' (`eval-expression').

  5. Yank the `graph-body-print' expression into the minibuffer with
     `C-y' (`yank)'.

  6. Press <RET> to evaluate the `graph-body-print' expression.

   Emacs will print a graph like this:

                         *
                     *   **
                     *  ****
                    *** ****
                   ********* *
                  ************
                 *************