求一个能够计算N阶行列式结果的程序代码，脑子实在是倒不过来了

#include"stdio.h"
main()
{
    int z,r,s,j,i;
    double a[20][20],m=1.0,k;
    printf("请输入阶数:");
        scanf("%d",&r);
        printf("请输入数字\n");
        for(i=0;i<r;i++)
        for(j=0;j<r;j++)
            scanf("%lf",&a[i][j]);
        for(z=0;z<r-1;z++)
        for(i=z;i<r-1;i++)
        {    if(a[z][z]==0)
                for(i=z;a[z][z]==0;i++)
                {
                    {
                        for(j=0;j<r;j++)
                        a[z][j]=a[z][j]+a[i+1][j];
                    }
                    if(a[z][z]!=0)break;
                }
                    {
                        k=-a[i+1][z]/a[z][z];
                        for(j=z;j<r;j++)
                        a[i+1][j]=k*(a[z][j])+a[i+1][j];
                    }
        }
         for(z=0;z<r;z++)
         m=m*(a[z][z]);
         printf("%f",m);

谢谢指导

给注视一下，不明白。谢谢