How to iterate over the product of several ranges or iterators? - iterator

Question

Is there a natural way in Rust to iterate over the "product" of several ranges or iterators?
This comes up when you're iterating over a multidimensional array, or perhaps some state space. For instance, I want to consider all possible values of a boolean tuple with 5 elements. Nesting 5 for loops is a bit unwieldy.

Answer 1

Here is a macro that does the job:
macro_rules! product {
($first:ident, $($next:ident),*) => (
$first.iter() $(
.flat_map(|e| std::iter::repeat(e)
.zip($next.iter()))
)*
);
}
fn main() {
let a = ['A', 'B', 'C'];
let b = [1, 4];
let c = [true, false];
let d = ['x', 'y'];
for (((a, b), c), d) in product![a, b, c, d] {
println!("{} {} {} {}", a, b, c, d);
}
}
Output:
A 1 true x
A 1 true y
A 1 false x
A 1 false y
A 4 true x
A 4 true y
etc...
Playpen example
The macro expands to the following
a.iter()
.flat_map(|e| std::iter::repeat(e).zip(b.iter()))
.flat_map(|e| std::iter::repeat(e).zip(c.iter()))
.flat_map(|e| std::iter::repeat(e).zip(d.iter()))
flat_map(|e| ... ) combines a sequence of iterators into an iterator. The e is an element yielded by an iterator.
std::iter::repeat(e) creates an iterator that repeats e.
.zip( ... ) iterates over two iterators simultaneously, yielding the elements of both as a pair.
Macros are a bit longer to explain, so it's best to read the macro chapter in the book

Answer 2

The itertools crate has a very ergonomic macro (iproduct!) for iterating over the product of iterators. Here is an example:
pub fn main() {
let a = ['A', 'B', 'C'];
let b = [1, 4];
let c = [true, false];
let d = ['x', 'y'];
for (a, b, c, d) in itertools::iproduct!(&a, &b, &c, &d) {
println!("{} {} {} {}", a, b, c, d);
}
}