How can I change the non-rectangular loops to rectangular

As I know that all loops must be rectangular, but in my code, the loop is non-rectangular,how can I change it to rectangular loop, can you advise me?

My code is as below:

for(i=0; i<n-1; i++)
        {
        
                for(j=i+1; j<n; j++)
                {
                        for(k=0,sum=0; k<n; k++)
                        {
                                if(k != i)
                                {
                                   sum += l[j][k]*u[k][i];
                                }
                        }
                        l[j][i] = (float)((a[j][i]-sum)/u[i][i]);
                }

                for(j=i+1; j<n; j++)
                {
                        for(k=0,sum=0; k<n; k++)
                        {
                                if(k != i+1)
                                {
                                  sum += l[i+1][k]*u[k][j];
                                }
                        }
                        u[i+1][j] = (float)((a[i+1][j]-sum));
                }
        }

Make it rectangular then add an if statement to ignore part some of the iterations.

for(i=0; i<n-1; i++) 
        { 
                for(j=0; j<n; j++) 
                { 
                    if (j > i) {
                        for(k=0,sum=0; k<n; k++) 
                        { 
                                if(k != i) 
                                { 
                                   sum += l[j][k]*u[k][i]; 
                                } 
                        } 
                        l[j][i] = (float)((a[j][i]-sum)/u[i][i]);
                     } 
                } 

                for(j=0; j<n; j++) 
                {
                    if (j > i) { 
                        for(k=0,sum=0; k<n; k++) 
                        { 
                                if(k != i+1) 
                                { 
                                  sum += l[i+1][k]*u[k][j]; 
                                } 
                        } 
                        u[i+1][j] = (float)((a[i+1][j]-sum)); 
                    }
                } 
        }

Hope this helps,
Mat