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15.1 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 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:

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