How would I transpose

float[3][2]

to

float[2][3]

with http://dlang.org/phobos/std_range.html#.transposed

On 02/19/2016 06:00 PM, maik klein wrote:
> How would I transpose
>
> float[3][2]
>
> to
>
> float[2][3]
>
> with http://dlang.org/phobos/std_range.html#.transposed
>
>

Because static arrays are not ranges, they must be used as slices with the help of []. The following code does the same thing in two different ways:

import std.stdio;
import std.range;
import std.algorithm;

void main() {
    float[3][2] arr = [ [1, 2, 3],
                        [4, 5, 6] ];

    // Method A
    {
        float[][] arr2;

        foreach (ref a; arr) {
            arr2 ~= a[];
        }

        writeln(arr2.transposed);
    }

    // Method B
    {
        auto r = arr[].map!((ref a) => a[]).array.transposed;
        writeln(r);
    }
}

Ali

On Saturday, 20 February 2016 at 02:22:12 UTC, Ali Çehreli wrote:
> On 02/19/2016 06:00 PM, maik klein wrote:
>> How would I transpose
>>
>> float[3][2]
>>
>> to
>>
>> float[2][3]
>>
>> with http://dlang.org/phobos/std_range.html#.transposed
>>
>>
>
> Because static arrays are not ranges, they must be used as slices with the help of []. The following code does the same thing in two different ways:
>
> import std.stdio;
> import std.range;
> import std.algorithm;
>
> void main() {
>     float[3][2] arr = [ [1, 2, 3],
>                         [4, 5, 6] ];
>
>     // Method A
>     {
>         float[][] arr2;
>
>         foreach (ref a; arr) {
>             arr2 ~= a[];
>         }
>
>         writeln(arr2.transposed);
>     }
>
>     // Method B
>     {
>         auto r = arr[].map!((ref a) => a[]).array.transposed;
>         writeln(r);
>     }
> }
>
> Ali

Your "Method B" is how I did it too but how do I convert it back to a static array of float[2][3]?

On Saturday, 20 February 2016 at 02:26:56 UTC, maik klein wrote:
> On Saturday, 20 February 2016 at 02:22:12 UTC, Ali Çehreli wrote:
>> On 02/19/2016 06:00 PM, maik klein wrote:
>>> How would I transpose
>>>
>>> float[3][2]
>>>
>>> to
>>>
>>> float[2][3]
>>>
>>> with http://dlang.org/phobos/std_range.html#.transposed
>>>
>>>
>>
>> Because static arrays are not ranges, they must be used as slices with the help of []. The following code does the same thing in two different ways:
>>
>> import std.stdio;
>> import std.range;
>> import std.algorithm;
>>
>> void main() {
>>     float[3][2] arr = [ [1, 2, 3],
>>                         [4, 5, 6] ];
>>
>>     // Method A
>>     {
>>         float[][] arr2;
>>
>>         foreach (ref a; arr) {
>>             arr2 ~= a[];
>>         }
>>
>>         writeln(arr2.transposed);
>>     }
>>
>>     // Method B
>>     {
>>         auto r = arr[].map!((ref a) => a[]).array.transposed;
>>         writeln(r);
>>     }
>> }
>>
>> Ali
>
> Your "Method B" is how I did it too but how do I convert it back to a static array of float[2][3]?

I don't see the point of using transposed which gives a range out if you want to put it back into a float[2][3] right away, just do a double foreach:

    void main(string[] args) {
        float[3][2] src = [[1, 2, 3], [2, 3, 4]];

        float[2][3] dst;

        foreach (i ; 0..src.length)
            foreach (j ; 0..src[0].length)
                dst[j][i] = src[i][j];

        assert(dst == [[1, 2], [2, 3], [3, 4]]);
    }

On 02/19/2016 06:26 PM, maik klein wrote:
> On Saturday, 20 February 2016 at 02:22:12 UTC, Ali Çehreli wrote:
>> On 02/19/2016 06:00 PM, maik klein wrote:
>>> How would I transpose
>>>
>>> float[3][2]
>>>
>>> to
>>>
>>> float[2][3]
>>>
>>> with http://dlang.org/phobos/std_range.html#.transposed

>>         auto r = arr[].map!((ref a) => a[]).array.transposed;

There is a problem there. Adding the following line

    writeln(r.array);

causes an error:

core.exception.AssertError@/usr/include/dmd/phobos/std/range/primitives.d(2207): Attempting to fetch the front of an empty array of float

Adding the following two lines does *not* preserve the range (first one does):

    writeln(r.save);
    writeln(r.save);

Did I not need (ref a) up there? I thought it would be a reference to a float[3] element.

>>         writeln(r);
>>     }
>> }
>>
>> Ali
>
> Your "Method B" is how I did it too but how do I convert it back to a
> static array of float[2][3]?

I don't know. :D

Ali

On Saturday, 20 February 2016 at 03:02:11 UTC, cym13 wrote:
> On Saturday, 20 February 2016 at 02:26:56 UTC, maik klein wrote:
>> On Saturday, 20 February 2016 at 02:22:12 UTC, Ali Çehreli wrote:
>>> [...]
>>
>> Your "Method B" is how I did it too but how do I convert it back to a static array of float[2][3]?
>
> I don't see the point of using transposed which gives a range out if you want to put it back into a float[2][3] right away, just do a double foreach:
>
>     void main(string[] args) {
>         float[3][2] src = [[1, 2, 3], [2, 3, 4]];
>
>         float[2][3] dst;
>
>         foreach (i ; 0..src.length)
>             foreach (j ; 0..src[0].length)
>                 dst[j][i] = src[i][j];
>
>         assert(dst == [[1, 2], [2, 3], [3, 4]]);
>     }

You are right. I restricted myself from using indices because I wanted to learn ranges but I think that working with matrices is actually much simpler by just using good old for loops.