A small question about CPP

Question

https://code.sololearn.com/c77aqVQCFsAA/?ref=app

#include <iostream>
#define M(a) ((a)*(a))
using namespace std;

int main() {
int i,j;
for (i=1,j=1;i<=5;i++)
cout<<M(j++)<<endl;
return 0;
}

How the define work in this code piece?
why the answer which provided by the text book turned to be 1 9 25 49 81 ?

But why the process got the results below?
2 12 30 56 90

Thanks alot ( ´▽` )

Answer 1

Yeah, I forgotton to mention as well:

https://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points

Multiple prefix/postfix operators used between two sequence points will cause undefined behavior in older compilers, so the result of:

j++*j++

in itself is compiler dependent. It's not surprising to get results which differ compared to your textbook.

Answer 2

Macros are ways the preprocessor substitute an identifier with another piece of identifier in a code before compilation.

#define M(a) ((a)*(a))

With the above #define directive, whatever is being passed into M(a) as a, will be translated to ((a)*(a)) in its literal sense. Hence, passing

j++

means that the resulting code of M(j++) will turn out to be

((j++)*(j++))

As a result, j is incremented twice per loop iteration.

Answer 3

Let's understand:
Давайте разбираться:

1. #include <iostream>
2. #define M(a) a*a
3. using namespace std;
4.
5. int main() {
6. int i, j;
7. for (i=1, j=1; i<=5; i++)
8. cout<<M(j++)<<endl;
9. return 0;
10. }

Macro M(j++) = M(a)
a*a replaces M(a) =>
=> M(j++) = (j++)*(j++) =>
As a result, we have:
В результате чего мы имеем:

Old compiler:
Старый компилятор:
1) (j=1) * j => cout<<"1" => j=1+1;
2) (j=2+1)*j => cout<<"9" => j=3+1;
3) (j=4+1)*j => cout<<"25" => j=5+1;
4) (j=6+1)*j => cout<<"49" => j=7+1;
5) (j=8+1)*j => cout<<"81" => j=9+1;

New compiler:
Новый компилятор:
1) (j=1) * (j=1+1) => cout<<"2";
2) (j=2+1) * (j=3+1) => cout<<"12";
3) (j=4+1) * (j=5+1) => cout<<"30";
4) (j=6+1) * (j=7+1) => cout<<"56";
5) (j=8+1) * (j=9+1) => cout<<"90";

https://code.sololearn.com/c365ec61RTu0/?ref=app

Answer 4

I had no idea this was compiler dependent.

Answer 5

Fermi Thanks alot! The compiler which used in the text book was too old!

Answer 6

Kiran Bodipati Thanks! I get it