Re: Function called twice

[Boris Carvajal via Digitalmars-d-learn]

On Friday, 2 August 2019 at 21:44:28 UTC, Jordan Wilson wrote:
Hello,
I don't quite understand why isEven is called twice in the 2nd 
example?

auto isEven(int n) {
        n.writeln;
        return (n % 2) == 0;
}

void main() {

        auto z = [1,2,3];
        
        // outputs 1 2 3
        z.map!(a => tuple!("number")(a))
         .filter!(a => a.number.isEven)
         .array;

        // outputs 1 2 2 3
        z.map!(a => tuple!("number","iseven")(a, a.isEven))
         .filter!(a => a.iseven)
         .array;

        return;
}

Thanks,

Jordan

The way map is designed is to call its predicate on each front 
call and filter calls it twice with the number 2.
https://github.com/dlang/phobos/blob/master/std/algorithm/iteration.d#L604

You can use "cache" to avoid the double front call on any range.

z.map!(a => tuple!("number","iseven")(a, a.isEven))
         .cache
         .filter!(a => a.iseven)
         .array;