Question

My implementation of insertion sort seems to be working with the exception of sorting the very first element. I have a small test case here. Can anyone tell me what is wrong with my algorithm?

#include <iostream>
#include <string>
#include <stdlib.h>
using namespace std;



void Insert(int *S, int k)
{
        int key = S[k];
        int j = k-1;
        while(j>0 && S[j] > key)
        {
                S[j+1] = S[j];
                j--;
        }

        S[j+1] = key;
}


void Insertionsort(int S[], int n)
{
        if(n>1)
                Insertionsort(S,n-1);
        Insert(S,n);

}

int main()
{
        srand ( time(NULL) );
        int S1_8[8];
        for(int i=0; i<8; i++)
                S1_8[i] = rand()%100;

        Insertionsort(S1_8,8);

        for(int i=0; i<8; i++)
        {
                cout << S1_8[i] << endl;
        }

        return 0;
}

Answer 1

The first time Insert is called, it is passed int key = S[8];

S[8] is not within array bounds.

Make that

void Insertionsort(int S[], int n)
{
        if(n>1)
                Insertionsort(S,n-1);
        Insert(S,n-1);

}

Also, in your while condition, it must be

while(j>=0 && S[j] > key)

Link to Code