## digitalmars.D.learn - How do you iterate "vertically" over a 2-D array?

• Chris Katko (33/33) Oct 09 2018 I have a 2-D array:
• Chris Katko (8/42) Oct 09 2018 So according to this:
• Stanislav Blinov (18/38) Oct 09 2018 It may be the same on paper, but not in memory.
• jmh530 (6/19) Oct 09 2018 mir has a lot of functionality that can be helpful here, byDim
```I have a 2-D array:

int data =
[
[1, 0, 1, 0, 0],
[1, 0, 1, 0, 0],
[1, 0, 1, 1, 1],
[1, 0, 0, 1, 0],
[1, 1, 1, 1, 0]
];

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?

int count_numbers(){/*...*/}

count_numbers(data); //horizontal sum of array[0..\$-1]
WORKS.
count_numbers(data[]); //vertical sum of array[0..\$-1]
HYPOTHETICAL

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.

Oh geez, did I just realize what I need? some sort of matrix
transpose function? I found this post;

https://forum.dlang.org/post/na8ikk\$2ojo\$1 digitalmars.com

The problem is though, even if I use [][], instead of  (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); //works
run_rle3(data.transposed); //nope [see error below]
run_rle3(data.transposed); //nope (didn't expect this to)
run_rle3((data.transposed)); //nope [see error below]

Error: function dmap.run_rle3 (int[] a) is not callable using
argument types (Transversal!(int[][], cast(TransverseOptions)0))
```
Oct 09 2018
```On Tuesday, 9 October 2018 at 10:52:47 UTC, Chris Katko wrote:
I have a 2-D array:

int data =
[
[1, 0, 1, 0, 0],
[1, 0, 1, 0, 0],
[1, 0, 1, 1, 1],
[1, 0, 0, 1, 0],
[1, 1, 1, 1, 0]
];

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?

int count_numbers(){/*...*/}

count_numbers(data); //horizontal sum of array[0..\$-1]
WORKS.
count_numbers(data[]); //vertical sum of array[0..\$-1]
HYPOTHETICAL

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.

Oh geez, did I just realize what I need? some sort of matrix
transpose function? I found this post;

https://forum.dlang.org/post/na8ikk\$2ojo\$1 digitalmars.com

The problem is though, even if I use [][], instead of 
(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); //works
run_rle3(data.transposed); //nope [see error below]
run_rle3(data.transposed); //nope (didn't expect this to)
run_rle3((data.transposed)); //nope [see error below]

Error: function dmap.run_rle3 (int[] a) is not callable using
argument types (Transversal!(int[][], cast(TransverseOptions)0))

So according to this:

https://dlang.org/library/std/range/transposed.html

It's trying to take my  as an argument for the
TransverseOptions variable? But even if I wrap it in parenthesis,
it still takes it! (See the last line of code.)

auto taco = (data.transposed);
run_rle3(taco); //same error, but <-------- HERE?
```
Oct 09 2018
```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 
(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); //works
run_rle3(data.transposed); //nope [see error below]
run_rle3(data.transposed); //nope (didn't expect this to)
run_rle3((data.transposed)); //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.
```
Oct 09 2018    jmh530 <john.michael.hall gmail.com> writes:
```On Tuesday, 9 October 2018 at 10:52:47 UTC, Chris Katko wrote:
I have a 2-D array:

int data =
[
[1, 0, 1, 0, 0],
[1, 0, 1, 0, 0],
[1, 0, 1, 1, 1],
[1, 0, 0, 1, 0],
[1, 1, 1, 1, 0]
];

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?

[snip]

mir has a lot of functionality that can be helpful here, byDim
 comes to mind. Of course it also has transposed, but you can
combine byDim with count to achieve the same thing as what you're
trying.

```
Oct 09 2018