noob needs help with ++ operator

[khantheprogrammer]

ok, i just need help understanding my compiler's methodology of analyzing this code. i know the "a++" increases the value of 'a' by 1. but i don't know why the following result is displayed. the answer comes out to be "7 6 7 5". what i was expecting was "7 7 7 7". in high school my professor told me that the compiler analyzes it from right to left, and then displays it from left to right. here's the code. i need step-to-step analysis. thanks in advance!

  #include<iostream>
  using namespace std;
  #define endp cin.ignore()
  int main()
  {
	int a=5;
        cout<<a<<endl<<a++<<endl<<a<<endl<<a++;
        endp;
	return 0;
  }

[Disch]

You cannot [safely] modify a variable multiple times in the same statement like that. The behavior is undefined.

Try this:

cout<<a<<endl; // prints 5
cout<<a++<<endl; // prints 5
cout<<a<<endl; // prints 6
cout<<a++; // print 6
// at this point, a==7 

[khantheprogrammer]

thanks for the quick reply. thing is, i am aware that i can use the variable one cout statement at a time. i'm only curious as to why my code outputs what it outputs. any thoughts?

[Disch]

i'm only curious as to why my code outputs what it outputs. any thoughts?

The rules of the C++ language do not explain the output.... the behavior is undefined. Looking for an answer is practically meaningless because the output can vary from compiler to compiler.

You might get "7 7 7 7"... or you might get "7 6 7 5" or you might get "1537 188993 48 0" or the program might crash. Literally anything can happen and still be within bounds of what the C++ language allows.


There is no deeper meaning. The only answer is "it's undefined"

[Chriscpp]

#include<iostream>
using namespace std;
#define endp cin.ignore()
int main()
{
    int a=5;
        cout<<++a<<endl<<a<<endl<<a<<endl<<++a;
        endp;
    return 0;
}

[Disch]

@Chriscpp:

That is also undefined.

If you modify a variable, you cannot read/modify it anywhere else on the same sequence point.

[Codeez]

What Disch said.

SomeBook wrote:

Most operators do not specify the order in which operands are evaluated: The compiler is free to evaluate either the left -or right- hand operand first.

Now you see why using operators that cause side effects in the same statement are not so good, be it increment or decrement.