Tuesday 7 September 2010

Plotting a basic 2D graph using C + gnuplot (Tutorial)

In this tutorial we will learn how to draw a graph using ANSI C language with the help of gnuplot utility.

Step 1 write a c program using any editor. (here file name is plot.c) 

/*
 *      file: plot.c
 *
 *      Copyright 2010 Rooparam Choudhary <rooparam@rishi.serc.iisc.ernet.in>
 *
 *      Date : 07.09.2010
 *
 *      This program is free software; you can redistribute it and/or modify
 *      it under the terms of the GNU General Public License as published by
 *      the Free Software Foundation; either version 2 of the License, or
 *      (at your option) any later version.
 *    
 *      This program is distributed in the hope that it will be useful,
 *      but WITHOUT ANY WARRANTY; without even the implied warranty of
 *      MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *      GNU General Public License for more details.
 *    
 *      You should have received a copy of the GNU General Public License
 *      along with this program; if not, write to the Free Software
 *      Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 *      MA 02110-1301, USA.
 *
 */


#include <stdio.h>

double func ( double x ) {
    if ( x < 0 )
        return 0 ;
    if ( x < 2 )
        return x * x ;

    return 16.0 / ( x * x ) ;
}

/*
 * output will be in 2 coloumns
 * 1 column contains x-axis values
 * 2 column contains y-axis {or f(x) } values corresponding to x-value
 */

int main ( ) {
    double x = 0.0;     // initial value
    double x_max = 5.0;
    double step = 0.01;
   
    printf ( "# x \t f(x) \n" );

    while ( x <= x_max ) {
        printf ( "%.3f \t %.3f \n", x, func(x) );
        x += step;
    }

    return 0;
}


Step 2 compile it and run it and grab the output in plot.txt file

            $ cc plot.c
            $ ./a.out >plot.txt

Step 3 Draw using gnuplot

            $ gnuplot
gnuplot> plot "./plot.txt" with lines



OK folks.
Tutorial is over.

This was just an introduction for plotting basic graphs. For more plotting tutorials and advanced plotting, just visit gnuplot .

See ya later.