Hi!

I'm trying to remove an item from an array of objects. But I get error messages when compiling (see below) which I do not understand. I figured I had to override opEquals for it to work, but no.
How do I get this to work?

Regards



    class A
    {
        this(string si, uint ui) { s = si; u = ui; }
        string s;
        uint u;

        override bool opEquals(Object obj)
        {
            A o = cast(A)obj;
            return (s == o.s) && (u == o.u);
        }
    }

    int main(string[] argv)
    {
        import std.stdio: writeln;
        import std.algorithm: remove;
        A a = new A("a", 1);
        A b = new A("b", 2);
        A[] as = [a, b];
        as.remove(a);
        writeln(as);

        return 0;
    }


Error:

/usr/include/dmd/phobos/std/algorithm/mutation.d(1503): Error: template std.range.primitives.popFrontExactly cannot deduce function from argument types !()(A[], A), candidates are:
/usr/include/dmd/phobos/std/range/primitives.d(1791):        std.range.primitives.popFrontExactly(Range)(ref Range r, size_t n) if (isInputRange!Range)
/usr/include/dmd/phobos/std/algorithm/mutation.d(1504): Error: template std.range.primitives.popFrontExactly cannot deduce function from argument types !()(A[], A), candidates are:
/usr/include/dmd/phobos/std/range/primitives.d(1791):        std.range.primitives.popFrontExactly(Range)(ref Range r, size_t n) if (isInputRange!Range)
/usr/include/dmd/phobos/std/algorithm/mutation.d(1505): Error: cannot implicitly convert expression (from) of type dummy.A to ulong
dummy.d(21): Error: template instance std.algorithm.mutation.remove!(cast(SwapStrategy)2, A[], A) error instantiating

On Wednesday, 9 December 2015 at 13:05:31 UTC, Tim K. wrote:
> Hi!
>
> I'm trying to remove an item from an array of objects. But I get error messages when compiling (see below) which I do not understand. I figured I had to override opEquals for it to work, but no.
> How do I get this to work?

You should read the documentation, there is two errors:

1) remove works with an index
2) remove does not remove in place

so with

~~~~~~~~~~~~~~~~~~~~~~~~~~
import std.stdio;
import std.algorithm;

class A
{
    this(string si, uint ui) { s = si; u = ui; }
    string s;
    uint u;

    override bool opEquals(Object obj) const
    {
        if (A o = cast(A)obj)
            return (s == o.s) && (u == o.u);
        else
            return false;
    }
}

void main(string[] argv)
{
    A a = new A("a", 1);
    A b = new A("b", 2);
    A[] as = [a, b];
    as = as.remove(0);
    writeln(as);
}
~~~~~~~~~~~~~~~~~~~~~~~~~~

also, off topic but 2 other advices/errors

3) opEquals can be 'const' because the method doesn't mutate the state of the object
4) your cast wasn't safe


http://dlang.org/phobos/std_algorithm_mutation.html#.remove

On Wednesday, 9 December 2015 at 13:13:36 UTC, BBaz wrote:

> 1) remove works with an index
I guess I did read it wrong. Sorry.
Is there a convenience function that allows me to remove an/all object(s) with a certain value from an array or do I need to write one myself?

> 2) remove does not remove in place
That's fine.

On Wednesday, 9 December 2015 at 13:23:00 UTC, Tim K. wrote:
> On Wednesday, 9 December 2015 at 13:13:36 UTC, BBaz wrote:
>
>> 1) remove works with an index
> I guess I did read it wrong. Sorry.
> Is there a convenience function that allows me to remove an/all object(s) with a certain value from an array or do I need to write one myself?
>
>> 2) remove does not remove in place
> That's fine.

void main(string[] argv)
{
    A a = new A("a", 1);
    A b = new A("b", 2);
    A[] as = [a, b];
    as = as.remove!(x => x == a);
    writeln(as);
}

If array is sorted you can find the index of that element and then remove it by index.

On Wednesday, 9 December 2015 at 13:13:36 UTC, BBaz wrote:
> 3) opEquals can be 'const' because the method doesn't mutate the state of the object
> 4) your cast wasn't safe
>
>
> http://dlang.org/phobos/std_algorithm_mutation.html#.remove

Maybe something like this works better:

...

    override bool opEquals(T)(T obj) const
    if (is(T : A))
    {
        return (s == o.s) && (u == o.u);
    }

...

On Wednesday, 9 December 2015 at 13:23:00 UTC, Tim K. wrote:
> On Wednesday, 9 December 2015 at 13:13:36 UTC, BBaz wrote:
>
> Is there a convenience function that allows me to remove an/all object(s) with a certain value from an array or do I need to write one myself?

Here are some elements of strategy:

     import std.algorithm;
     import std.array;

     auto arr = [2, 3, 4, 5, 3, 2, 4];

     // Remove all
     arr = arr.remove!(x => x==3);
     assert(arr == [2, 4, 5, 2, 4];);

     // remove all (filter works lazilly, we use array to get an array nonetheless)
     // filter is useful when composing functions, not so much here
     arr = arr.filter!(x => x==2).array;
     assert(arr == [4, 5, 4]);

     // Remove one, throw if the element is not found,
     // we use countUntil to get the index.
     arr = arr.remove(arr.countUntil(4));
     assert(arr == [5, 4]);

also you may be interested in reading this:

http://p0nce.github.io/d-idioms/#Adding-or-removing-an-element-from-arrays