Next: , Up: Readying a Graph

Printing the Columns of a Graph

Since Emacs is designed to be flexible and work with all kinds of terminals, including character-only terminals, the graph will need to be made from one of the `typewriter' symbols. An asterisk will do; as we enhance the graph-printing function, we can make the choice of symbol a user option.

We can call this function graph-body-print; it will take a numbers-list as its only argument. At this stage, we will not label the graph, but only print its body.

The graph-body-print function inserts a vertical column of asterisks for each element in the numbers-list. The height of each line is determined by the value of that element of the numbers-list.

Inserting columns is a repetitive act; that means that this function can be written either with a while loop or recursively.

Our first challenge is to discover how to print a column of asterisks. Usually, in Emacs, we print characters onto a screen horizontally, line by line, by typing. We have two routes we can follow: write our own column-insertion function or discover whether one exists in Emacs.

To see whether there is one in Emacs, we can use the M-x apropos command. This command is like the C-h a (command-apropos) command, except that the latter finds only those functions that are commands. The M-x apropos command lists all symbols that match a regular expression, including functions that are not interactive. What we want to look for is some command that prints or inserts columns. Very likely, the name of the function will contain either the word `print' or the word `insert' or the word `column'. Therefore, we can simply type M-x apropos RET print\|insert\|column RET and look at the result. On my system, this command once too takes quite some time, and then produced a list of 79 functions and variables. Now it does not take much time at all and produces a list of 211 functions and variables. Scanning down the list, the only function that looks as if it might do the job is insert-rectangle.

Indeed, this is the function we want; its documentation says:

     insert-rectangle:
     Insert text of RECTANGLE with upper left corner at point.
     RECTANGLE's first line is inserted at point,
     its second line is inserted at a point vertically under point, etc.
     RECTANGLE should be a list of strings.
     After this command, the mark is at the upper left corner
     and point is at the lower right corner.

We can run a quick test, to make sure it does what we expect of it.

Here is the result of placing the cursor after the insert-rectangle expression and typing C-u C-x C-e (eval-last-sexp). The function inserts the strings ‘"first"’, ‘"second"’, and ‘"third"’ at and below point. Also the function returns nil.

     (insert-rectangle '("first" "second" "third"))first
                                                   second
                                                   thirdnil

Of course, we won't be inserting the text of the insert-rectangle expression itself into the buffer in which we are making the graph, but will call the function from our program. We shall, however, have to make sure that point is in the buffer at the place where the insert-rectangle function will insert its column of strings.

If you are reading this in Info, you can see how this works by switching to another buffer, such as the *scratch* buffer, placing point somewhere in the buffer, typing M-:, typing the insert-rectangle expression into the minibuffer at the prompt, and then typing <RET>. This causes Emacs to evaluate the expression in the minibuffer, but to use as the value of point the position of point in the *scratch* buffer. (M-: is the keybinding for eval-expression. Also, nil does not appear in the *scratch* buffer since the expression is evaluated in the minibuffer.)

We find when we do this that point ends up at the end of the last inserted line—that is to say, this function moves point as a side-effect. If we were to repeat the command, with point at this position, the next insertion would be below and to the right of the previous insertion. We don't want this! If we are going to make a bar graph, the columns need to be beside each other.

So we discover that each cycle of the column-inserting while loop must reposition point to the place we want it, and that place will be at the top, not the bottom, of the column. Moreover, we remember that when we print a graph, we do not expect all the columns to be the same height. This means that the top of each column may be at a different height from the previous one. We cannot simply reposition point to the same line each time, but moved over to the right—or perhaps we can...

We are planning to make the columns of the bar graph out of asterisks. The number of asterisks in the column is the number specified by the current element of the numbers-list. We need to construct a list of asterisks of the right length for each call to insert-rectangle. If this list consists solely of the requisite number of asterisks, then we will have position point the right number of lines above the base for the graph to print correctly. This could be difficult.

Alternatively, if we can figure out some way to pass insert-rectangle a list of the same length each time, then we can place point on the same line each time, but move it over one column to the right for each new column. If we do this, however, some of the entries in the list passed to insert-rectangle must be blanks rather than asterisks. For example, if the maximum height of the graph is 5, but the height of the column is 3, then insert-rectangle requires an argument that looks like this:

     (" " " " "*" "*" "*")

This last proposal is not so difficult, so long as we can determine the column height. There are two ways for us to specify the column height: we can arbitrarily state what it will be, which would work fine for graphs of that height; or we can search through the list of numbers and use the maximum height of the list as the maximum height of the graph. If the latter operation were difficult, then the former procedure would be easiest, but there is a function built into Emacs that determines the maximum of its arguments. We can use that function. The function is called max and it returns the largest of all its arguments, which must be numbers. Thus, for example,

     (max  3 4 6 5 7 3)

returns 7. (A corresponding function called min returns the smallest of all its arguments.) However, we cannot simply call max on the numbers-list; the max function expects numbers as its argument, not a list of numbers. Thus, the following expression,

     (max  '(3 4 6 5 7 3))

produces the following error message;

     Wrong type of argument:  number-or-marker-p, (3 4 6 5 7 3)

We need a function that passes a list of arguments to a function. This function is apply. This function `applies' its first argument (a function) to its remaining arguments, the last of which may be a list.

