A C++ Program - Looks Simple yet not simple enough to Understand !

Question

I'm having difficult time getting this program. How it prints true two when it shud print true one ? As a = 10, so if(a==a--) condition supposed to be true. how its treating the condition as false ? Someone please have a look at the code. it's just making me mad !

https://code.sololearn.com/c331j9216uhX/?ref=app

Answer 1

Based on your code:

#include <iostream>
using namespace std;

int main()
{
int a=10;
cout << a << a-- << endl;

a=10;
cout << a << --a << endl;

return 0;
}

This should clear things up for you. :>
You see, when a variable exists at different instances on the same line with post/pre-fix operators on them, different compiler does different dank stuff to interpret this shat. We have had a thread about this sometime ago.

E.g. In C++, b = b++; does nothing while Java may interpret it as a net b++.

Answer 2

Decrement operator has higher precedence.

a == a--
First, a-- is executed. It sets "a" to 9 but returns 10. The "a" at the left has the value 9, which is not equal to 10 returned by a-- and that's why first if block doesn't run.

a == --a
First, --a is executed. It sets "a" to 9 and returns 9. The "a" at the left has the value 9, which is equal to 9 returned by --a and that's why second if block runs.

Answer 3

The decrement has the highest priority in your sample.

( a == a--) → ( a == (a--) )
1) a = 10;
2) a == (a--)
3) a == 10 // postfix-- return old value (not a reference) of variable
4) a -= 1
5) 9 == 10
6) false

( a == --a) → ( a == (--a) )
1) a = 10
2) a == (--a)
3) a -= 1
4) a == a // prefix-- return reference to variable
5) 9 == 9
6) true

Answer 4

Thankyou for your valuable replies @JPM7. Those really helped a lot actually.

Answer 5

i can understand that.

Answer 6

So the differences between a-- and --a
The primary value of a-- is 9, because it takes the value of a (which equals 10) and reduces it by one.
The primary value of --a is still 10. It also takes the value of a and reduces it by one, but during the checking if condition it still 10.