Constructor issue <Unable to read memory> - c++

Question

I have to create a class Histogram and make operations on this class. The input can be one dimensional array or a two dimensional array. The problem appears when i convert the array into a matrix. This what i have tried so far. The error is <Unable to read memory>
histrogram.h
#ifndef HISTOGRAM_H
#define HISTOGRAM_H
#include<iostream>
class Histogram
{
private:
int** matrix;
int lines;
void SortMatrix();
public:
Histogram(){ }
Histogram(int elements[], int elementsNr);
Histogram(int** m, int l);
void Print();
};
#endif
historgram.cpp
#include"histogram.h"
using namespace std;
Histogram::Histogram(int** m, int l)
{
matrix=m;
lines=l;
SortMatrix();
}
Histogram::Histogram(int elements[], int elementsNr)
{
lines=0;
//initialize matrix : elementrNr lines and 2 columns
int** matrix=new int*[elementsNr];
for(int i=0;i<elementsNr;i++)
{
matrix[i]=new int[2];
matrix[i][0]=INT_MIN;
matrix[i][1]=INT_MIN;
}
//search each element from the array in the matrix
bool found=false;
for(int i=0;i<elementsNr;i++)
{
found=false;
for(int j=0;j<elementsNr;j++)
{
//the element was found in the matrix ( on the first column )
if(matrix[j][0] == elements[i])
{
matrix[j][1]++;
found=true;
break;
}
}
if(!found)
{
matrix[lines][0]=elements[i];
matrix[lines][1]=1;
lines++;
}
}
SortMatrix();
}
void Histogram::SortMatrix()
{
bool flag=true;
int temp;
for(int i=0;(i<lines) && flag;i++)
{
flag=false;
if(matrix[i+1][0]>matrix[i][0])
{
temp=matrix[i][0];
matrix[i][0]=matrix[i+1][0];
matrix[i+1][0]=temp;
flag=true;
}
}
}
void Histogram::Print()
{
for(int i=0;i<lines;i++)
{
cout<<matrix[i][0]<<" : " <<matrix[i][1]<<endl;
}
}
main.cpp
#include"histogram.h"
#include<iostream>
using namespace std;
int main()
{
int arr[]={6,7,3,1,3,2,4,4,7,5,1,1,5,6,6,4,5};
Histogram h(arr,17);
h.Print();
}

Answer 1

Here
int** matrix=new int*[elementsNr];
replace with
matrix=new int*[elementsNr];
becausematrix is already a member variable. You are creating a new temporary variable double pointer named matrix and allocating memory to it rather than your member variable matrix

Answer 2

A couple of people have already given you advice about how to fix some of the problems with this code. I'll give slightly different advice that may initially seem a bit brutal by comparison, but I'll try to demonstrate how it's honestly useful rather than nasty.
I would throw out your existing code with the possible exception of what you have in main, and start over, using an std::map. What you're doing right now is basically trying to re-create the capabilities that std::map already provides (and even when your code is fixed, it's not doing the job as well as std::map does right out of the box).
Using map, your whole program comes out to something like this:
std::ostream &operator<<(std::ostream &os, std::pair<int, int> const &d) {
return os << d.first << " : " << d.second;
}
int main() {
std::map<int, int> h;
for (int i=0; i<17; i++)
++h[arr[i]];
std::copy(h.begin(), h.end(),
std::ostream_iterator<std::pair<int, int> >(std::cout, "\n"));
return 0;
}
If you want to maintain virtually the same interface as your histogram class provided, it's pretty easy to do that -- the for loop goes into the constructor, the copy into print (and SortMatrix disappears, because a map is always sorted).
By doing this, you change from an O(N2) algorithm to an O(N log N) algorithm. The bugs others have pointed out disappear completely, because the code that contained them is no longer needed. The only real disadvantage I can see is that the result will probably use a bit more memory -- it uses a balanced tree with individually allocated nodes, which is likely to introduce a fair amount of overhead for nodes that only contain 2 ints (and a bit for balancing). I can't quite imagine worrying about this though -- long before you have enough nodes for the memory usage to become significant, you have way too many to present to even consider presenting to the user.

Answer 3

#mathematician1975 already provided an answer for the main problem. There's another bug in SortMatrix(): you only swap the elements of the first column, therefore after sorting, the counts (in the second column) will not be correct anymore. You'll have to insert
temp=matrix[i][1];
matrix[i][1]=matrix[i+1][1];
matrix[i+1][1]=temp;
to get it working.