adding arrays using malloc in c++ - c++

Question

i dont know whats wrong with my code but im getting same value of "sum" on the
screen..
assume that m and n are entered equal ....enter image description here
#include<stdio.h>
#include<malloc.h>
#include<iostream>
#include<stdlib.h>
using namespace std;
int main()
{
int n,m;
int *ptr1, *ptr2, *sum;
cout<<" enter the size of 1st and 2nd array : "<<endl;
cin>>n>>m;
ptr1=(int*)malloc(n*sizeof(int));
ptr2=(int*)malloc(m*sizeof(int));
sum=(int*)malloc((n)*sizeof(int));
cout<<"enter 1st array element :";
for(int i=0;i<n;i++)
{
cin>>*(ptr1+i) ;
}
cout<<"enter 2st array element :";
for(int i=0;i<m;i++)
{
cin>>*(ptr2+i);
}
for(int j=0;j<m||j<n;j++)
{
*(sum+j) = (*(ptr1) + *(ptr2)) ;
}
cout<<" the sum is "<<endl;
for(int j=0;j<m||j<n;j++)
{
cout<<*(sum+j)<<endl;
}
}

Answer 1

First, the reason you get the same number springs from where you form the sums.
In this loop
for (int j = 0; j<m || j<n; j++)
{
*(sum + j) = (*(ptr1)+*(ptr2));
}
you find the sum of the contents of ptr1 and ptr2 over and over which never change - this is always the first two numbers.
So, we could iterate over the arrays by indexing in j along as follows
for (int j = 0; j<m || j<n; j++)
{
*(sum + j) = (*(ptr1 + j) + *(ptr2 + j));
}
BUT what happens if m!=n? You'll walk off the end of the array.
If you change the loop to
for (int j = 0; j<m && j<n; j++)
{
*(sum + j) = (*(ptr1 + j) + *(ptr2 + j));
}
then you find the sum for pairs of numbers up to the smaller of m and n.
You will have to do likewise with the display of the results
for (int j = 0; j<m && j<n; j++)
{
cout << *(sum + j) << endl;
}
However, I believe you wanted to either display n numbers, regardless of which is bigger, or perhaps assume a 0 if there is no element. Also, I notice you have malloced and not freed - perhaps using a C++ array rather than C-style arrays is better? I'll come to that in a moment.
Let's do the C appraoch and have a 0 if we go beyond the end of an array.
This will work, but can be tidied up - comments inline about some important things
#include<stdlib.h>
#include <algorithm> //for std::max
#include <iostream>
using namespace std;
int main()
{
int n, m;
int *ptr1, *ptr2, *sum;
cout << " enter the size of 1st and 2nd array : " << endl;
cin >> n >> m;
ptr1 = (int*)malloc(n * sizeof(int));
ptr2 = (int*)malloc(m * sizeof(int));
sum = (int*)malloc((std::max(n, m)) * sizeof(int));
// ^--- sum big enough for biggest "array"
// data entry as before - omitted for brevity
for (int j = 0; j<m || j<n; j++)
{
*(sum + j) = 0;
if (j < n)
*(sum + j) += *(ptr1 + j);
if (j < m)
*(sum + j) += *(ptr2 + j);
}
cout << " the sum is " << endl;
for (int j = 0; std::max(n, m); j++)//however big it is
{
cout << *(sum + j) << endl;
}
free(ptr1); //tidy up
free(ptr2);
free(sum);
}
I know you said you wanted to use malloc and perhaps this is a practise with pointers, but consider using C++ idioms (at least you won't forget to free things you have maoolced this way).
Let's nudge your code towards using a std::vector:
First the include and the input:
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
int n, m;
vector<int> data1, data2, sum;
cout << " enter the size of 1st and 2nd array : " << endl;
cin >> n >> m;
cout << "enter 1st array element :";
for (int i = 0; i<n; i++)
{
int number;
cin >> number;
data1.push_back(number); //there is a neater way, but start simple
}
cout << "enter 2st array element :";
for (int i = 0; i<m; i++)
{
int number;
cin >> number;
data2.push_back(number);
}
This post shows a way to neaten up the data entry. However, let's do something simple and get the sum:
for (int j = 0; j < std::max(m, n); j++)
{
int number = 0;
if (j < n)
number += data1[j];
if (j < m)
number += data2[j];
sum.push_back(number);
}
And now for a C++ way to do output
cout << " the sum is " << endl;
for (auto item : sum)
{
cout << item << '\n';
}
}
Finally, let's have a brief think about the sum.
If you now #include <iterator> you can use an algorithm to put your sum into sum
std::transform(data1.begin(), data1.end(),
data2.begin(), std::back_inserter(sum), std::plus<int>());
However, note this won't fill with zeros. You could either make the vectors the same size, filled with zeros first, or lookup/discover ways to zip vectors of different sizes. Or stick with ifs in a loop as I demonstrated above.
Avoid malloc in C++. Just saying.

Answer 2

I highly encourage you to use a modern cpp data structure like a vector for storing your data. Thus, you don't have to worry about malloc and can access them far more easyly.
But now to your question: Your summation for loop is broken. Use
for(int j=0;j<m||j<n;j++)
{
*(sum+j) = (*(ptr1+j) + *(ptr2+j)) ;
}
Best regrads, Georg