Create array from range

Oct 09, 2021

Greg Strong

Oct 10, 2021

H. S. Teoh

Oct 10, 2021

jfondren

Oct 10, 2021

Ali Çehreli

On Sat, Oct 09, 2021 at 11:58:14PM +0000, Greg Strong via Digitalmars-d-learn wrote: > This should be a simple question, but I'm having difficult finding an answer. How do I filter some elements of an array into a new array? The filter! function returns a range, but I can't seems to assign it to a new array. I get: > > Cannot implicitly convert expression of type FilterResult!(__lambda10 ... > > Nothing I try to construct a new array seems to work. You can use the .array function (from std.array) to create an array out of the filtered range: -------------- int[] myArray = [ ... ]; int[] filteredArray = myArray.filter!(e => ...).array; -------------- T -- Chance favours the prepared mind. -- Louis Pasteur

October 10, 2021

Re: Create array from range

Posted by jfondren
in reply to Greg Strong

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jfondren

Posted in reply to Greg Strong

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On Saturday, 9 October 2021 at 23:58:14 UTC, Greg Strong wrote:

This should be a simple question, but I'm having difficult finding an answer. How do I filter some elements of an array into a new array? The filter! function returns a range, but I can't seems to assign it to a new array. I get:

Cannot implicitly convert expression of type FilterResult!(__lambda10 ...

Nothing I try to construct a new array seems to work.

std.array.array does this:

https://dlang.org/phobos/std_array.html

and some other functions in that module are similar.

A value of output ranges like FilterResult is that you can decide how or whether to allocate an array of the results: you could allocate a new array with .array or you could consume the results as they come, or you could out pick a particular result, or you could populate a static array...

int example(R)(R range) @nogc {
    import std.range : take, enumerate;

    int[5] ints;

    foreach (size_t i, n; range.enumerate.take(ints.length))
        ints[i] = n;

    // just to do something with the array
    int sum;
    foreach (n; ints)
        sum += n;
    return sum;
}

unittest {
    import std.range : iota;
    import std.algorithm : filter;

    assert(20 == iota(100).filter!"a%2==0".example);
}

On 10/9/21 4:58 PM, Greg Strong wrote: > How do I filter some elements of an array into a new array? This doesn't answer your specific question but std.algorithm.remove may be usable in some cases: https://dlang.org/phobos/std_algorithm_mutation.html#remove If it matters, the following solution does not allocate new memory. import std.stdio; import std.algorithm; void main() { auto arr = [ 1, 2, 3, 4, 5, 6, 7 ]; // The default strategy is SwapStrategy.stable arr = arr.remove!(a => a % 2, SwapStrategy.unstable); writeln(arr); } The original array is modified. SwapStrategy.unstable will shuffle elements around. The program prints [6, 2, 4] Ali

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