How do I put two increment statements in a C++ 'for' loop?

Question

I would like to increment two variables in a for-loop condition instead of one.

So something like:

for (int i = 0; i != 5; ++i and ++j)      do_something(i, j); 

What is the syntax for this?

Answer 1

A common idiom is to use the comma operator which evaluates both operands, and returns the second operand. Thus:

for(int i = 0; i != 5; ++i,++j)      do_something(i,j); 

But is it really a comma operator?

Now having wrote that, a commenter suggested it was actually some special syntactic sugar in the for statement, and not a comma operator at all. I checked that in GCC as follows:

int i=0; int a=5; int x=0;  for(i; i<5; x=i++,a++){     printf("i=%d a=%d x=%d\n",i,a,x); } 

I was expecting x to pick up the original value of a, so it should have displayed 5,6,7.. for x. What I got was this

i=0 a=5 x=0 i=1 a=6 x=0 i=2 a=7 x=1 i=3 a=8 x=2 i=4 a=9 x=3 

However, if I bracketed the expression to force the parser into really seeing a comma operator, I get this

int main(){     int i=0;     int a=5;     int x=0;      for(i=0; i<5; x=(i++,a++)){         printf("i=%d a=%d x=%d\n",i,a,x);     } }  i=0 a=5 x=0 i=1 a=6 x=5 i=2 a=7 x=6 i=3 a=8 x=7 i=4 a=9 x=8 

Initially I thought that this showed it wasn't behaving as a comma operator at all, but as it turns out, this is simply a precedence issue - the comma operator has the lowest possible precedence, so the expression x=i++,a++ is effectively parsed as (x=i++),a++

Thanks for all the comments, it was an interesting learning experience, and I've been using C for many years!