boolean function call within if statement

Question

I am a bit confused regarding my program below. In my if statement, I am calling the boolean function twice. For the i, and j variable I spicked, the "else" statement should be executed as both function calls will yield true. But my "i" variable is not being modified (it outputs as 1001 instead of 1000), and I have no idea why. The j variable is being modified as expected. The first function call passes in "j" variable, and the second function call passes in "i" variable. Could someone explain why the "i" variable is not being modified?

#include <iostream>
using namespace std;

const int MINCOLOR = 0;
const int MAXCOLOR = 1000;

bool clipColor(int &amountColor);


int main()
{
    int i=1001;
    int j = 3333;
    int k;
    bool check;

    if (clipColor(j) == false && clipColor(i) == false)
    {
        check = false;
    }
    else 
    {
        check = true;
    }

    cout << i << " " << j << " " << check << " " << endl;
    return 0;
}


bool clipColor(int &amountColor)       
{
  if (amountColor > MAXCOLOR)
  {
    amountColor = MAXCOLOR;
    return true;
  }
  else if (amountColor < MINCOLOR)
  {
    amountColor = MINCOLOR;
    return true;
  }
  else
  {
    return false;
  }
}

Dolda2000 · Accepted Answer · 2016-02-14 05:17:38Z

1

This is because of how the && and || operators work. For &&, if the first operand is false, then the second operand is never even evaluated. Therefore, the second function call is not made.

This is known as short-circuit evaluation, and is a feature not only of C and C++, but of most programming languages.

If you explicitly want to evaluate both sides, then for bool returns, the single & operator arguably does what you want. By specification, it does bit-wise AND, but since bool values only use exactly one bit, the result should be exactly what you need.

By the way, clipColor(j) == false is the same as !clipColor(j).

edited Feb 14, 2016 at 5:17

answered Feb 14, 2016 at 5:13

Dolda2000

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13 Comments

πάντα ῥεῖ Over a year ago

I'm pretty sure we have a dupe for that question.

roulette01 Over a year ago

Is there any way to make it so the second function call is evaluated though?

roulette01 Over a year ago

Thanks guys. I just wanted to condense some code. I guess I need to perform the two function calls before the if statement and assign them to 2 separate variables and then check the 2 variables instead.

roulette01 Over a year ago

Thanks Dolda2000. I'll try that.

roulette01 Over a year ago

Is using the single "&" to do what I wanted to do considered bad practice? Since we usually don't see single ampersands in conditions.

|

K'Prime · Accepted Answer · 2016-02-14 05:27:24Z

0

In your code

if (clipColor(j) == false && clipColor(i) == false)
{
    check = false;
}
else 
{
    check = true;
}

The function clipcolor(j) is executing first and returning true. As a result there is no need for clipColor(i) to execute because whatever value clipColor(i) returns, the condition (clipColor(j) == false && clipColor(i) == false) will always be false.

answered Feb 14, 2016 at 5:27

K'Prime

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