C program works only a part -

i need make program says have fill "1" in odd rows , "0" in ones. tried , wrote code, seem works part. please, can explain me method?

#include <stdlib.h> #include <stdio.h> #include <math.h>  main() {   int r, c;   {   printf("insert number of rows in matrix: ");   scanf("%d", &r);   printf("insert number of colums in matrix: ");   scanf("%d", &c);   } while(r<=0||c<=0);    r--; c--;   int mat[r][c];   int a, b;    for(a=0; a<=r; a++)   {     for(b=0; b<=c; b++)     {       if(a%2==0)       {mat[a][b]=0;}       else {mat[a][b]=1;}     }   }    printf("\noutput:\n\n");     for(a=0; a<=r; a++){       for(b=0; b<=c; b++){         printf("%d", mat[a][b]);       }       printf("\n");     }    return 0; } 

input: 2, 3

output: 001 111

you don't use number of rows , columns correctly. true in c, array n entries can access indices 0 through n - 1 , index n 1 beyond range of array. still have define array actual size:

// don't adjust c , r here  int mat[r][c]; 

when use dimension in for loop, can ensure never access mat[r][c] using < instead of <= condition:

for (a = 0; < r; a++) {     (b = 0; b < c; b++) {         if (a % 2 == 0) {             mat[a][b] = 0;         } else {             mat[a][b] = 1;         }     } }