debugging - 2D array referencing whole index, not individual cell in C -


i can't seem figure out problem is. i'm writing 2d random walk simulation , thought use 2d array simulate grid it's on. however, in code, when try reference particular cell in array increment it's value, bin[3][4], increments value of whole index i.e. bin[1][4],bin[2][4],bin[3][4],etc. ideas on how fix or i'm doing wrong? thanks.

#include <stdio.h> #include <math.h>  #define total 10.  /*total iterations*/ #define walk 5  /*total # of steps in walk*/ #define lambda 1.0 /*step size*/ #define binsize 0.1 /*current chosen values bug checking simplicity*/  main() {   float range = walk*lambda*2.; /*2 times max range, positive*/   int n,prob,i,k,j,placex,placey,bins;   double valuex,valuey, a; /*value starts @ half range values +*/   bins=(range/binsize);    int bin[bins][bins];   for(i=0;i<=bins;i++) /*zero out bin*/   {     for(j=0;j<=bins;j++)     {       bin[i][j]=0;     }   }    for(k=0;k<total;k++)   {     valuex=range/2.;     valuey=range/2.;      for(n=1;n<=walk;n++)     {       prob= rand(4) % 100+1;       if(prob<=25)       {         valuex=valuex+pow(lambda,n);       }       else if(prob>25 && prob<=50)       {         valuex=valuex-pow(lambda,n);       }       else if(prob>50 && prob<=75)       {         valuey=valuey+pow(lambda,n);       }       else if(prob>75)       {         valuey=valuey-pow(lambda,n);       }     }         placex=floor(valuex/binsize+0.5);     placey=floor(valuey/binsize+0.5);      bin[placex][placey]=++bin[placex][placey];      printf("%d %d %d\n",placex,placey,bin[placex][placey]); /* bug checking. prints bin numbers value should go , value of element.*/   }    for(i=0;i<=bins;i++)   {     for(j=0;j<=bins;j++)     {       a=bin[i][j]/total;       //printf("%lf %lf %lf\n",i*binsize-range/2.,j*binsize-range/2.,a); /*format input igor*/     }   } }

figured out. going out of array bounds. loops go bins arrays indexed 0 bins -1. took out = <= in loops worked. end code looked this. in general going out of array bounds has unpredictable behavior.

#include <stdio.h> #include <math.h>  #define total 10.  /*total iterations*/ #define walk 5  /*total # of steps in walk*/ #define lambda 1.0 /*step size*/ #define binsize 0.1 /*current chosen values bug checking simplicity*/  main()     {    float range = walk * lambda * 2.; /*2 times max range, positive*/    int n, prob, i, k, j, placex, placey, bins;    double valuex, valuey, a; /*value starts @ half range values +*/     bins = (range / binsize);     int bin[bins][bins];     /*zero out bin*/    for(i = 0; < bins; i++) {       for(j = 0; j < bins; j++)           bin[i][j] = 0;    }     for(k = 0; k < total; k++) {       valuex = range / 2. - 1;       valuey = range / 2. - 1;        for(n = 1; n <= walk; n++) {          prob = rand(4) % 100 + 1;          if(prob<=25)             valuex += pow(lambda, n);          else if(prob <= 50)             valuex -= pow(lambda, n);          else if(prob <= 75)             valuey += pow(lambda, n);          else              valuey -= pow(lambda, n);       }           placex = floor(valuex / binsize + 0.5);        placey = floor(valuey / binsize + 0.5);        bin[placex][placey] += 1;        printf("%d %d %d\n", placex, placey, bin[placex][placey]); /* bug checking. prints bin numbers value should go , value of element.*/    }     for(i = 0; < bins; i++) {     printf("\n");       for(j = 0 ;j < bins; j++) {         if (bin[i][j] == 0)         printf("  ");         else         printf("%d ", bin[i][j]);          = bin[i][j] / total;         }     } }

the loops added @ bottom output pattern.


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