Insertion Sort Algorithm in C

Question

I have a question about the following code for Insertion Sort:

void insertion(Item a[], int ell, int r)                
{               
    int i;          
        for (i=ell+1; i<=r; i++)            
           compexch(a[ell], a[i]);          
    {           
        for (i=ell+2; i<=r; i++)        
        {       
            int j=i;    
            Item v=a[i];    
            while(less (v, a[j-1])) 
            {   
                a[j]=a[j-1];
                j--;
            }   
            a[j]=v;
        }       

    }           
}

Ok, so my question is specifically about the while loop portion-I see that j is decremented and want to know what happens when j=0 and the a[-1] occurs. I do not understand how we can allow a negative index-what if the information we compare happens to work out and the while loops continues to run? Thanks.

Answer 1

I assume compexch(x,y) does something like if (less(y,x)) { Item t = x; x=y; y=t } if (less(y,x)) { Item t = x; x=y; y=t } if (less(y,x)) { Item t = x; x=y; y=t } . So after the first for loop finishes, then a[ell] contains the least Item out of a[ell+1],...,a[r] . Now j is initialized with the value i , which is at least ell+2 , so we have j > ell on entry to the while loop. If the while loop doesn't terminate sooner, we will eventually get down to j == ell . Since a[ell] has already been set to the least element in the range, less(v, a[ell]) will necessarily return false, and the loop will terminate then.

So j will never decrease to a value less than ell .