On Saturday, 28 December 2013 at 22:55:39 UTC, Ivan Kazmenko
wrote:
Another quick question, two of them.
1. This is a minimal example of trying the permutations of a
character array.
-----
import std.algorithm;
void main () {
char [] a;
do { } while (nextPermutation(a));
}
-----
This gives a compile error. However, it works when I change
"char [] a" to "dchar [] a". Why?
I understand that permuting a char [] array might be wrong way
to go when dealing with Unicode. But what if, at this point of
the program, I am sure I'm dealing with ASCII and just want
efficiency? Should I convert to ubyte [] somehow - what's the
expected way then?
Because next permutation (AFAIK) works on ranges with
*assignable* elements, and "char[]" is not such a range: It is a
read-only range of dchars.
Arguably, the implementation *could* work for it, *even* while
taking into consideration unicode (using the underlying UTF8
knowledge). You should file an ER for that. But currently, this
is not the case, so you have to look for a workaround.
The "cast (ubyte [])" works, but "to!(ubyte [])" fails at
runtime, expecting a string representation of the array, not
its raw contents.
You can use "representation" to "conveniently" transform a
string/wstring/dstring to its corresponding numeric type
(ubyte/ushort/uint). That *should* work.
"Unfortunatly", "to!T(string)" often does a parse. As convenient
as it is, I think the added ambiguity makes it often wrong.
2. Why does nextPermutation hang up for empty arrays? I
suppose that's a bug?
I suppose so. Please file it. If it is deemed "illegal", at the
very least, it should throw.
Ivan Kazmenko.
