Hello,

I'm working with a simple piece of code in the D language and encountered a result that has left me puzzled. The code snippet is as follows:

import std.stdio:writeln;

void main() {
    int i = 5;
    writeln("Result: ", i + ++i);
}

When I run this, it surprisingly outputs 11. I tried something similar in C before and it gave me 12. I’m curious, why is there a difference? How is i + ++i evaluated in D that it ends up giving 11 instead of 12?

Is there something about operator precedence or evaluation order in D that I'm missing? I'd really appreciate it if someone could break it down for me or point me towards some resources to get a better understanding of what's going on.

Thanks a bunch!

On Friday, 23 August 2024 at 08:58:16 UTC, Me'vâ wrote:

import std.stdio:writeln;

void main() {
    int i = 5;
    writeln("Result: ", i + ++i);
}

D: 5 + 6
C++: undefined, could be 6 + 6 if the increment is done first. g++ gives me a warning with -Wall:

inc.cxx: In function ‘int main()’:
inc.cxx:30:26: warning: operation on ‘i’ may be undefined [-Wsequence-point]
   30 |         std::cout << i + ++i << "\n";
      |                          ^~~

See https://dlang.org/spec/expression.html#order-of-evaluation

So in D, the value of i on the left is always read before the increment on the right.

On Friday, 23 August 2024 at 09:42:38 UTC, Nick Treleaven wrote:

You asked about C, for some reason I used C++. But it's the same in C, and the error happens with gcc -Wall.

On Friday, 23 August 2024 at 09:49:27 UTC, Nick Treleaven wrote:

I think the ++ might’ve infected your thought process.

On Friday, 23 August 2024 at 08:58:16 UTC, Me'vâ wrote:

writeln("Result: ", i + ++i);

I would definitely expect 11 as result (but I still have K&R on my book shelf, maybe I'm a bit biased). So, when you get 12 with C, I would consider that an error.