Re: std.algorithm.reduce on an array of structs

[Daniel Kozak via Digitalmars-d-learn]

On Thursday, 11 September 2014 at 13:06:05 UTC, Colin wrote:

I have this test code:

struct Thing {
    uint x;
}

void main(){
    uint[] ar1 = [1, 2, 3, 4, 5];
    auto min1 = ar1.reduce!((a,b) => a < b);
    writefln("%s", min1);  // prints 1 as expected

    Thing[] ar2 = [Thing(1), Thing(2), Thing(4)];

auto min2 = ar2.reduce!((a,b) => a.x < b.x); // <- Wont Compile

    writefln("%s", min2);
}

The line with "Wont Compile" on it has this error message:

/usr/include/dmd/phobos/std/algorithm.d(770): Error: cannot implicitly convert expression (__lambda2(result, front(_param_1))) of type bool to Thing /usr/include/dmd/phobos/std/algorithm.d(791): Error: template instance t.main.reduce!((a, b) => a.x < b.x).reduce!(Thing, Thing[]) error instantiating

t.d(16):        instantiated from here: reduce!(Thing[])


Any idea what I'm doing wrong here?

To me, the operation on ar2 should be pretty much identical to ar1, except for the use of the struct.

You are try to put uint to Thing. This is corect version:

import std.stdio;
import std.algorithm;

struct Thing {
        uint x;
}

void main(){
        uint[] ar1 = [1, 2, 3, 4, 5];
        auto min1 = ar1.reduce!((a,b) => a < b);
        writefln("%s", min1);  // prints 1 as expected
        
        Thing[] ar2 = [Thing(1), Thing(2), Thing(4)];
        auto min2 = ar2.reduce!((a,b) => a.x < b.x ? a : b);  //  <-
Wont Compile
        writefln("%s", min2);
}