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digitalmars.D.learn - D slicing

reply "Colin Grogan" <grogan.colin gmail.com> writes:
Hi all.

Wondering what way I'd go about this, I want to slice an array 
into two arrays. First array containing every even index (i.e. 
0,2,4,6,8..$)
Second slice containing every odd index (i.e. 1,3,5,7,9..$) <-- 
be some issue with using $ depending on if orig length is odd or 
even. Can work that out easily enough...

Reading the articles on array slicing its not clear if its 
possible.

Ideally, I could do something like the following:

auto orig = [1,2,3,4,5,6,7];
auto sliceEven = orig[0..$..2];
auto sliceOdd = orig[1..$..2];

But I dont think thats possible?

Any idears?
Jun 17 2013
next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
Colin Grogan:


 Reading the articles on array slicing its not clear if its 
 possible.


I presume Walter thinks that slicing with a stride is a not 
common enough operation to put it into D. His choices on such 
things are a bit arbitrary.

One way to do it:


import std.stdio, std.array, std.range;

void main() {
     auto orig = [1, 2, 3, 4, 5, 6, 7];
     auto sliceOdd = orig.stride(2).array;
     sliceOdd.writeln;
     auto sliceEven = orig[1..$].stride(2).array;
     sliceEven.writeln;
}


stride() is just a function used with UFCS.

Don't use ".array" if you just need a lazy sequence.

Bye,
bearophile
Jun 17 2013
prev sibling next sibling parent "Andrej Mitrovic" <andrej.mitrovich gmail.com> writes:
On Monday, 17 June 2013 at 23:34:46 UTC, Colin Grogan wrote:

 auto orig = [1,2,3,4,5,6,7];
 auto sliceEven = orig[0..$..2];
 auto sliceOdd = orig[1..$..2];



 But I dont think thats possible?


Not with arrays, they must be contiguous. But you can use ranges 
instead:

-----
import std.stdio;
import std.range;

void main()
{
     auto orig = [1,2,3,4,5,6,7];
     auto sliceEven = orig.stride(2);
     auto sliceOdd = orig.drop(1).stride(2);

     writeln(sliceEven);
     writeln(sliceOdd);
}
-----

You can convert the ranges into arrays by calling .array on them 
(and importing std.array first), but this will cause allocations.
Jun 17 2013
prev sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 06/17/2013 04:34 PM, Colin Grogan wrote:


 Wondering what way I'd go about this, I want to slice an array into two
 arrays. First array containing every even index (i.e. 0,2,4,6,8..$)
 Second slice containing every odd index (i.e. 1,3,5,7,9..$) <-- be some
 issue with using $ depending on if orig length is odd or even. Can work
 that out easily enough...

 Reading the articles on array slicing its not clear if its possible.

 Ideally, I could do something like the following:

 auto orig = [1,2,3,4,5,6,7];
 auto sliceEven = orig[0..$..2];
 auto sliceOdd = orig[1..$..2];


If you want the data sit where it is but simply have different views in 
it, then you must use ranges. There are multiple ways.

Here is one using std.range.stride:

import std.stdio;
import std.range;

void main()
{
     auto orig = [0, 1, 2, 3, 4, 5, 6, 7];

     auto sliceEven = orig.stride(2);
     auto sliceOdd = orig.dropOne.stride(2);

     writeln(sliceEven);
     writeln(sliceOdd);
}

The output:

[0, 2, 4, 6]
[1, 3, 5, 7]

Or you can generate the indexes and then get a view that way:

     auto sliceEven = orig.indexed(iota(0, orig.length, 2));
     auto sliceOdd = orig.indexed(iota(1, orig.length, 2));

Ali
Jun 17 2013
parent "Colin Grogan" <grogan.colin gmail.com> writes:
On Monday, 17 June 2013 at 23:48:36 UTC, Ali Çehreli wrote:

 On 06/17/2013 04:34 PM, Colin Grogan wrote:


 Wondering what way I'd go about this, I want to slice an


 array into two

 arrays. First array containing every even index (i.e.


 0,2,4,6,8..$)

 Second slice containing every odd index (i.e. 1,3,5,7,9..$)


 <-- be some

 issue with using $ depending on if orig length is odd or


 even. Can work

 that out easily enough...

 Reading the articles on array slicing its not clear if its


 possible.

 Ideally, I could do something like the following:

 auto orig = [1,2,3,4,5,6,7];
 auto sliceEven = orig[0..$..2];
 auto sliceOdd = orig[1..$..2];


 If you want the data sit where it is but simply have different 
 views in it, then you must use ranges. There are multiple ways.

 Here is one using std.range.stride:

 import std.stdio;
 import std.range;

 void main()
 {
     auto orig = [0, 1, 2, 3, 4, 5, 6, 7];

     auto sliceEven = orig.stride(2);
     auto sliceOdd = orig.dropOne.stride(2);

     writeln(sliceEven);
     writeln(sliceOdd);
 }

 The output:

 [0, 2, 4, 6]
 [1, 3, 5, 7]

 Or you can generate the indexes and then get a view that way:

     auto sliceEven = orig.indexed(iota(0, orig.length, 2));
     auto sliceOdd = orig.indexed(iota(1, orig.length, 2));

 Ali


Thats perfect folks. Should have known to look in std.range.
This works for me perfectly.

3 answers all within a couple minutes of each other, what a good 
community! ;)
Jun 18 2013