We have slices, do we have Strides?

[Steven Schveighoffer]

On Tue, 09 Aug 2011 11:29:52 -0400, Kai Meyer <kai at unixlords.com> wrote:

> On 08/08/2011 05:25 PM, Steven Schveighoffer wrote:
>> On Mon, 08 Aug 2011 18:33:55 -0400, Kai Meyer <kai at unixlords.com> wrote:
>>
>>> On 08/08/2011 12:55 PM, Jonathan M Davis wrote:
>>>>> I have a problem I'd really like to use Strides for to simplify my
>>>>> code.
>>>>> Currently, I do this:
>>>>> foreach(n; 0..chunks)
>>>>> comp_arr[n] = values[(n * step_size) + n]
>>>>> if(!all_same(comp_arr, comp_arr[0]))
>>>>>
>>>>> It would eliminate an entire 2 lines of code for each time I want
>>>>> strides, to be able to do this:
>>>>> if(!all_same(bytes[i..$..step_size])
>>>>>
>>>>> Meaning, start with i, grab all elements at i + block_size * n until
>>>>> block_size * n> bytes.length. Right?
>>>>>
>>>>> -Kai Meyer
>>>>
>>>> Would std.range.stride work for you?
>>>>
>>>> - Jonathan M Davis
>>>
>>> It would, if there was a way to give it an offset:
>>>
>>> int[] a = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 ];
>>
>> fixed:
>>
>>> assert(equal(stride(a, 3), [ 1, 4, 7, 10 ][]));
>>> assert(equal(stride(a[1..$], 3), [ 2, 5, 8, 11 ][]));
>>> assert(equal(stride(a[2..$], 3), [ 3, 6, 9 ][]));
>>> assert(equal(stride(a[3..$], 3), [ 4, 7, 10 ][]));
>>
>> -Steve
>
> Doh, how do I extract the array from the return value of stride?
>
> import std.range;
> int do_something(int[] values)
> {
>      return values[0];
> }
> void main(string[] args)
> {
>          int[] a = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 ];
>              do_something(stride(a,3));
> }
>
>
> I get:
> moo.d(9): Error: function moo.do_something (int[] values) is not  
> callable using argument types (Result)
> moo.d(9): Error: cannot implicitly convert expression (stride(a,3LU)) of  
> type Result to int[]

Two options, first, you can change do_something to accept a range  
(depending on what the real implementation of do_something is, you might  
need to require a random-access range, but that dummy implementation could  
accept any input range, just return x.front):

int do_something(R)(R values) if (isInputRange!R && is(ElementType!R :  
int))
{
    return values.front; // maybe need random access in real code?
}

Second option, convert range to an array.  This incurs a memory allocation  
to create the array:

import std.array;

...
void main(...)
{
    ...
    do_something(array(stride(a, 3)));
}

doc for std.array.array:  
http://www.d-programming-language.org/phobos/std_array.html#array

-Steve