The One-Line Trick for Iterating Backwards in C That Almost Everyone Gets Wrong
A size_t index counting down to zero looks obvious but breaks: i >= 0 is always true, and an empty buffer wraps i to SIZE_MAX. Switching to ssize_t works but drags in signed-conversion traps, a POSIX-only type, and promotion issues on 32-bit. The clean fix is `for(size_t i = count; i--; )`, relying on post-decrement and well-defined unsigned wraparound to keep i in bounds.
So much simpler! Let's break it down: The key is the postdecrement operator on i.
- kenferry
I know it's "too clever", but if I'm writing code no one else will ever see,
while (i --> 0) {
// use array[i]
}
easy to read (if not understand), and correct!
- Joker_vD
That's the price of innovation, I guess; before C invented its "for"-loop, other languages had their equivalents generally looking like this:
FOR var = start [DOWN] TO end BY step DO ... END
No custom increment expressions, and "step" even usually was only allowed to be a constant (in which case, the language usually didn't have DOWN specifier and used the step's sign instead). Then Dennis Ritchie invented C, and ditched the traditional "do"-loop (Fortran started that construct with DO, so it was called DO loop; Algol used FOR instead, but most people preferred the Fortran's nomenclature... until C came to dominate, but that wouldn't happen for yet another 20 years) in favour of his invention because he felt that easily iterating through linked lists or strings was a much more pressing concern:
for(p=head; p; p=p->next) { ... }
for(s=str; *s; s++) { ... }
The downside, of course, is that now you can't straightforwardly write
for size_t i = count - 1 downto 0 do
...
end
- bugake
For the uninformed: counting backwards in a for loop is faster because comparing an unsigned integer against 0 is generally faster than comparing it against any other value.
It also says not to use `for (i >= 0)` because any unsigned integer is >= 0, and the loop will never end.
- kccqzy
This is why I prefer to use C++ and its reversed iterators. The rbegin and rend accessors are there in the containers from the beginning. It’s less error prone than the C version using indices. (Of course some situations require me to use indices, so I do use the i-->0 trick, but it is much rarer.)
- randomblock1
I thought most people used the explicit version:
for (size_t i = count; i > 0; --i) {
// use array[i - 1]
}
You can also do something like:
size_t index = i - 1;
to not have to use i-1. I believe the compiler optimizes that out anyway to be identical.