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Programming in Emacs Lisp

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C.1 The print-graph Varlist

In writing the print-graph function, the first task is to write the varlist in the let expression. (We will leave aside for the moment any thoughts about making the function interactive or about the contents of its documentation string.)

The varlist should set several values. Clearly, the top of the label for the vertical axis must be at least the height of the graph, which means that we must obtain this information here. Note that the print-graph-body function also requires this information. There is no reason to calculate the height of the graph in two different places, so we should change print-graph-body from the way we defined it earlier to take advantage of the calculation.

Similarly, both the function for printing the X axis labels and the print-graph-body function need to learn the value of the width of each symbol. We can perform the calculation here and change the definition for print-graph-body from the way we defined it in the previous chapter.

The length of the label for the horizontal axis must be at least as long as the graph. However, this information is used only in the function that prints the horizontal axis, so it does not need to be calculated here.

These thoughts lead us directly to the following form for the varlist in the let for print-graph:

(let ((height (apply 'max numbers-list)) ; First version.
      (symbol-width (length graph-blank)))

As we shall see, this expression is not quite right.

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  Copyright 2003   by The Free Software Foundation     Updated Jun 2003