why the below merge sort code doesnt work? - c++

Question

What should be changed in the below code? It shows the following error
ERROR: ld.so: object '/home/bot/funcs.so' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Error in `/home/bot/dcb7b4b8f54571a4467c2113b7856878': free(): invalid next size (fast): 0x0000000000c350b0
#include <iostream>
using namespace std;
int merge(int *left,int nl,int *right,int nr,int *a)
{
int i=0,j=0,k=0;
while(i<nl && j<nr)
{
if(left[i]<right[j])
{
a[k]=left[i];
i++;
}
else
{
a[k]=right[j];
j++;
}
k++;
}
while(i<nl)
{
a[k]=left[i];
i++;
}
while(j<nr)
{
a[k]=right[j];
j++;
}
}
int mergesort(int *a,int n)
{
if(n<2) return 0;
int mid=n/2;
int *left=new int[mid];
int *right=new int[n-mid];
for(int i=0;i<=mid-1;i++)
{
left[i]=a[i];
}
for(int i=mid;i<=n-1;i++)
{
right[i]=a[i];
}
mergesort(left,mid);
mergesort(right,n-mid);
merge(left,mid,right,n-mid,a);
delete[]left;
delete[]right;
}
int main() {
//code
int a[]={10,7,8,9,4,2,3,6,5,1};
int n=sizeof(a)/sizeof(a[0]);
mergesort(a,n);
for(int i=0;i<10;i++)
{
cout<<a[i]<<"\t";
}
return 0;
}

Answer 1

Fixes noted in comments, seems to be working now. A one time allocation of a temp array either in main or in a helper function would be faster. Using mutually recursive functions (one merges AtoA, the other merges AtoTemp, they call each other), eliminates having to copy data. I can post an example later if wanted. Bottom up merge sort would be slightly faster.
#include <iostream>
using namespace std;
// fix return type to void
void merge(int *left,int nl,int *right,int nr,int *a)
{
int i=0,j=0,k=0;
while(i<nl && j<nr)
{
if(left[i]<right[j])
{
a[k]=left[i];
i++;
}
else
{
a[k]=right[j];
j++;
}
k++;
}
while(i<nl)
{
a[k]=left[i];
i++;
k++; // fix
}
while(j<nr)
{
a[k]=right[j];
j++;
k++; // fix
}
}
// fix return type to void
void mergesort(int *a,int n)
{
if(n<2) return; // fix: change return type to void
int mid=n/2;
int *left=new int[mid];
int *right=new int[n-mid];
for(int i=0;i<=mid-1;i++)
{
left[i]=a[i];
}
for(int i=mid;i<=n-1;i++)
{
right[i-mid]=a[i]; // fix
}
mergesort(left,mid);
mergesort(right,n-mid);
merge(left,mid,right,n-mid,a);
delete[]left;
delete[]right;
}
int main() {
//code
int a[]={10,7,8,9,4,2,3,6,5,1};
int n=sizeof(a)/sizeof(a[0]);
mergesort(a,n);
for(int i=0;i<10;i++)
{
cout<<a[i]<<"\t";
}
cout << endl; // added this not needed
return 0;
}
Cleanup, changed output to not use tabs:
#include <iostream>
#include <iomanip> // for std::setw()
using namespace std;
void merge(int *left,int nl,int *right,int nr,int *a)
{
int i=0,j=0,k=0;
while(i<nl && j<nr)
{
if(left[i]<right[j])
a[k++]=left[i++];
else
a[k++]=right[j++];
}
while(i<nl)
a[k++]=left[i++];
while(j<nr)
a[k++]=right[j++];
}
void mergesort(int *a,int n)
{
if(n<2)
return;
int mid=n/2;
int *left=new int[mid];
int *right=new int[n-mid];
for(int i=0;i<=mid-1;i++)
left[i]=a[i];
for(int i=mid;i<=n-1;i++)
right[i-mid]=a[i];
mergesort(left,mid);
mergesort(right,n-mid);
merge(left,mid,right,n-mid,a);
delete[]left;
delete[]right;
}
int main() {
int a[]={10,7,8,9,4,2,3,6,5,1};
int n=sizeof(a)/sizeof(a[0]);
mergesort(a,n);
for(int i=0;i<10;i++)
cout<<setw(2)<<a[i]<<" "; // 2 digit field output
cout << endl;
return 0;
}

Answer 2

You should use std::vector instead of raw pointer:
typedef std::vector<int> ivector;
void merge( const ivector &l, const ivector &r, ivector &to )
{
ivector_const::iterator il = l.begin();
ivector_const::iterator ir = r.begin();
ivector::iterator ito = to.begin();
while( true ) {
if( il == l.end() ) {
if( ir == r.end() )
return;
*ito++ = *ir++;
} else {
if( ir == r.end() || *il < *ir )
*ito++ = *il++;
else
*ito++ = *ir++;
}
}
}
void mergesort( ivector &v )
{
if(v.size()<2) return;
ivector::iterator imid = v.begin() + v.size() / 2;
ivector left( v.begin(), imid );
ivector right( imid, v.end() );
mergesort(left);
mergesort(right);
merge( left, right, v );
}
int main() {
//code
ivector a ={10,7,8,9,4,2,3,6,5,1};
mergesort(a);
for(size_t i=0;i<a.size();i++)
{
cout<<a[i]<<"\t";
}
return 0;
}
Note: I did not validate your algorithm just rewrote it with std::vector instead of raw pointer. You can notice how simpler your function become when you use proper data type.