Re: setIntersection of struct range

[Steven Schveighoffer]

On Tue, 11 Aug 2009 19:35:40 -0400, Jesse Phillips  
<jessekphillip...@gmail.com> wrote:

I am trying to obtain a rang that is the intersection of two other  
ranges. To do this I am using the _setIntersection()_ function.

    import std.algorithm;
    import std.stdio;

    struct S {
        string label;
    }

    void main() {
        auto s1 = new S[2];
        auto s2 = new S[2];

        s1[0].label = "fish";
        s1[1].label = "bar";
        s2[0].label = "foo";
        s2[1].label = "fish";

        foreach(str; setIntersection(s1,s2))
            writeln(str);
    }

The code above generates this error:

     C:\opt\dmd\windows\bin\..\..\src\phobos\std\functional.d(191):
          Error: static assert  "Bad binary function q{a < b}.
             You need to use a valid D expression using symbols a of  
type S and b of type S."

You did not define a way to compare two S structs.  Try redefining S like  
this:

struct S {
  string label;
  int opCmp(ref const S s2) const {
     if(label < s2.label)
       return -1;
     if(label > s2.label)
       return 1;
     return 0;
  }
}


So I attempted an intersection of string arrays, and received a  
different error. I'm not sure if I am at fault or the compiler.  
Shouldn't these work?

    import std.algorithm;
    import std.stdio;
    import std.array;

    struct S {
        string label;
    }

    void main() {
        auto s1 = ["fish", "bar"];
        auto s2 = ["foo", "fish"];

        foreach(str; setIntersection(s1,s2))
            writeln(str);
    }

Which ended up with:

    test.d(13): Error: template std.algorithm.setIntersection(alias less  
= "a < b",Rs...)
        if (allSatisfy!(isInputRange,Rs)) does not match any function  
template declaration
    test.d(13): Error: template std.algorithm.setIntersection(alias less  
= "a < b",Rs...)
        if (allSatisfy!(isInputRange,Rs)) cannot deduce template  
function from
        argument types !()(immutable(char)[][2u],immutable(char)[][2u])

I think it's expecting dynamc arrays, not static ones.  Auto is making  
them static.

Try explicitly defining s1 and s2 as string[] or slice the literal, typing  
them as dynamic arrays like this:

auto s1 = ["fish", "bar"][];
auto s2 = ["foo", "fish"][];

    test.d(13): Error: foreach: int is not an aggregate type

This is a dummy error because it couldn't figure out the type of  
setIntersection, so it just assumes int (a quirk of dmd).

-Steve