Error while calculating sum of two matrices in c - c

Question

So I'm new to C and was trying to write a program to add two 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 the number of rows and columns for matrix ");
scanf("%i%i",&m,&n);
printf("enter elements of matrix one :");
for(i=0;i<m;i++){
for(j=0;j<n;j++){
scanf("%i",&mat1[i][j]);
}
}
printf("enter the 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 the 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 i used a function to enter the values in two the matrix, it produced the following error
25 24 C:\Users\Hp\my-programs\matrix-entry-function.cpp [Error] invalid types 'int[int]' for array subscript
25 35 C:\Users\Hp\my-programs\matrix-entry-function.cpp [Error] invalid types 'int[int]' for array subscript
code:
#include <stdio.h>
int mat_entry(int m,int n)
{
printf("enter the rows and 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 the %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]);
}
}
}

Answer 1

Your program 1 happened not to produce error. You are unlucky. You invoked undefined behavior by using values in uninitialized variables having automatic storage duration, which are indeterminate. You must declare mat1, mat2 and result after reading m and n.
int main(){
int m,n,o,p,i,j;
printf("enter the number of rows and columns for matrix ");
scanf("%i%i",&m,&n);
/* move these declaretions after reading number of rows and columns */
int mat1[m][n];
int mat2[m][n];
int result[m][n];
printf("enter elements of matrix one :");
Your program 2 invokes undefined behavior by using return values of functions with no return statements. Also using [] operator with two operands having type int is wrong. E1[E2] is equivalent to (*((E1)+(E2))) (N1570 6.5.2.1 Array subscripting, paragraph 2), so exactly one operand of it must be a pointer (including one converted from arrays).
To return matrixes as return values of functions, consider using structures and allocating memory for elements dinamically.

Answer 2

You have used
int mat1=mat_entry(a,b);
int mat2=mat_entry(c,d);
instead of sending parameters from one function to another, which would work only when the variables you take are pointers, here you have to either use mat1 and mat2 as pointers or you have to make them integer matrices.

Answer 3

There isn't one thing that you are doing wrong my friend.
You are not even using dynamic memory allocation but still trying to declare matrices with values which are not defined yet.Try to run your code,even your first code won't run.
i have corrected your code for you :
#include <stdio.h>
int main(){
int m,n;
printf("enter the number of rows and columns for matrix ");
scanf("%i%i",&m,&n);
int mat1[m][n];
int mat2[m][n];
int result [m][n];
int i,j;
printf("enter elements of matrix one :");
for(i=0;i<m;i++){
for(j=0;j<n;j++){
scanf("%d",&mat1[i][j]);
}
}
printf("enter the elements of matrix two:");
for(i=0;i<m;i++){
for(j=0;j<n;j++){
scanf("%d",&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 the matrices are");
for(i=0;i<m;i++){
for(j=0;j<n;j++){
printf("%d",result[i][j]);
}
}
return 0;
}
I don't think it would be that difficult to put this functionality into a function . Try it out

Answer 4

It is impossible to write a function that makes a matrix with user-supplied dimensions, unless you use dynamic allocation (which you probably don't want to, at this stage).
Luckily, you don't need that! Since all you want is adding matrices, they should have the same dimensions, and you only want to ask the user once what the dimensions are. Afterwards, fill both matrices, and do the "adding" part.
That is:
int main()
{
int a,b,c,d,e,f,m,n;
printf("enter the rows and columns of matrix ");
scanf("%i%i",&m,&n);
int res[m][n];
int mat1[m][n];
int mat2[m][n];
mat_entry(m, n, mat1); // fill matrix 1
mat_entry(m, n, mat2); // fill matrix 2
for(int i=0;i<m;i++)
{
for(int j=0;j<n;j++)
{
res[i][j]=(mat1[i][j]+mat2[i][j]);
}
}
}
The function mat_entry has no return value (i.e. void), but it receives the matrix to fill by pointer (the details are a bit complicated, but the syntax is straightforward):
void mat_entry(int m,int n, int mat[m][n])
{
...
}
The code inside is the same as you suggested, minus the asking for dimensions:
for(int i=0;i<m;i++)
{
for(int j=0;j<n;j++)
{
printf("\nenter the %i'th element %i'th row :",j+1,i+1);
scanf("%i",&mat[i][j]);
}
}