PGPLOT help please!!

**shae** · 11-27-2018

Hi, I'm brand new to C programming and coding in general and I am getting
" %PGPLOT, invalid x limits in PGENV: XMIN = XMAX."
when I run my program.
This is my code:

Code:

#include <stdio.h>
#include <math.h>


/*float N(float S[n], float I[n], float R[n])
{
  N[n] = 400.0;
  N[n] = S[n] + I[n] + R[n];
  return N[n];
}*/


float dSdt(float N, float S, float I, float R, float mu, float beta)
  {
    return mu*N - (beta*S*I) - mu*S; 
  }
  float dIdt(float S, float I, float R, float mu, float beta, float gam)  
  {
    return (beta*S*I) - gam*I - mu*I;
  }
  float dRdt(float I, float R, float mu, float gam)
  {
    return gam*I - mu*R;
  }


int main ()
{
  
  int i;
  int n=20;
  int t=10;
  float S[n], I[n], R[n];
  S[0]=1.;
  I[0]=0.;
  R[0]=0.;
  //N=S+I+R therefore N=1 *** explain in write up this means that i can delete all "/N" because its just dividing by 1
  float N, mu, beta, gam; 
  


  float Sk1, Sk2, Sk3, Sk4, Ik1, Ik2, Ik3, Ik4, Rk1, Rk2, Rk3, Rk4;
  float h = (float)t/(float)n;
  /*printf("Input numbers for x0, y0, h, and n.\n");
  scanf("%f %f %f %f %f", &x0, &y0, &h, &n0); */
  for (i=1;i<=n;i++) //??
    {
      Sk1 = dSdt(N, S[i-1], I[i-1], R[i-1], mu, beta);
      Ik1 = dIdt(S[i-1], I[i-1], R[i-1], mu, beta, gam);
      Rk1 = dRdt(I[i-1], R[i-1], mu, gam);


      Sk2 = dSdt(N, S[i-1]+Sk1*h/2.0, I[i-1]+Ik1*h/2.0, R[i-1]+Rk1*h/2.0, mu, beta);
      Ik2 = dIdt(S[i-1]+Sk1*h/2.0, I[i-1]+Ik1*h/2.0, R[i-1]+Rk1*h/2.0, mu, beta, gam);
      Rk2 = dRdt(I[i-1]+Ik1*h/2.0, R[i-1]+Rk1*h/2.0, mu, gam);


      Sk3 = dSdt(N, S[i-1]+Sk2*h/2.0, I[i-1]+Ik2*h/2.0, R[i-1]+Rk2*h/2.0, mu, beta);
      Ik3 = dIdt(S[i-1]+Sk2*h/2.0, I[i-1]+Ik2*h/2.0, R[i-1]+Rk2*h/2.0, mu, beta, gam);
      Rk3 = dRdt(I[i-1]+Ik2*h/2.0, R[i-1]+Rk2*h/2.0, mu, gam);


      Sk4 = dSdt(N, S[i-1]+Sk3*h/2.0, I[i-1]+Ik3*h/2.0, R[i-1]+Rk3*h/2.0, mu, beta); 
      Ik4 = dIdt(S[i-1]+Sk3*h/2.0, I[i-1]+Ik3*h/2.0, R[i-1]+Rk3*h/2.0, mu, beta, gam);
      Rk4 = dRdt(I[i-1]+Ik3*h/2.0, R[i-1]+Rk3*h/2.0, mu, gam);


      S[i] = S[i-1] + (h/6.0)*(Sk1 + 2.0*Sk2 + 2.0*Sk3 + Sk4);
      I[i] = I[i-1] + (h/6.0)*(Ik1 + 2.0*Ik2 + 2.0*Ik3 + Ik4);
      R[i] = R[i-1] + (h/6.0)*(Rk1 + 2.0*Rk2 + 2.0*Rk3 + Rk4);
    }


/*----------------------------------------------------------------------------------------------------------------*/
/*PLOT IT*/
/*----------------------------------------------------------------------------------------------------------------*/


 // Open a plot window
  if (!cpgopen("/XWINDOW")) return 1;


  // Set-up plot axes
  cpgenv(0.,1.,0.,1.,0,1);


  // Label axes
  cpglab("x", "y", "project");


  //set colour to yellow
  cpgsci(7);


  //plot
  cpgline(n+1,t,S);
  // Pause and then close plot window
  cpgclos();


}

Any help would be so greatly appreciated!!! Thank you so much in advance!

**john.c** · 11-27-2018

cpgline's second parameter is supposed to be a pointer to a float, but you are passing an int. One way to fix it:

Code:

  float ft = t;
  cpgline(n+1, &ft, S);

Or maybe you should make t a float in the first place (and pass it's address like I did with ft above).

**Salem** · 11-27-2018

There are a bunch of problems before you even get to plotting.

Code:

$ gcc -Wall -Wextra foo.c
foo.c: In function ‘dSdt’:
foo.c:13:45: warning: unused parameter ‘R’ [-Wunused-parameter]
 float dSdt(float N, float S, float I, float R, float mu, float beta)
                                             ^
foo.c: In function ‘dIdt’:
foo.c:17:38: warning: unused parameter ‘R’ [-Wunused-parameter]
   float dIdt(float S, float I, float R, float mu, float beta, float gam)
                                      ^
foo.c: In function ‘main’:
foo.c:48:11: warning: ‘N’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       Sk1 = dSdt(N, S[i-1], I[i-1], R[i-1], mu, beta);
           ^
foo.c:48:11: warning: ‘mu’ may be used uninitialized in this function [-Wmaybe-uninitialized]
foo.c:48:11: warning: ‘beta’ may be used uninitialized in this function [-Wmaybe-uninitialized]
foo.c:49:11: warning: ‘gam’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       Ik1 = dIdt(S[i-1], I[i-1], R[i-1], mu, beta, gam);
           ^

All those maybe-uninitialized are actually uninitialised, which means you're computing with garbage.

