Determinant of a Matrix in C, Troubleshooting - c

Question

main.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "matrix.h"
int main(){
//Prompt the user for the size of matrix to be calculated.
printf("Welcome to the matrix determinant calculator!\n\n");
printf("Please select the matrix size you would like to input: \n");
printf("\t (A): 2x2 matrix\n");
printf("\t (B): 3x3 matrix\n\n");
char selection; //Stores matrix size selection
scanf(" %c", &selection);
int size; //Size of matrix
//Uses selection from user to determine value to assign to 'size'
if (selection == 'A' || selection == 'a'){
size = 2;
}
else if (selection == 'B' || selection == 'b'){
size = 3;
}
else{
printf("Your selection is invalid. Please start over.\n");
return 0;
}
printf("\nYou have selected a %dx%d matrix.\n\n", size, size);
//Initialize pointer array
int* matrix_ptr = (int*) malloc(size * sizeof(int*));
int** matrix = &matrix_ptr;
for (int i = 0; i < size; i++){
matrix[i] = (int*)malloc(size * sizeof(int));
}
readMatrix(matrix, size); //Sets up matrix by taking input from user
int calc = determinant(matrix, size); //Calculates determinant
printf("The %dx%d matrix is: \n\n", size, size);
//Displays the matrix on the console
for (int row = 0; row < size; row++){
for (int col = 0; col < size; col++){
printf("%d\t", matrix[row][col]);
}
printf("\n");
}
//Deletes stored data
for (int i = 0; i < size; i++){
free(matrix[i]);
}
free(matrix);
printf("\nThe determinant of the matrix is: %d\n", calc);
return 0;
}
determinant.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "matrix.h"
#include "determinant.h"
int determinant(int** matrix, int size){
int detm_calc; //Determinant calculation variable
//Determine which formula to use - 2x2 or 3x3 matrix.
if (size == 2){ //2x2 case
int a = matrix[0][0];
int b = matrix[0][1];
int c = matrix[1][0];
int d = matrix[1][1];
detm_calc = (a*d) - (b*c);
}
else{ //3x3 case
int a = matrix[0][0];
int b = matrix[0][1];
int c = matrix[0][2];
int d = matrix[1][0];
int e = matrix[1][1];
int f = matrix[1][2];
int g = matrix[2][0];
int h = matrix[2][1];
int i = matrix[2][2];
detm_calc = a*(e*i - f*h) - b*(d*i - f*g) + c*(d*h - e*g);
}
return detm_calc;
}
determinant.h
#ifndef DETERMINANT_H
#define DETERMINANT_H
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "matrix.h"
int determinant(int**, int);
#endif
matrix.c
#include <stdio.h>
#include <stdlib.h>
#include "matrix.h"
#include "determinant.h"
void readMatrix(int** matrix, int size){
for (int i = 0; i < size; i++){
for (int j = 0; j < size; j++){
printf("Please enter the integer for row %d column %d:\t", i+1, j+1);
scanf("%d", &matrix[i][j]);
}
printf("\n");
}
}
matrix.h
#ifndef MATRIX_H
#define MATRIX_H
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "determinant.h"
void readMatrix(int**, int);
#endif
Makefile
determinant: main.o determinant.o matrix.o
gcc main.o determinant.o matrix.o -o determinant.out
main.o: main.c
gcc -c main.c
determinant.o: determinant.c determinant.h
gcc -c determinant.c
matrix.o: matrix.c matrix.h
gcc -c matrix.c
The code shown above is supposed to create a determinant of a matrix however, it is not processing correctly as it infinitely loops. I assume something is wrong with the Makefile or the translation from C++ to C. Are there any notable errors that I am unable to spot? Thanks!

Answer 1

You are using
int* matrix_ptr = (int*) malloc(size * sizeof(int*));
int** matrix = &matrix_ptr;
Problems with this:
You are initializing matrix_ptr as a pointer to int, but allocating the memory for size*sizeof(int*), as though it would be an array of int*. So, allocated memory will depend on your hardware, and could be a mismatch with int memory size resulting in the wrong size of memory allocation.
int** matrix = &matrix_ptr; causes matrix to point to matrix_ptr, so when you are trying to write into matrix[1] in a loop - you are trying to write into an unallocated memory block, which will result in a segmentation fault.
int** matrix = (int**)malloc(size*sizeof(int*)); instead of these two lines will solve your problem
Note: I do not understand where it loops (I got a segfault with your code) and how the makefile can result in a runtime problem in your situation.