Segmentation Fault During Matrix Multiplication-CodePudding

I am new to C coding, and am trying to implement standard matrix multiplication. My code works fine for square matrices, but refuses to accept a column vector. Here is my attempt at the code. Any help would be much appreciated.

//---------------------------------------IMPORTING NECESSARY C PACKAGES AND DEFINING EXECUTION CONSTANTS-------------------------------------------//
#include <stdio.h> // Standard input output library
#include <math.h> // Mathematical function library
#include <stdlib.h> // General purpose standard library

#define true 1
#define false 0
typedef long double numeric; // Using the long double datatype to avoid overflows during computations
//-------------------------------------------------------------------------------------------------------------------------------------------------//

//----------------------------------------------------------------FUNCTION DECLERATION-------------------------------------------------------------//
numeric **create_matrix(int x, int y); // To dynamically allocate memory and create a matrix
void input_matrix(numeric **matrix, int m, int n); // To accept a matrix
void print_matrix(numeric **l, int x, int y); // To print a matrix
numeric **standard_matrix_multiplication(int m, int n, int l); // To multiply two matrices
//-------------------------------------------------------------------------------------------------------------------------------------------------//

//------------------------------------------------------------------DRIVER CODE--------------------------------------------------------------------//
int main(int argc, char *argv[]) {
  int m, n, l; int choice;
  printf("Enter the matrix operation to be performed using the corresponding index number.\n");
  printf("\n");
  printf("1.\tMatrix Multiplication");
  printf("\n");
  scanf("%d", &choice);

  switch(choice) {
    case 1 :
    printf("Enter the number of rows in the first matrix\n");
    scanf("%d", &m);
    printf("Enter the number of columns in the first matrix\n");
    scanf("%d", &n);
    printf("Enter the number of columns in the second matrix\n");
    scanf("%d", &l);
    printf("Enter both matrices.\n");
    numeric **matrix_x;
    matrix_x = create_matrix(m, l);
    matrix_x = standard_matrix_multiplication(m, n, l);
    print_matrix(matrix_x, m, l);
    break;
  }
}
//-------------------------------------------------------------------------------------------------------------------------------------------------//

//----------------------------------------------------------MATRIX MULTIPLICATION IMPLEMENTATIONS--------------------------------------------------//
numeric **standard_matrix_multiplication(int m, int n, int l) {
  numeric **matrix_a; numeric **matrix_b; numeric **matrix_k;
  matrix_a = create_matrix(m, n);
  matrix_b = create_matrix(n, l);
  matrix_k = create_matrix(m, l);
  input_matrix(matrix_a, m, n);
  print_matrix(matrix_a, m, n);
  input_matrix(matrix_b, n, l);
  for(int i = 0; i < m; i  ) {
    for (int j = 0; j < n; j   ) {
      for (int k = 0; k < l; k  ) {
        matrix_k[i][j]  = matrix_a[i][k] * matrix_b[k][j];
      }
    }
  }
  return matrix_k;
}
//-------------------------------------------------------------------------------------------------------------------------------------------------//

//---------------------------------------------------------------HELPER FUNCTIONS------------------------------------------------------------------//
numeric **create_matrix(int x, int y) {
  numeric **matrix = (numeric**)malloc(x * sizeof(numeric*)); // Dynamically creating an array of pointers
  for (int i = 0; i < y; i  ) {
    matrix[i] = (numeric*)malloc(y * sizeof(numeric)); // Dynamically allocating memory for each columns of the matrix
  }
  return matrix;
}

void input_matrix(numeric **matrix, int m, int n) {
  printf("Enter the elements of the matrix, row wise.\n"); // Instructing the user on matrix entry
  printf("For example, to enter the matrix\n");
  printf("\t\t1\t2\n");
  printf("\t\t3\t4\n");
  printf("enter 1, 2, 3, and 4 in that order.\n");

  for (int i = 0; i < m; i  ) { // Iterating through the rows and columns of the matrix
    for (int j = 0; j < n; j  ) {
      scanf("%Lf", &matrix[i][j]); // Accepting each element
    }
  }
}

void print_matrix(numeric **l, int x, int y) { // To print a matrix
    for (int i = 0; i < x; i  ) {
        for (int j = 0; j < y; j  ) {
            printf("%0.10Lf\t", l[i][j]); // Printing numeric type values
        }
    printf("\n");
    }
  printf("\n");
}

As of now, I have only written one switch case, and that is for matrix multiplication. So I chose 1. I gave 2, 1, 2 as my inputs for the number of rows in the first matrix, number of columns in the first matrix, and number of columns in the second matrix respectively. I have given a print statement in line 52, and it isn't executing it for the above input, giving a segmentation fault instead. Could someone please help me out?

CodePudding user response：

Yes there are some issues with your code that gives segfault error during runtime. The Matrix multiplication logic part of the code needs to be corrected as follows

for(int i = 0; i < m; i  ) {
for (int j = 0; j < l; j   ) {
  for (int k = 0; k < n; k  ) {
    matrix_k[i][j]  = matrix_a[i][k] * matrix_b[k][j];

because k should iterate till the no of columns in 1st matrix which is n but not till l.

