Error while calculating sum of two matrices in c -

so i'm new c , trying write program add 2 matrices

program 1

#include <stdio.h> int main(){ int m,n,o,p,i,j; int mat1[m][n]; int mat2[m][n]; int result[m][n]; printf("enter number of rows , columns matrix "); scanf("%i%i",&m,&n); printf("enter elements of matrix 1 :"); for(i=0;i<m;i++){     for(j=0;j<n;j++){         scanf("%i",&mat1[i][j]);     } } printf("enter elements of matrix two:"); for(i=0;i<m;i++){     for(j=0;j<n;j++){         scanf("%i",&mat2[i][j]);     } } for(i=0;i<m;i++){     for(j=0;j<n;j++){         result[i][j]=mat1[i][j]+mat2[i][j];     } }  printf("the sum of matrices are"); for(i=0;i<m;i++){     for(j=0;j<n;j++){         printf("%i",result[i][j]);     } } return 0;  } 

this produced no error

method two

when used function enter values in 2 matrix, produced following error

25  24  c:\users\hp\my-programs\matrix-entry-function.cpp   [error] invalid types 'int[int]' array subscript 25  35  c:\users\hp\my-programs\matrix-entry-function.cpp   [error] invalid types 'int[int]' array subscript 

code:

#include <stdio.h>  int mat_entry(int m,int n) {     printf("enter rows , columns of matrix ");     scanf("%i%i",&m,&n);     int mat[m][n];     for(int i=0;i<m;i++)     {         for(int j=0;j<n;j++)         {             printf("\nenter %i'th element %i'th row :",j+1,i+1);             scanf("%i",&mat[i][j]);         }     } }  int main() {     int a,b,c,d,e,f,m,n;     int res[m][n];     int mat1=mat_entry(a,b);     int mat2=mat_entry(c,d);      for(int i=0;i<m;i++)     {         for(int j=0;j<n;j++)         {             res[i][j]=(mat1[i][j]+mat2[i][j]);         }     } } 

your program 1 happened not produce error. unlucky. invoked undefined behavior using values in uninitialized variables having automatic storage duration, indeterminate. must declare mat1, mat2 , result after reading m , n.

int main(){ int m,n,o,p,i,j; printf("enter number of rows , columns matrix "); scanf("%i%i",&m,&n); /* move these declaretions after reading number of rows , columns */ int mat1[m][n]; int mat2[m][n]; int result[m][n]; printf("enter elements of matrix 1 :"); 

your program 2 invokes undefined behavior using return values of functions no return statements. using [] operator 2 operands having type int wrong. e1[e2] equivalent (*((e1)+(e2))) (n1570 6.5.2.1 array subscripting, paragraph 2), 1 operand of must pointer (including 1 converted arrays).

to return matrixes return values of functions, consider using structures , allocating memory elements dinamically.