Help with Ranges

Steven Schveighoffer schveiguy at
Mon Jul 27 18:15:26 UTC 2020

On 7/27/20 1:10 PM, Charles wrote:

> Still, I'm confused since, as 
> far as I know, map wraps its source, i.e. the array in this case, which 
> is sortable. It seems to me the only reason I can't sort MapResult is 
> because it doesn't have the proper interface.

Let's talk about a concrete example, so you can see why:

int[] arr = [21, 83, 45, 60];

auto m =!(a => a % 10);

Sort is going to look at m as a random-access range of integers. But the 
integer you get from m[2] for instance is going to be 5. So you can 
compare e.g. m[2] and m[3] (5 and 0), but how do you *WRITE* back to the 
original array? All you have as an interface is non-writable 5 and 0, 
not the original array members 45 and 60. In other words, if you swapped 
5 and 0, map can't do the inverse transform here (and even if it could, 
0 and 5 map to millions of possible original values).

What it seems like you are possibly interested in doing is to sort the 
original array based on a transformation. But in your original post you 
said "doSomething is some predicate that does a lot of processing on 
each element", so I assumed, e.g. something like this is not valid for 
your use case:

arr.sort!((a, b) => doSomething(a) < doSomething(b))

as it would be very expensive assuming doSomething is expensive.

You can use H.S. Teoh's suggestion and use schwartzSort (in fact, it 
does something almost exactly the same as what I wrote, except it sorts 
the original elements).

However, for something like a % 10, I'd much rather just do it without 
allocating another array.

So it really depends on what your requirements are, which you haven't 
fully identified.


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