And there is a slight correction needed in create_matrix function. You use y (i.e. no of columns in the matrix) in your for instead of x (no of rows in matrix).

If x < y, you end up outside of the memory you allocated.

If x > y, you end up with uninitialized pointers.

So change it as follows

for (int i = 0; i < x; i  ) {
matrix[i] = (numeric*)malloc(y * sizeof(numeric));

After these corrections try executing the code, you should get the expected results without any segfault errors

Here is the complete working code

#include <stdio.h> // Standard input output library
#include <math.h> // Mathematical function library
#include <stdlib.h> // General purpose standard library

#define true 1
#define false 0
typedef long double numeric; // Using the long double datatype to avoid overflows during computations
//-------------------------------------------------------------------------------------------------------------------------------------------------//

//----------------------------------------------------------------FUNCTION DECLERATION-------------------------------------------------------------//
numeric **create_matrix(int x, int y); // To dynamically allocate memory and create a matrix
void input_matrix(numeric **matrix, int m, int n); // To accept a matrix
void print_matrix(numeric **l, int x, int y); // To print a matrix
numeric **standard_matrix_multiplication(int m, int n, int l); // To multiply two matrices
//-------------------------------------------------------------------------------------------------------------------------------------------------//

//------------------------------------------------------------------DRIVER CODE--------------------------------------------------------------------//
int main(int argc, char *argv[]) {
  int m, n, l; int choice;
  printf("Enter the matrix operation to be performed using the corresponding index number.\n");
  printf("\n");
  printf("1.\tMatrix Multiplication");
  printf("\n");
  scanf("%d", &choice);

  switch(choice) {
    case 1 :
    printf("Enter the number of rows in the first matrix\n");
    scanf("%d", &m);
    printf("Enter the number of columns in the first matrix\n");
    scanf("%d", &n);
    printf("Enter the number of columns in the second matrix\n");
    scanf("%d", &l);
    printf("Enter both matrices.\n");
    numeric **matrix_x;
    matrix_x = create_matrix(m, l);
    matrix_x = standard_matrix_multiplication(m, n, l);
    print_matrix(matrix_x, m, l);
    break;
  }
}
//-------------------------------------------------------------------------------------------------------------------------------------------------//

//----------------------------------------------------------MATRIX MULTIPLICATION IMPLEMENTATIONS--------------------------------------------------//
numeric **standard_matrix_multiplication(int m, int n, int l) {
  numeric **matrix_a; numeric **matrix_b; numeric **matrix_k;
  matrix_a = create_matrix(m, n);
  matrix_b = create_matrix(n, l);
  matrix_k = create_matrix(m, l);
  input_matrix(matrix_a, m, n);
  print_matrix(matrix_a, m, n);
  input_matrix(matrix_b, n, l);
  //print_matrix(matrix_b, n, l);
  for(int i = 0; i < m; i  ) {
    for (int j = 0; j < l; j   ) {
      for (int k = 0; k < n; k  ) {
        matrix_k[i][j]  = matrix_a[i][k] * matrix_b[k][j];

      }
    }
  }
  return matrix_k;
}
//-------------------------------------------------------------------------------------------------------------------------------------------------//

//---------------------------------------------------------------HELPER FUNCTIONS------------------------------------------------------------------//
numeric **create_matrix(int x, int y) {
  numeric **matrix = (numeric**)malloc(x * sizeof(numeric*)); // Dynamically creating an array of pointers
  for (int i = 0; i < x; i  ) {
    matrix[i] = (numeric*)malloc(y * sizeof(numeric)); // Dynamically allocating memory for each columns of the matrix
  }
  return matrix;
}

void input_matrix(numeric **matrix, int m, int n) {
  printf("Enter the elements of the matrix, row wise.\n"); // Instructing the user on matrix entry
  printf("For example, to enter the matrix\n");
  printf("\t\t1\t2\n");
  printf("\t\t3\t4\n");
  printf("enter 1, 2, 3, and 4 in that order.\n");

  for (int i = 0; i < m; i  ) { // Iterating through the rows and columns of the matrix
    for (int j = 0; j < n; j  ) {
      scanf("%Lf", &matrix[i][j]); // Accepting each element
    }
  }
}

void print_matrix(numeric **l, int x, int y) { // To print a matrix
    for (int i = 0; i < x; i  ) {
        for (int j = 0; j < y; j  ) {
            printf("%0.10Lf\t", l[i][j]); // Printing numeric type values
        }
    printf("\n");
    }
  printf("\n");
}

CodePudding user response：

You are not allocating enough memory for matrix_k. You are allocating memory for m rows and l columns, but you are trying to access m rows and n columns. You need to allocate memory for m rows and n columns. You can do this by changing the line

matrix_k = create_matrix(m, l);

matrix_k = create_matrix(m, n);

This is the same problem you had in your other question.