#include<stdio.h>
int main(){
  int a[3][3],b[3][3],c[3][3],i,j;
  printf("Enter the First matrix->");
  for(i=0;i<3;i++)
      for(j=0;j<3;j++)
           scanf("%d",&a[i][j]);
  printf("\nEnter the Second matrix->");
  for(i=0;i<3;i++)
      for(j=0;j<3;j++)
           scanf("%d",&b[i][j]);
  printf("\nThe First matrix is\n");
  for(i=0;i<3;i++){
      printf("\n");
      for(j=0;j<3;j++)
           printf("%d\t",a[i][j]);
  }
  printf("\nThe Second matrix is\n");
  for(i=0;i<3;i++){
      printf("\n");
      for(j=0;j<3;j++)
      printf("%d\t",b[i][j]);
   }
   for(i=0;i<3;i++)
       for(j=0;j<3;j++)
            c[i][j]=a[i][j]+b[i][j];
   printf("\nThe Addition of two matrix is\n");
   for(i=0;i<3;i++){
       printf("\n");
       for(j=0;j<3;j++)
            printf("%d\t",c[i][j]);
   }
   return 0;
}

#include<stdio.h>
int main(){
  int a[3][3],b[3][3],c[3][3],i,j;
  printf("Enter the First matrix->");
  for(i=0;i<3;i++)
      for(j=0;j<3;j++)
           scanf("%d",&a[i][j]);
  printf("\nEnter the Second matrix->");
  for(i=0;i<3;i++)
      for(j=0;j<3;j++)
           scanf("%d",&b[i][j]);
  printf("\nThe First matrix is\n");
  for(i=0;i<3;i++){
      printf("\n");
      for(j=0;j<3;j++)
           printf("%d\t",a[i][j]);
  }
  printf("\nThe Second matrix is\n");
  for(i=0;i<3;i++){
      printf("\n");
      for(j=0;j<3;j++)
      printf("%d\t",b[i][j]);
   }
   for(i=0;i<3;i++)
       for(j=0;j<3;j++)
            c[i][j]=a[i][j]-b[i][j];
   printf("\nThe Subtraction of two matrix is\n");
   for(i=0;i<3;i++){
       printf("\n");
       for(j=0;j<3;j++)
            printf("%d\t",c[i][j]);
   }
   return 0;
}

#include<stdio.h>
int main(){
  int a[5][5],b[5][5],c[5][5],i,j,k,sum=0,m,n,o,p;
  printf("\nEnter the row and column of first matrix");
  scanf("%d %d",&m,&n);
  printf("\nEnter the row and column of second matrix");
  scanf("%d %d",&o,&p);
  if(n!=o){
      printf("Matrix mutiplication is not possible");
      printf("\nColumn of first matrix must be same as row of second matrix");
  }
  else{
      printf("\nEnter the First matrix->");
      for(i=0;i<m;i++)
      for(j=0;j<n;j++)
           scanf("%d",&a[i][j]);
      printf("\nEnter the Second matrix->");
      for(i=0;i<o;i++)
      for(j=0;j<p;j++)
           scanf("%d",&b[i][j]);
      printf("\nThe First matrix is\n");
      for(i=0;i<m;i++){
      printf("\n");
      for(j=0;j<n;j++){
           printf("%d\t",a[i][j]);
      }
      }
      printf("\nThe Second matrix is\n");
      for(i=0;i<o;i++){
      printf("\n");
      for(j=0;j<p;j++){
           printf("%d\t",b[i][j]);
      }       
      }
      for(i=0;i<m;i++)
      for(j=0;j<p;j++)
           c[i][j]=0;
      for(i=0;i<m;i++){ //row of first matrix
      for(j=0;j<p;j++){  //column of second matrix
           sum=0;
           for(k=0;k<n;k++)
               sum=sum+a[i][k]*b[k][j];
           c[i][j]=sum;
      }
      }
  }
  printf("\nThe multiplication of two matrix is\n");
  for(i=0;i<m;i++){
      printf("\n");
      for(j=0;j<p;j++){
           printf("%d\t",c[i][j]);
      }
  }
  return 0;
}

#include<stdio.h>

int main(){

  int a[10][10],i,j,sum=0,m,n;

  printf("\nEnter the row and column of matrix: ");
  scanf("%d %d",&m,&n);

  printf("\nEnter the elements of matrix: ");
  for(i=0;i<m;i++)
      for(j=0;j<n;j++)
           scanf("%d",&a[i][j]);
  printf("\nThe matrix is\n");

  for(i=0;i<m;i++){
      printf("\n");
      for(j=0;j<m;j++){
      printf("%d\t",a[i][j]);
      }
 }
 for(i=0;i<m;i++){
     for(j=0;j<n;j++){
          if(i==j)
              sum=sum+a[i][j];
     }
 }
 printf("\n\nSum of the diagonal elements of a matrix is: %d",sum);

 return 0;
}
Sample output:
Enter the row and column of matrix: 3 3
Enter the elements of matrix: 2
3
5
6
7
9
2
6
7
The matrix is
2       3       5
6       7       9
2       6       7
Sum of the diagonal elements of a matrix is: 16

#include<stdio.h>
int main(){
  int a[10][10],b[10][10],i,j,k=0,m,n;
  printf("\nEnter the row and column of matrix");
  scanf("%d %d",&m,&n);
  printf("\nEnter the First matrix->");
  for(i=0;i<m;i++)
      for(j=0;j<n;j++)
           scanf("%d",&a[i][j]);
  printf("\nThe matrix is\n");
  for(i=0;i<m;i++){
      printf("\n");
      for(j=0;j<m;j++){
           printf("%d\t",a[i][j]);
      }
  }
  for(i=0;i<m;i++)
      for(j=0;j<n;j++)
           b[i][j]=0;
  for(i=0;i<m;i++){
      for(j=0;j<n;j++){
           b[i][j]=a[j][i];
           printf("\n%d",b[i][j]);
      }
  }
  printf("\n\nTraspose of a matrix is -> ");
  for(i=0;i<m;i++){
      printf("\n");
      for(j=0;j<m;j++){
           printf("%d\t",b[i][j]);
      }
  }
  return 0;
}