Merge Sort Algorithm? - c++

Question

I can't figure out what is wrong with my MergeSort function. This is my code:
void Merge(int* A, int p, int q, int r)
{
int B[100], i = 0, j = 0, k = 0;
i = p;
k = p;
j = q + 1;
while (i <= q && j <= r)
{
if (A[i] <= A[j])
{
B[k] = A[i];
k++;
i++;
}
else
{
B[k] = A[j];
k++;
j++;
}
}
while (i <= q)
{
B[k] = A[i];
k++;
i++;
}
while (j <= r)
{
B[k] = A[j];
k++;
j++;
}
for (i = p; i < r; i++)
{
A[i] = B[i];
}
}
void MergeSort(int A[], int p, int r)
{
int q;
if (p < r)
{
q = (p + r) / 2;
MergeSort(A, p, q);
MergeSort(A, q + 1, r);
Merge(A, p, q, r);
}
}
int main(void)
{
int N[10];
N[0] = 4;
N[1] = 5;
N[2] = 8;
N[3] = 12;
N[4] = 7;
N[5] = 3;
N[6] = 23;
N[7] = 1;
N[8] = 90;
N[9] = 26;
MergeSort(N, 0, 9);
for (int i = 0; i < 10; i++)
{
cout << N[i] << endl;
}
}
The output of program is: 1, 3, 1, 4, 5, 7, 7, 7, 26, 26, which is obviously wrong. However I just don't see what is wrong in code, to me everthing looks good. I googled some C++ codes of MargeSort and try to debug it but i can't find mistake. Anyone see it?

Answer 1

//you have just one err,in last for you missed = :
//err: for(i = p; i < r; i++)
//correct: for(i = p; i <= r; i++)
#include <iostream>
using namespace std;
void Merge(int* A, int p, int q, int r)
{
int B[100], i = 0, j = 0, k = 0;
i = p;
k = p;
j = q + 1;
while (i <= q && j <= r)
{
if (A[i] <= A[j])
{
B[k] = A[i];
k++;
i++;
}
else
{
B[k] = A[j];
k++;
j++;
}
}
while (i <= q)
{
B[k] = A[i];
k++;
i++;
}
while (j <= r)
{
B[k] = A[j];
k++;
j++;
}
for (i = p; i <= r; i++)
{
A[i] = B[i];
}
}
void MergeSort(int A[], int p, int r)
{
int q;
if (p < r)
{
q = (p + r) / 2;
MergeSort(A, p, q);
MergeSort(A, q + 1, r);
Merge(A, p, q, r);
}
}
int main(void)
{
int A[10]={9,8,7,6,5,4,3,2,1,0};
MergeSort(A,0,9);
for (int i = 0; i<10; i++)
{
cout<<A[i]<<",";
}
}
//this is cleaned version of your code:
#include <iostream>
using namespace std;
void Merge(int* A, int p, int q, int r)
{
int* B=new int[r-p+1];
int i = p;
int j = q + 1;
int k = 0;
while (i <= q && j <= r) B[k++] = (A[i] <= A[j])? A[i++] : A[j++];
while (i <= q) B[k++] = A[i++];
while (j <= r) B[k++] = A[j++];
for (i = p; i <= r; i++) A[i] = B[i-p];
delete B;
}
void MergeSort(int* A, int p, int r)
{
if (p >= r) return;
int q = (p + r) / 2;
MergeSort(A, p, q);
MergeSort(A, q + 1, r);
Merge(A, p, q, r);
}
int main(void)
{
int A[15]={10,11,12,13,14,0,8,7,6,5,4,3,2,1,9};
MergeSort(A,0,14);
for (int i = 0; i<15; i++) cout<<A[i]<<",";
}
//this is another answer:
#include <iostream>
#include <climits>
using namespace std;
static void Merge(int* A, int p, int q, int r)
{
int n1 = q - p + 1;// A[p..q]
int n2 = r - q;// A[q+1..r]
int* L = new int[n1 + 1];
int* R = new int[n2 + 1];
int i, j;
for (i = 0; i < n1; i++)
L[i] = A[p + i];
for (i = 0; i < n2; i++)
R[i] = A[q + i+1];
L[n1] = INT_MAX;
R[n2] = INT_MAX;
i = 0;
j = 0;
for (int k = p; k <= r; k++)
A[k] = L[i] <= R[j] ? L[i++] : R[j++];
delete L;
delete R;
}
static void Ascending(int* A, int p, int r)
{
if (p >= r) return;
int q = (p + r) / 2;
Ascending(A, p, q);
Ascending(A, q + 1, r);
Merge(A, p, q, r);
}
int main()
{
int A[10]={9,8,7,6,5,4,3,2,1,0};
Ascending(A,0,9);
for (int i = 0; i<10; i++)
{
cout<<A[i]<<",";
}
}