How to plot data by c program?

Question

I am a mechanical engineer who has only limited knowledge in C programming. I wrote some code in order to make simulations, and I want to visualize the simulation results. At the moment I am using Dev-C for writing my codes. With fopen and fprintf commands I generate a .dat file which includes the results. Then I open GNUPLOT program and import my .dat file to plot the results. This takes time and I have to wait till the end of the simulation. Is there an easy way to connect my plotter with Dev-C, so my plotter starts plotting data during the simulation? Any library or etc. ?

Solution 1

Since you already know gnuplot, the simplest thing to do may be to just call gnuplot from your program and pipe the data to it:

FILE *gnuplot = popen("gnuplot", "w");
fprintf(gnuplot, "plot '-'\n");
for (i = 0; i < count; i++)
    fprintf(gnuplot, "%g %g\n", x[i], y[i]);
fprintf(gnuplot, "e\n");
fflush(gnuplot);

Solution 2

OK, one solution, as you are writing out to a file, would be to just make a system() call when you write out to the file, and call gnuplot.

But, that means you should change the filename each time, but I fear if you do that that it won't look correct, since you are sending small amounts of data each time.

http://www.gnu.org/software/libc/manual/html_node/System-Calls.html

I have never used gnuplot, but if you look at this page (http://gnuplot-tricks.blogspot.com/) you may find some tricks that would enable this to work well.

But, I think, unless you are going to plot everything yourself, and skip gnuplot, then you may just need to wait, as you are.

You may find the C interface to gnuplot may help you:

http://ndevilla.free.fr/gnuplot/

Solution 3

I've been using PLPlot for plotting from C and have found it both effective and easy. It's cross platform, open source, and supports a rich array of plot capabilities. I'd recommend having a look at the examples to get started.

Solution 4

pbPlots is very easy to use and works with all C compilers.

Download pbPlots.c/h and supportLib.c/h from the github page and include them in your build.

Here is a complete example that will produce a plot in a PNG-file.

#include "pbPlots.h"
#include "supportLib.h"

int main(){

    double x [] = {-1, 0, 1, 2, 3, 4, 5, 6};
    double y [] = {-5, -4, -3, -2, 1, 0, 1, 2};

    RGBABitmapImageReference *imageRef = CreateRGBABitmapImageReference();

    DrawScatterPlot(imageRef, 600, 400, x, 8, y, 8);

    size_t length;
    double *pngData = ConvertToPNG(&length, imageRef->image);
    WriteToFile(pngData, length, "plot.png");

    return 0;
 }

There are a lot more options available, these are described on the github page.

View more solutions

Comments

[James Black]

Do you have data to plot if the simulation isn't done?

[ArjunShankar]

What exactly takes time? Manually executing gnuplot and importing the data? Or does gnuplot itself take a lot of time plotting it? If its the first, you could easily get rid of it by executing gnuplot from the simulator itself. e.g. in POSIX-likes, there is the exec function.

[Hot Licks]

I wonder if you couldn't use a pipe instead of a file.

[Eddy_Em]

As I know, such popular plotting libraries like gnuplot & mathGL needs all data be ready before plotting.

[CrazyHorse]

Hi all, thanks for replying. I will give quick info. The thing what takes time is everytime writing the gnuplot commands manually. By some commands I wrote in my C programs, I want gnuplot to automatically plot my data. Since it is hard, it is not really important whether during or end of simulation... Since the other things I read here, I am working on "_popen" command. But by somehow the program says: "popen undeclared (first use in this function)". (I am using win7 and I added #include <stdio.h>) Any ideas?

[mgilson]

I've used this one in a unix environment and when using cygwin on windows. It always worked for me :).

[CrazyHorse]

@Edgar Hi Edgar, thanks for answer. Calling gnuplot from my program and piping the data to it is exactly what I want to do. I added your code into my program (ofcourse I also changed X[i] and y[i]). But I got this compiling error: "'popen' undecleared (first use in this function). I am using Win 7, do you have any idea why I got this error? Thanks.

[CrazyHorse]

Hi Paddy, thanks for answer. I checked the webpage you linked. Actually I was using this method (preparing make file, bash file) during my master work in a company. THere we were using Linux and the programs were all set by IT. Now in my personal computer I use Windows, so I need to learn everything (with my poor knowledge) this time for windows platform. Is there anyway to do those things in windows?

[paddy]

The most promising way is to install Cygwin, but you could have a crack at installing Perl for Windows. There's a ported version kicking around. It's nice if you can stick to the tools and methods that you know rather than learn a whole new (crippled, in the case of Windows) toolset.

[Edgar Bonet]

Since popen() is POSIX, on Windows you may try with an underscore: _popen().

[CrazyHorse]

@Edgar, That also doesn't work. The code you wrote seems the easiest approach to my goal but I can't compile it :)

[sitilge]

Prepending set nokey; to plot removes the annoying legend.

[le_lemon]

I added the pbPlots.c/h and supportLib.c/h files to a CLion project but when I try to run your example I get an error: C:/Users/flori/CLionProjects/Plottest/main.c:9: undefined reference to `CreateRGBABitmapImageReference' (+ a lot more unresolved references) ... what did I do wrong ?

[Martin Johansen]

@le_lemon, did you include pbPlots.c and supportLib.c in the build? In C, the compiler uses the .h-files while the linker uses the .c files. The linker needs to know which C files are included. The error comes form the linker not getting the compiled .c-files.

[M-Chen-3]

Please explain how this relates to the question and how it works.

[Admin]

@m-chen-3 You can find set of examples at Dislin examples. Just click one of plot tutorial then select your language (C).

[Chef Gladiator]

@MartinJohansen are you the author?

[Martin Johansen]

@ChefGladiator, yes, I am the author.