On Tuesday, 9 October 2018 at 10:52:47 UTC, Chris Katko wrote:
I have a 2-D array:
1 - Is there a way to foreach vertically through that? (that
is, rotated 90 degrees from the usual iteration.)
2 - Is there a way to send that "vertical slice" to a function?
Not built-in in the language, no.
I'm working on an RLE encoding function and I need both
vertical and horizontal strips. It just occurred to me that the
problem is essentially the same, if you can "transpose" the
matrix.
It may be the same on paper, but not in memory.
Oh geez, did I just realize what I need? some sort of matrix
transpose function? I found this post;
The problem is though, even if I use [][], instead of [5][5]
(or try using his conversion function), I can call
transposed... but I can't call my function with it!
void run_rle3(int [] a){/*...*/}
run_rle3(data[0]); //works
run_rle3(data.transposed[0]); //nope [see error below]
run_rle3(data[0].transposed); //nope (didn't expect this to)
run_rle3((data.transposed)[0]); //nope [see error below]
Error: function dmap.run_rle3 (int[] a) is not callable using
argument types (Transversal!(int[][], cast(TransverseOptions)0))
That's because your `run_rle3` function takes an `int[]`, which
is a contiguous slice of ints. `transposed` does not return an
`int[]`, it returns a lazy range (i.e. a struct with a range
interface). Moreover, the range that `transposed` returns isn't
even random-access, even though the source may be.
If you don't need random access and are just iterating over all
elements, you can just change the signature of `run_rle3` to:
void run_rle3(R)(auto ref R a) { /* ... */ }
However, I'd first think hard about doing this. Iterating
"vertically" you essentially skip (N-1) elements on each
iteration, elements that would still be pulled into the CPU cache
only to be ignored. If you're going to be iterating over every
column this way, it may be more efficient to just eagerly
transpose the whole array and use it as an array without any
extra "transposed" abstractions.
