poiter returning wierd number

**Jin-Ting Lyu** · 06-22-2016

I did a exercise,but got a wierd result.
Here is the question:
Write a function that takes two input arguments and provides two separate results to the caller, one that is the result of multiplying the two arguments, the other the result of adding them. Since you can directly return only one value from a function, you'll need the second value to be returned through a pointer or reference parameter.
Here is my solution:

Code:

#include <iostream>

using namespace std;


int calculate(int x,int y,int *sum)
{
    *sum = x + y;


    return x*y;
}
int main()
{
    int num1,num2,sum;
    cout<<"enter number 1:";
    cin>>num1;
    cout<<"enter number 2:";
    cin>>num2;
    cout<<calculate(num1,num2,&sum)<<endl<<sum;
    return 0;
}

If I put num1=4 and num2=5,It would cout 20 .
4309744
Where is 4309744 coming from?
Is that address of variable sum? But I think my sytanx is right.
Any help would be appreciated.

**GReaper** · 06-22-2016

You have experienced undefined behavior. In a nutshell, don't use it like that. Never should you change a variable that is used later( or earlier ) inside the same cout statement.

**Elkvis** · 06-22-2016

The main problem is that sum is uninitialized, and in this case, gets evaluated before calculate(...), so it's printing the random garbage value that it just happens to have, before you assign anything to it.

**sean_cantab** · 06-22-2016

To the OP: your calculate() has 3 arguments but you're required to use 2. One way is to use pointers to capture the return values of sum and product of the 2 integers and then pass them to console in the following manner:

Code:

#include<iostream>
#include<cstdlib>
using namespace std;


void multi_return(int x, int y);


int main(){
    int x, y; 
    cout<<"Enter 2 numbers: "<<endl;
    while(cin>>x>>y){
        multi_return(x, y);
    }
}
void multi_return(int x, int y){
    int * p = (int*)malloc(sizeof(int)*2);
    p[0]=x + y;
    p[1]=x*y;
    cout<<"Sum of the numbers is: "<<p[0]<<endl;
    cout<<"Product of the numbers is: "<<p[1]<<endl;
}

If you search using something like "c++ 2 return values from function" you'll find other ways of doing this like using structure, templates, etc.

**Elkvis** · 06-22-2016

Originally Posted by sean_cantab

To the OP: your calculate() has 3 arguments but you're required to use 2.

Incorrect. The requirement is that the function accept two input arguments. That does not implicitly mean two arguments, in total. It could be interpreted to mean that the output parameter should be in addition to the input parameters.

**whiteflags** · 06-22-2016

Your code relies on a particular order of evaluation.

The compiler has opted to insert sum before it was changed by calculate(), which it is allowed to do. See Sequence point - Wikipedia, the free encyclopedia for examples of the behavior.

There are a number of fixes, including this one:

Code:

cout<<calculate(num1,num2,&sum)<<endl;
cout<<sum<<endl;

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