Thread: Adding two matrix using malloc

This is my code without malloc. I need to change the array size so there is no max size for each matrix. I must dynamically allocate space for all arrays used. So I need to use malloc to create my arrays. So I cant use int A[rows][cols].

Code:

/* This program asks the user for 2 matrices called A and B, as integers,
   and displays their sum, C. The max dimension of each matrix is 100. */

#include<stdio.h>

// Construct function
void construct()
{
   int m, n, i, j;// Variables
   int first[100][100], second[100][100], sum[100][100];// Matrices variables

   printf("Please enter the number of rows: ");
   scanf("%d",&m);
   printf("Please enter the number of columns: ");
   scanf("%d",&n);

   // User enters m x n amount whole numbers for the Matrix A
   printf("Enter Matrix A\n");
   for(i =0; i < m; i++)
      for(j =0; j < n; j++)
         scanf("%d",&first[i][j]);

   // User enters m x n amount whole numbers for the Matrix B
   printf("Enter Matrix B\n");
   for(i =0; i < m; i++)
      for(j =0; j < n; j++)
            scanf("%d",&second[i][j]);

   // Adds the sum of Matrix A and Matrix B
   for(i =0; i < m; i++)
      for(j =0; j < n; j++)
         sum[i][j]= first[i][j]+ second[i][j];

   // Display the sum of Matrix A and Matrix B
   printf("A + B =\n");
   for(i =0; i < m; i++)
   {
      for(j =0; j < n; j++)
         printf("%d ", sum[i][j]);

      printf("\n");// Prints new line
   }

   return;
}

// Main Function
int main()
{
    construct();// Calls construct function
    return 0;
}

Can someone help guide me through this? Im having a hard time understanding/using malloc. Should first, second, and sum be single pointers or double? How do I scan the matrix correctly? And how do I add them properly? Do I have to change my for loops?
Any suggestions for the next step to be?

Code:

/* This program asks the user for 2 matrices called A and B, as integers,
   and displays their sum, C. The max dimension of each matrix is 100. */


#include <stdio.h>
#include <stdlib.h>


// Construct function
void construct()
{
   int m, n, i, j;      // m = rows; n = columns
   int *first, *second, *sum; // first = matrix1; second = matrix2


   printf("Please enter the number of rows: ");
   scanf("%d", &m);
   printf("Please enter the number of columns: ");
   scanf("%d", &n);


   first  = (int*)malloc(m * n * sizeof(int));
   second = (int*)malloc(m * n * sizeof(int));
   sum    = (int*)malloc(m * n * sizeof(int));




   // User enters m x n amount whole numbers for the Matrix A
   printf("Enter Matrix A\n");
   for (i = 0; i < m; i++)
      for (j = 0; j < n; j++)
         scanf("%d", *(*(first+i)+j));


   // User enters m x n amount whole numbers for the Matrix B
   printf("Enter Matrix B\n");
   for (i = 0; i < m; i++)
      for (j = 0; j < n; j++)
            scanf("%d", second[i][j];


   // Adds the sum of Matrix A and Matrix B
   for (i = 0; i < m; i++)
      for (j = 0; j < n; j++)
         *(sum[i][j])= *(first[i][j]) + *(second[i][j]);


// Display the sum of Matrix A and Matrix B
   printf("A + B =\n");
   for (i = 0; i < m; i++)
   {
      for (j = 0; j < n; j++)
         printf("%d ", *(sum + i + j));


      printf("\n");
   }


   return ;
}


// Main Function
int main()
{
    construct(); // Calls construct function
    return 0;
}

Originally Posted by machine392

Code:

   printf("Enter Matrix A\n");
   for (i = 0; i < m; i++)
      for (j = 0; j < n; j++)
         scanf("%d", *(*(first+i)+j));

You have m rows and n columns in first. The element on row i,column j in first is therefore first[n*i + j], and the pointer to this is &(first[n*i+j]).

For example, if m==2 and n==5, the elements are arranged as

Code:

first[0] first[1] first[2] first[3] first[4]
first[5] first[6] first[7] first[8] first[9]

which is also called row-major order.

Also, you should always check the return value from scanf(); it returns the number of successful conversions (here 1). So, the above code snippet should really look like

Code:

   printf("Enter Matrix A\n");
   for (i = 0; i < m; i++)
      for (j = 0; j < n; j++)
         if (scanf("%d", &(first[n*i+j])) != 1) {
            printf("Not an integer (row %d, column %d for matrix A).\n", i+1, j+1);
            return EXIT_FAILURE;
         }

Thank you so much it worked!