Thread: floating exception problems

hello! I am fairly new to C and am trying to write a code that solves ODEs and graphs them. I seem to have run into a "floating exception". I have been using gdb but I don't think I fully understand what it is telling me/how to go about fixing it. Any help would be great!

My code is below thanks!

Code:

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

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//DEFINE DIFFERENTIAL EQUATIONS
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

float dSdt(float B, float N, float I, float S)
{
     return (-((B/N)*I)*S);
}
float dIdt(float y, float B, float N, float I, float S)
{
     return ((B/N)*I*S)-y*I;
}
float dRdt(float y, float I)
{
     return y*I;
}

int main (){

//~~~~~~~~~~~~~~~~~~~~~
//RUNGE KUTTA
//~~~~~~~~~~~~~~~~~~~~~~
  
  int DAYS = 15;
  int i;
  float t[i];
  t[0]=0.;
  int n = 100;

// step height h 
    float h = (float)DAYS/(float)n; 
 
//rk   
    float Sk1,Ik1,Rk1;
    float Sk2,Ik2,Rk2;
    float Sk3,Ik3,Rk3;
    float Sk4,Ik4,Rk4;

//define arrays i need to graph at end
    float S[n];
    float I[n];
    float R[n];

//define and set parameters
  float u=0.02;
  float y=36.;
  float B=10.;

const float N = 100;

//set initial conditions
    S[0]=30.;
    I[0]=30.;
    R[0]=40.;

// Apply Runge Kutta
    for (i=1; i<=n; i++) 
    { 

//k1
        Sk1 = h*dSdt(N,B,S[i-1],I[i-1]); 
        Ik1 = h*dIdt(y,B,N,I[i-1],S[i-1]);
        Rk1 = h*dRdt(y,I[i-1]);

//k2 
        Sk2 = h*dSdt(N,B,I[i-1] + 0.5*Ik1, S[i-1] + 0.5*Sk1); 
        Ik2 = h*dIdt(y,B,N,I[i-1]+ 0.5*Ik1,S[i-1] + 0.5*Sk1);
        Rk2 = h*dRdt(y,I[i-1]+ 0.5*Ik1);

//k3 
        Sk3 = h*dSdt(N,B,S[i-1]+ 0.5*Sk2,I[i-1]+ 0.5*Ik2); 
        Ik3 = h*dIdt(y,B,N,I[i-1]+ 0.5*Ik2,S[i-1]+ 0.5*Sk2);
        Rk3 = h*dRdt(y,I[i-1]+ 0.5*Ik2);

//k4
        Sk4 = h*dSdt(N,B,S[i-1]+ 0.5*Sk3,I[i-1]+ 0.5*Ik3);
        Ik4 = h*dIdt(y,B,N,I[i-1]+ 0.5*Ik3,S[i-1]+ 0.5*Sk3);
        Rk4 = h*dRdt(y,I[i-1]+ 0.5*Ik3);
         
// Update next value of funtions
        S[i] = S[i-1] + (1.0/6.0)*(Sk1 + 2.*Sk2 + 2.*Sk3 + Sk4);
        I[i] = I[i-1] + (1.0/6.0)*(Ik1 + 2.*Ik2 + 2.*Ik3 + Ik4);
        R[i] = R[i-1] + (1.0/6.0)*(Rk1 + 2.*Rk2 + 2.*Rk3 + Rk4);
  
// Update next value of t
        t[i] = t[i-1] + h; 
   } 
 
//~~~~~~~~~~~~~~~~~~~
//PLOT GRAPHS
//~~~~~~~~~~~~~~~~~~~

// Open a plot window
  if (!cpgopen("/XWINDOW")) return 1;
//set up plot axes
  cpgenv(0.,1000.,0.,100.,0,1);
// Label axes
  cpglab("Time (days)", "Fraction of people", "SIR Model");

  // Change plot colour to colour 4 (BLUE)
  cpgsci(4);
  // Plot S
  cpgline(n+1,t,S);

  //change colour of derivative
   cpgsci(7);
  // Plot I
  cpgline(n+1,t,I);

  //change colour of derivative
   cpgsci(5);
  // Plot R
  cpgline(n+1,t,R);

  cpgclos();

  return 0;
}

Thanks for your answer! Sorry about any confusion with the variables, will defienitely keep that in mind for next time! So t should be of some size to do with n right? I was thinking [n+1] but it didn't seem to help (I also tried changing the array sizes of S,I and R to the same but I am still a little bit stuck...)

I thought that might be happening! Thanks so much! I did some research and got some better initial conditions and the program runs!

please fix the size of array always.