> S[i] = S[i-1] + (h/6.0)*(Sk1 + 2.0*Sk2 + 2.0*Sk3 + Sk4);
> I[i] = I[i-1] + (h/6.0)*(Ik1 + 2.0*Ik2 + 2.0*Ik3 + Ik4);
> R[i] = R[i-1] + (h/6.0)*(Rk1 + 2.0*Rk2 + 2.0*Rk3 + Rk4);
On the last iteration of the loop, when i == n, the assignments are out of bounds.

**shae** · 11-27-2018

Thank you both very much for your input! I have updated my code to change t to a float and also to fix the problems that have been pointed out. I am unfortunately getting the same response when I try to run my program. Any other insights would be greatly appreciated!!

Code:

#include <stdio.h>
#include <math.h>


/*float N(float S[n], float I[n], float R[n])
{
  N[n] = 400.0;
  N[n] = S[n] + I[n] + R[n];
  return N[n];
}*/


float dSdt(float N, float S, float I, float mu, float beta)
  {
    return mu*N - (beta*S*I) - mu*S; 
  }
  float dIdt(float S, float I, float mu, float beta, float gam)  
  {
    return (beta*S*I) - gam*I - mu*I;
  }
  float dRdt(float I, float R, float mu, float gam)
  {
    return gam*I - mu*R;
  }


int main ()
{
  
  int i;
  int n=100;
  int t=0;
  float S[n], I[n], R[n];
  S[0]=1.;
  I[0]=0.;
  R[0]=0.;
  


  //N=S+I+R therefore N=1 *** explain in write up this means that i can delete all "/N" because its just dividing by 1
  float N, mu, beta, gam; 
  N = 1.;


  float Sk1, Sk2, Sk3, Sk4, Ik1, Ik2, Ik3, Ik4, Rk1, Rk2, Rk3, Rk4;
  float h = (float)t/(float)n;
  /*printf("Input numbers for x0, y0, h, and n.\n");
  scanf("%f %f %f %f %f", &x0, &y0, &h, &n0); */
  for (i=1;i<=n;i++) //??
    {
      Sk1 = dSdt(N, S[i-1], I[i-1], mu, beta);
      Ik1 = dIdt(S[i-1], I[i-1], mu, beta, gam);
      Rk1 = dRdt(I[i-1], R[i-1], mu, gam);


      Sk2 = dSdt(N, S[i-1]+Sk1*h/2.0, I[i-1]+Ik1*h/2.0, mu, beta);
      Ik2 = dIdt(S[i-1]+Sk1*h/2.0, I[i-1]+Ik1*h/2.0, mu, beta, gam);
      Rk2 = dRdt(I[i-1]+Ik1*h/2.0, R[i-1]+Rk1*h/2.0, mu, gam);


      Sk3 = dSdt(N, S[i-1]+Sk2*h/2.0, I[i-1]+Ik2*h/2.0, mu, beta);
      Ik3 = dIdt(S[i-1]+Sk2*h/2.0, I[i-1]+Ik2*h/2.0, mu, beta, gam);
      Rk3 = dRdt(I[i-1]+Ik2*h/2.0, R[i-1]+Rk2*h/2.0, mu, gam);


      Sk4 = dSdt(N, S[i-1]+Sk3*h/2.0, I[i-1]+Ik3*h/2.0, mu, beta); 
      Ik4 = dIdt(S[i-1]+Sk3*h/2.0, I[i-1]+Ik3*h/2.0, mu, beta, gam);
      Rk4 = dRdt(I[i-1]+Ik3*h/2.0, R[i-1]+Rk3*h/2.0, mu, gam);


      S[i] = S[i-1] + (h/6.0)*(Sk1 + 2.0*Sk2 + 2.0*Sk3 + Sk4);
      I[i] = I[i-1] + (h/6.0)*(Ik1 + 2.0*Ik2 + 2.0*Ik3 + Ik4);
      R[i] = R[i-1] + (h/6.0)*(Rk1 + 2.0*Rk2 + 2.0*Rk3 + Rk4);
    }


/*----------------------------------------------------------------------------------------------------------------*/
/*PLOT IT*/
/*----------------------------------------------------------------------------------------------------------------*/


 // Open a plot window
  if (!cpgopen("/XWINDOW")) return 1;


  // Set-up plot axes
  cpgenv(0.,10.,0.,1.,0,1);


  // Label axes
  cpglab("x", "y", "project");


  //set colour to yellow
  cpgsci(7);


  //plot
  float ft = t;
  cpgline(n+1, &ft, S);
  
  // Pause and then close plot window
  cpgclos();


}

Thanks again in advance!

**john.c** · 11-27-2018

You ignored Salem's last point and are still accessing S[n], I[n] and R[n] (which are out-of-bounds) on the last iteration of your loop.

**shae** · 11-28-2018

Oh right, thank you!!

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