A simple question on Conditional Operator

[qmzh85]

Hi guys:

I have a fairly simple question regarding the conditional operator.

For a conditional operator, the definition is given as:

condition ? expression 1 : expression 2

which means if condition is true, then execute expression 1, otherwise execute expression 2. It's easy to understand.

I wrote a simple program to find elements in a vector<int> that have odd value and double the value of each such element. (From C++ primer, 5th ed, Exercise 4.21).

Below is my code:

#include <iostream>
#include <vector>

using std::cin;     using std::cout;    using std::endl;    using std::vector;

int main(void)
{
    vector<int> vec;
    int element;

    cout << "Enter the values for the vector, EOF to quit:" << endl;

    while (cin >> element)
        vec.push_back(element);

    for (auto iter = vec.begin(); iter != vec.end(); ++iter) {
        (*iter) = (*iter) % 2 ? 2 * (*iter) : (*iter);
        cout << *iter << " ";
    }

    return 0;
}

The above code works correctly. If I input 1, 2, 3, 4, 5 it will yield 2, 2, 6, 4, 10.

But why can't I even simplify line 17, just write (*iter) % 2 ? 2 * (*iter) : (*iter);? In this way, the odd value will not be doubled. The vector remains unchanged.

My understanding is: I test (*iter) % 2, if it is true, then it indicates that (*iter) is odd and double it, otherwise keep (*iter) unchanged. Why I have to use assign operator = to make the code work?

I am a newbie in C++, just want to clarify each concept I have encountered.

Thanks.

[Cubbi]

The expression 2 * (*iter) calculates and *returns* the new value (double whatever iter is pointing to). Unless you store that result somewhere, as you do now with (*iter) =, that result is simply lost.

You could double what iter points to with (*iter) *= 2 (one of the very many ways to do that)

[qmzh85]

Thanks, Cubbl.

I originally thought that 2 * (*iter) will automatically change the element pointed by iter but apparently it's not.