Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!tank!eecae!cps3xx!usenet
From: usenet@cps3xx.UUCP (Usenet file owner)
Newsgroups: comp.lang.c
Subject: Re: Reading in Functions
Message-ID: <5038@cps3xx.UUCP>
Date: 19 Oct 89 02:43:12 GMT
References: <1197@utkcs2.cs.utk.edu>
Reply-To: porkka@frith.UUCP (Joe Porkka)
Organization: Michigan State University
Lines: 49

In article <1197@utkcs2.cs.utk.edu> wozniak@utkux1.utk.edu (Bryon Lape) writes:
>
>	How does one write a procedure in C so that the user can type in
>a formula from the keyboard and the programme will graph it?  I can
>handle the graphing part, but what I want to be able to do is is have a
>programme that will read in a function and graph the result.

Sounds like you have to write an expression evaluation program.
Then you could do this:
	scanf("%s",buffer);
	tree=parse(buffer);
	for(x=min;x<=max;x+=step) {
		y= eval(x,tree);
		plot(x,y);
	}

You can find out how to turn the expression into a tree in
books about compilers (maybe interpreters too).

Example:
	y=x+3*x

Tree:
		add
	       /   \
	      /     \
	     x       *
		    / \
		   /   \
		  3     x


Then the eval() function does an postorder left-then right traversal
of tree. This means:

float eval(struct tree)
{
	float t1,t2, calc();
	t1=eval(tree.left);
	t2=eval(tree.right);
	val = calc(t1,t2,tree.function); This function
					 can be a big switch statement
					 to perfrom the appropriate
					 function.
	return val;
}
If you only have to evaulate once, you could do the computation
while building the tree, instaed of makeing it a second pass.
 Joe Porkka   porkka@frith.egr.msu.edu