How to increment takes place in C++ ?

Question

1.00/5 (1 vote)

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#include<iostream>
    using namespace std;

    void sum(int a,int b,int &c)
    {
        c = a++ + b++;
    }
    int main()
    {
        int a = 10, b= 20,c;

        sum(a,b,c);
        cout<<a<<"  "<<b<<" "<<c<<endl;
        return 0;
    }

What I have tried:

value of c should have been assigned as 33 but instead it's just 30
Please help to understand this

Posted 19-Aug-19 6:53am

Sam Bown

Updated 19-Aug-19 15:52pm

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4 solutions

Solution 1

There'a a bunch of issues here, but the answer being wrong is because those postfix ++ operators are executed AFTER the + expression is evaluated in your sum method.

So it's adding 10 and 20 together and setting the result to 30, THEN it increments the values of a and b. But, since a and b are passed "by value" and not "by reference" like c is, the values of a and b in your main method will always be 10 and 20.

Posted 19-Aug-19 7:07am

Dave Kreskowiak

Solution 3

Quote:
value of c should have been assigned as 33 but instead it's just 30

30 is, the correct result for this code!
Your code means:

C++

void sum(int a,int b,int &c)
{
    //c = a++ + b++;
    c = a + b; // 30 is the correct result!
    a = a + 1;
    b = b + 1;
}

which is post increment.
If you use pre increment, the correct result is 32:

C++

void sum(int a,int b,int &c)
{
    //c = ++a + ++b;
    a = a + 1;
    b = b + 1;
    c = a + b; // 32 is the correct result!
}

Posted 19-Aug-19 8:27am

Patrice T

Solution 4

Try:

c = (a++) + (b++);

I think.

Posted 19-Aug-19 15:52pm

Ron Anders

Comments

Richard MacCutchan 20-Aug-19 3:57am

Try it.

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OriginalGriff · Accepted Answer · 2019-08-19T08:03:00

Solution 2

Have a look here: Why does x = ++x + x++ give me the wrong answer?[^] - it explains what is happening, even if you aren't doing really silly things with pre- and post- increment.

Posted 19-Aug-19 8:03am

OriginalGriff