• d-bugmail puremagic.com (54/63) May 15 2011 http://d.puremagic.com/issues/show_bug.cgi?id=6004

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=6004

           Summary: std.range.unzip()
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc



In Python and Haskell there is a way to invert the operation of zip(), this is
a commonly used feature/function:

Python (V2.6.6):

 s = "abc"
 d = [1, 2, 3]
 z = zip(s, d)
 z



[('a', 1), ('b', 2), ('c', 3)]
 zip(*z)



[('a', 'b', 'c'), (1, 2, 3)]



Haskell (GHCI v.7.0.2):

Prelude> let s = "abc"
Prelude> let d = [1, 2, 3]
Prelude> let z = zip s d
Prelude> z
[('a',1),('b',2),('c',3)]
Prelude> unzip z
("abc",[1,2,3])



A D example shows where it's useful:

import std.stdio, std.typecons, std.algorithm, std.range;

Tuple!(int,int) divMod(int n, int m) {
    return tuple(n / m, n % m);
}

void main() {
    auto r = map!((m){ return divMod(20,m); })(iota(1, 20));
    int[] firsts, seconds;
    foreach (t; r) {
        firsts ~= t[0];
        seconds ~= t[1];
    }
    writeln(firsts);
    writeln(seconds);
}


Output:
[20, 10, 6, 5, 4, 3, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1]
[0, 0, 2, 0, 0, 2, 6, 4, 2, 0, 9, 8, 7, 6, 5, 4, 3, 2, 1]


Similar code in Python shell:

 r = (divmod(20, m) for m in xrange(1,20))
 firsts, seconds = zip(*r)
 firsts



(20, 10, 6, 5, 4, 3, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1)
 seconds



(0, 0, 2, 0, 0, 2, 6, 4, 2, 0, 9, 8, 7, 6, 5, 4, 3, 2, 1)


In Phobos a higher-order function to perform this operation may be named
"unzip" as in Haskell. This unzip is generalizable to zips of more than two
sequences:

unzip(zip([1,2,3], "abc", [0.5,1.0,1.5]))

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