Document updating an element of a 2D array

[tcNickolas]

At Copy-and-Update Expressions page, we have examples of updating UDTs and 1D arrays, but not of 2D arrays. It is really hard for a new user to figure out the syntax for 2D arrays, so an example would help a lot.

[cgranade]

I think part of what may make this confusing is that we don't have 2D arrays in Q# currently (indeed, this issue is great feedback on #39, as one of the motivations for that QEP is to reduce confusion in representing multidimensional data). Thus, at the moment, the best option available for representing multidimensional data is to use an array of arrays, e.g.:

mutable identity = ConstantArray(3, ConstantArray(3, 0));
for idx in 0..2 {
    // Use w/= to replace the `idx`th row.
    set identity w/= idx <-
        // We want the `idx`th row to be identical to its original value,
        // except in the `idx`th column. That re
        (identity[0] w/ idx <- 1);
}

Hopefully once QEP 3 is implemented, that will look a bit simpler by allowing replacement of elements directly without having to worry about jaggedness of nested arrays:

let identity = [| 0, size = (3, 3) |]
    w/ (0, 0) <- 1
    w/ (1, 1) <- 1
    w/ (2, 2) <- 1;

[TonyHoldroyd]

Thank you for that, very helpful. And I'll look forward to QEP 3