string to source(?)

**Draco** · 10-19-2003

I'm trying to make a simple graphing program.

Code:

for(x=-10;x<11;x++)
  {
   y=(-(x*x))+4;
   putpixel(buffer,x+320,y+240,(255,255,255));
   blit(buffer,screen,0,0,0,0,640,480);
   delay(100);
  }

I want to take this piece of code and change it from graphing hard-coded functions into a function that will take an equation from the user and graph that. My only problem is that I can't think of how to use a string and then have the program read it like it was hard-coded. Am I using the wrong approach?

**Azuth** · 10-19-2003

I'm not entirely sure what you mean. Data can be assigned to variables, you then use these variables as part of your equations.

The data in these variables can be read from users by a number of means, if this is what you're having trouble with take a look at the 'fgets' function. The function 'atoi' may also be helpful.

**DrZoidberg** · 10-19-2003

What kind of euations?
You can limit it to simple equations that contains no brackets and only +,-,*,/.
That should not be very difficult.

Try this code. But it can only process simple equations. numbers can only be one digit. e.g. 6*x+4 or x*x-5

Code:

#include <stdio.h>

int main() {
    char input[80];
    char temp[10];
    int i,j;
    char op;
    double y,x;
    
    printf("Input formula: y(x)=");
    fgets(input, 80, stdin);
    do {
    printf("Input x(0-9): ");
    fgets(temp, 10, stdin);
    sscanf(temp,"%lf",&x);
    } while (x<0 || x>9);
    y=0;
    j=0;
    op='+';
    for(i=0; input[i]!='\0'; i++) {
        if(input[i]=='+' || input[i]=='-' || input[i]=='*' || input[i]=='/') {
            op=input[i];
            i++;
        }
        do {
            if(input[i]=='\0') break;
            if(input[i]=='x') j=x;
            else j=input[i]-'0';
            if(j<0 || j>9) i++;
        } while(j<0 || j>9);
        if(input[i]=='\0') break;
        switch(op) {
            case '+': y+=j; break;
            case '-': y-=j; break;
            case '*': y*=j; break;
            case '/': y/=j; break;
        }
    }
    printf("\ny = %lf", y);
}

**Brian** · 10-20-2003

You cannot turn string to source, because your program does not have a built in compiler

.

**twm** · 10-20-2003

You could always use another language that has this functionality built in:

Code:

#! /usr/bin/perl

# Simple simple evaluate and filter
chomp($expr = <STDIN>);
print eval $expr;

Then you can pipe the output of this script through your C program, use atoi on the string you get, and you have an evaluated expression with way less effort than it would take to write it in C.

**vikasgp** · 10-20-2003

I was trying to do the same thing sometime ago. If you want to graph arbitrary functions, you'll have to "parse" the user-given expression. If all you want are simple equations like just +,-,*,/ etc., then you can just hand-code something like in the posting above. Hand-coding gets harder as you try to parse more complex expressions. One way out is to use yacc (or GNU bison), which will handle the parsing for you. In fact, you'll be writing a compiler-like program for a tiny language.

You should be able to find details about the different methods of parsing and about yacc in any compiler design book. If you want the definitive reference, get the "Dragon Book" -- "Principles of Compiler Design", Aho, et. al. You can also try the GNU bison info pages.

HTH

Update: Oh, and also the last chapter of "The Unix Programming Environment" by Rob Pike and Kernighan(?) is about the design of a compiler using yacc for a 4-function calculator that grows into a small programming language by the end of the chapter.

**twm** · 10-20-2003

>Hand-coding gets harder as you try to parse more complex expressions.
If you do it in C that is. Perl's eval can parse any valid Perl expression (and any mathematical expression that this program would require). So why bother reinventing things that have already been created for no reason? I don't know what OS Draco uses or I would have suggested bc/dc instead of a relatively portable Perl solution.

**Draco** · 10-20-2003

I use windowsxp. Thanks for the code DrZoidBerg. I like twm's suggestion too, but could you please explain to me where I would use the code segment, and what it accomplishes in the piping?

**twm** · 10-20-2003

>I like twm's suggestion too
You like me, you really like me!

>could you please explain to me where I would use the code segment
It's not a segment my young Perl patawan, it's a complete program. What you need to do is call it from your C program like so (for me, for you it may be different because I use UNIX and everyone here seems to like Windows):

Code:

/* Includes */

/* Some shtuff */
        int  result;
        char sresult[80];
        FILE *pipey = popen("eva", "r"); /* I called the script eva, deal */

        fgets(sresult, 80, pipey);
        result = atoi(sresult);
        /* Tada! You have the result of an expression. I hope...I haven't tested it,
           but if I got lucky then everything I've said is true. :) */

>and what it accomplishes in the piping?
Well, piping in this case is the easiest thing to do. Since writing an expression parser in C is a mite complex, the coolness of my two line Perl script is worth a little PVC in your program.

If you're using Windows, I think you still may have popen in some shape or form.

**Draco** · 10-21-2003

thanks, that clears things up. And so you know, it's padawan

**twm** · 10-22-2003

>And so you know, it's padawan

I uh...knew that...yea.

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