For example,

     (apply 'max 3 4 7 3 '(4 8 5))

returns 8.

(Incidentally, I don't know how you would learn of this function without a book such as this. It is possible to discover other functions, like search-forward or insert-rectangle, by guessing at a part of their names and then using apropos. Even though its base in metaphor is clear—`apply' its first argument to the rest—I doubt a novice would come up with that particular word when using apropos or other aid. Of course, I could be wrong; after all, the function was first named by someone who had to invent it.)

The second and subsequent arguments to apply are optional, so we can use apply to call a function and pass the elements of a list to it, like this, which also returns 8:

     (apply 'max '(4 8 5))

This latter way is how we will use apply. The recursive-lengths-list-many-files function returns a numbers' list to which we can apply max (we could also apply max to the sorted numbers' list; it does not matter whether the list is sorted or not.)

Hence, the operation for finding the maximum height of the graph is this:

     (setq max-graph-height (apply 'max numbers-list))

Now we can return to the question of how to create a list of strings for a column of the graph. Told the maximum height of the graph and the number of asterisks that should appear in the column, the function should return a list of strings for the insert-rectangle command to insert.

Each column is made up of asterisks or blanks. Since the function is passed the value of the height of the column and the number of asterisks in the column, the number of blanks can be found by subtracting the number of asterisks from the height of the column. Given the number of blanks and the number of asterisks, two while loops can be used to construct the list:

     ;;; First version.
     (defun column-of-graph (max-graph-height actual-height)
       "Return list of strings that is one column of a graph."
       (let ((insert-list nil)
             (number-of-top-blanks
              (- max-graph-height actual-height)))
     
         ;; Fill in asterisks.
         (while (> actual-height 0)
           (setq insert-list (cons "*" insert-list))
           (setq actual-height (1- actual-height)))
     
         ;; Fill in blanks.
         (while (> number-of-top-blanks 0)
           (setq insert-list (cons " " insert-list))
           (setq number-of-top-blanks
                 (1- number-of-top-blanks)))
     
         ;; Return whole list.
         insert-list))

If you install this function and then evaluate the following expression you will see that it returns the list as desired:

     (column-of-graph 5 3)

returns

     (" " " " "*" "*" "*")

As written, column-of-graph contains a major flaw: the symbols used for the blank and for the marked entries in the column are `hard-coded' as a space and asterisk. This is fine for a prototype, but you, or another user, may wish to use other symbols. For example, in testing the graph function, you many want to use a period in place of the space, to make sure the point is being repositioned properly each time the insert-rectangle function is called; or you might want to substitute a ‘+’ sign or other symbol for the asterisk. You might even want to make a graph-column that is more than one display column wide. The program should be more flexible. The way to do that is to replace the blank and the asterisk with two variables that we can call graph-blank and graph-symbol and define those variables separately.

Also, the documentation is not well written. These considerations lead us to the second version of the function:

     (defvar graph-symbol "*"
       "String used as symbol in graph, usually an asterisk.")
     
     (defvar graph-blank " "
       "String used as blank in graph, usually a blank space.
     graph-blank must be the same number of columns wide
     as graph-symbol.")

(For an explanation of defvar, see Initializing a Variable with defvar.)

     ;;; Second version.
     (defun column-of-graph (max-graph-height actual-height)
       "Return MAX-GRAPH-HEIGHT strings; ACTUAL-HEIGHT are graph-symbols.
     The graph-symbols are contiguous entries at the end
     of the list.
     The list will be inserted as one column of a graph.
     The strings are either graph-blank or graph-symbol."
     
       (let ((insert-list nil)
             (number-of-top-blanks
              (- max-graph-height actual-height)))
     
         ;; Fill in graph-symbols.
         (while (> actual-height 0)
           (setq insert-list (cons graph-symbol insert-list))
           (setq actual-height (1- actual-height)))
     
         ;; Fill in graph-blanks.
         (while (> number-of-top-blanks 0)
           (setq insert-list (cons graph-blank insert-list))
           (setq number-of-top-blanks
                 (1- number-of-top-blanks)))
     
         ;; Return whole list.
         insert-list))

If we wished, we could rewrite column-of-graph a third time to provide optionally for a line graph as well as for a bar graph. This would not be hard to do. One way to think of a line graph is that it is no more than a bar graph in which the part of each bar that is below the top is blank. To construct a column for a line graph, the function first constructs a list of blanks that is one shorter than the value, then it uses cons to attach a graph symbol to the list; then it uses cons again to attach the `top blanks' to the list.

It is easy to see how to write such a function, but since we don't need it, we will not do it. But the job could be done, and if it were done, it would be done with column-of-graph. Even more important, it is worth noting that few changes would have to be made anywhere else. The enhancement, if we ever wish to make it, is simple.

Now, finally, we come to our first actual graph printing function. This prints the body of a graph, not the labels for the vertical and horizontal axes, so we can call this graph-body-print.