Why isn't the ++y part executing?

Question

So here is the question I am given , I need to tell the output :

#include <iostream>
using namespace std;
int main()
{
    int x = 10;
    int y = 20;
    if(x++ > 10 && ++y > 20 ){
    cout << "Inside if ";
    } else{
    cout << "Inside else ";
    }
    cout << x << “ “ << y;
}

The ans given is Inside else 11 20 I checked with complier this is the correct answer but according to me the answer should be Inside else 11 21.

Why is this happening ? Why isn't the ++y part executing ?

I also tried y++ I still get same answer.

Short circuit evaluation. That is how both && and || are required to work in C++. — Avi Berger, Jan 19 '23 at 21:38
***Why isn't the ++y part executing?*** Because (x++ > 10) is not true — drescherjm, Jan 19 '23 at 21:39
Because `++x` increments before the comparison and not after. `x++` increments after. (++x > 10) is evaluated as (11 > 10) while (x++ > 10) is evaluated as (10 > 10) — drescherjm, Jan 19 '23 at 21:42
"if I change it to ++x why does it work" because `x++ > 10` is false but `++x > 10` is true — Slava, Jan 19 '23 at 21:43

score 7 · Answer 1 · answered Jan 19 '23 at 21:38

7

When you write x++, that means two things:

return current value
increment x

Since current value is 10 and 10 > 10 is false, the part after &&, including ++y, is not evaluated.

An alternative would be prefix increment, i.e., ++x.

answered Jan 19 '23 at 21:38

lorro

10,687
23
36

score 4 · Accepted Answer · edited Jan 19 '23 at 22:06

If the first operand of the logical AND (&&) operator evaluates to false, the second operand is not evaluated, because the value of the expression is already known.

From the C++ 20 Standard (7.6.14 Logical AND operator)

1 The && operator groups left-to-right. The operands are both contextually converted to bool (7.3). The result is true if both operands are true and false otherwise. Unlike &, && guarantees left-to-right evaluation: the second operand is not evaluated if the first operand is false.

Also, the value of an expression with the post-increment operator is the value of its operand before incrementing.

From the C++ 20 Standard (7.6.1.6 Increment and decrement_

1 The value of a postfix ++ expression is the value of its operand...

So, in this if statement:

if(x++ > 10 && ++y > 20 ){

the left operand of the logical AND operator x++ > 10 evaluates to false. However, the side effect of the post-increment operator is applied to the variable x. The second operand ++y > 20 is not evaluated.

So, the control will be passed to the else statement, and within its sub-statement x will be equal to 11 and y will keep its original value 20.

Why isn't the ++y part executing?

2 Answers2