Why does this code have different outputs if pointers are incremented differently c-CodePudding

#include <iostream>
using namespace std;
int main() {
  int num=10;
  int *ptr=NULL;
  ptr=&num;
  num=(*ptr)  ; //it should increase to 11
  num=(*ptr)  ; //it should increase to 12 but im getting 10
                //if i dont initialize num and just use (*ptr)   it gives me 11
  cout<<num<<endl;
    return 0;
}

I want to know why is this happening and why am I getting 10 as output.

CodePudding user response：

why is this happening

Because you are using post-increment operator instead of pre-increment operator.

Replace (*ptr) with:

num =   (*ptr);//uses pre-increment operator

And you will get 12 as output at the end of the program which can be seen here.

Alternative solution

You can also just write (*ptr) ; without doing assignment to num. So in this case code would look like:

int main() {
  int num=10;
  int *ptr=NULL;
  ptr=&num;
  (*ptr)  ; //no need for assignment to num
  (*ptr)  ; //no need for assignment to num
                
  cout<<num<<endl;
    return 0;
}

CodePudding user response：

(*ptr) increases num to 11 but returns its previous value (10), because the is postfix.

So with num = (*ptr) , you are temporarily increasing num to 11, but then (re)assigning it with 10.

CodePudding user response：

It is caused by assigning to num. operator returns old value and then increments. However, then the old value is assigned to num.