Determinant calculation error

Question

#include<iostream.h>
#include<conio.h>
#include<math.h>

double Det (double a[2][2], int size);
void form(double a[2][2], int c, int size);

double b[2][2];

double Det(double a[2][2], int size) {
  if (size==1) {
    return a[0][0];
  }
  else {
    double ans = 0.0;
    int c;
    for (c=0;c<size;c++) {
      form(a,c,size);
      ans += pow(-1.0,(double)c)*Det(b,size-1);
    }
    return ans;
  }
}

void form(double a[2][2], int c, int size) {
  int i,j=0,k,l=0;
  for (i=0;i<size;i++) {
    for (j=0;j<size;j++) {
      if ((i!=0)&&(j!=c)) {
        if (l==size-1) {
           k++;
           l=0;
        }
        b[k][l]=a[i][j];
        l++;
      }
    }
  }
}

int main() {
    double mat[2][2] = {{1.0,2.0},{3.0,7.0}};
    cout << Det(mat,2);
    getch();
    return 0 ;
}

I am writing a program in C++ to calculate the determinant of a matrix using recursion. I know that there are many better algorithms to do the same. However, I have been asked to implement this one. As a test case, I have used the matrix specified in the code. When I this run this code, I get an answer of -4 instead of 1. I do not understand what is wrong with this code. Could someone please help? Thanks in advance.

That is some crazy indentation. – chris Sep 24 '13 at 16:36 — chris, Sep 24 '13 at 16:36
`iostream.h` ? *How old is your toolchain??* – WhozCraig Sep 24 '13 at 16:58 — WhozCraig, Sep 24 '13 at 16:58

score 0 · Answer 1 · answered Sep 24 '13 at 16:54

you made several mistakes in your code, use the one below

#include<iostream>
#include<math.h>
using namespace std;
double Det (double a[2][2], int size);
void form(double a[2][2], int c, int size);
double b[2][2];
double Det(double a[2][2], int size)
{
  if(size==1)
    return a[0][0];
  else
  {
    double ans=0.0;
    int c;
    for(c=0;c<size;c++)
    {
      form(a,c,size);
      ans+=pow(-1.0,(double)c) * a[0][c] * Det(b,size-1);
    }
    return ans;
  }
}
void form(double a[2][2], int c, int size)
{
  int i,j=0,k = 0,l=0;
  for(i=0;i<size;i++)
  {
    for(j=0;j<size;j++)
    {
      if((i!=0)&&(j!=c))
      {
        if(l==size-1)
        {
           k++;
           l=0;
        }
        b[k][l]=a[i][j];
        l++;
      }
    }
  }
}
int main()
{
    double mat[2][2]={{1.0,2.0},{3.0,7.0}};
    cout<<Det(mat,2);
    return 0 ;
}

This gives you 1. The mistakes, you forget to initialize k to 0, and you forget to multiply a[0][c].

Determinant calculation error

1 Answers1