On Thu, 08 Sep 2011 03:33:24 -0400, bearophile wrote:
> std.range.indexed of DMD 2.055 is very nice. But it signature shows
> constraints that I have had problems to work with, maybe the error is
> just mine, I don't know:
>
>
> import std.algorithm, std.range, std.array; void main() {
> auto data = [7, 6, 5, 4, 3, 2, 1, 0]; auto indices = [6, 1, 7];
>
> //auto idxSet = array(uniq(indices.sort())); // OK
In that case, idxSet is a RandomAccessRange.
> auto idxSet = uniq(indices.sort()); // not OK?
indices.sort() is a RandomAccessRange but uniq is not, as it needs to
walk the elements lazily.
The documentation of indexed() says: "Source must be a random access
range. The returned range will be bidirectional or random-access if
Indices is bidirectional or random-access, respectively."
> // std.range.indexed() constraints
> static assert(isInputRange!(typeof(idxSet))); // OK static assert(
> is(typeof((int[]).init[ElementType!(typeof(idxSet)).init])) ); // OK
>
> sort(indexed(data, idxSet));
Because idxSet is not RandomAccessRange, indexed() does not provide
opIndex() and doesn't match sort()'s requirement.
Here is an excerpt of Indexed from range.d:
struct Indexed(Source, Indices)
if(isRandomAccessRange!Source && isInputRange!Indices &&
is(typeof(Source.init[ElementType!(Indices).init])))
{
// ...
static if(isRandomAccessRange!Indices)
{
/// Ditto
auto ref opIndex(size_t index)
{
return _source[_indices[index]];
}
// ...
}
> assert(data == [7, 0, 5, 4, 3, 2, 1, 6]);
> }
>
>
> Bye and thank you,
> bearophile
It's unfortunate that template the error messages cannot be more
helpful. :-/
Ali
