Re: how to return map(fn)

[Steven Schveighoffer via Digitalmars-d-learn]

On Wednesday, 23 February 2022 at 16:48:00 UTC, steve wrote:
Thank you both a lot for your help. I am new to D so all of 
this is incredibly helpful. This seems like an amazing 
community!

@Ali I will have a look at std.functional as I think this is 
really what I was looking for. Until then, I have solved the 
problem with a simple class (e.g. below). I'm sure its not a 
particularly good way of doing it but it will work for now :). 
Also thank you for your book, its amazing!


```d
class Mapper
{
    float function(float) fn;

    this(float function(float) func)  {this.fn = func;}

    float[] opCall(float[] x){
        auto arr = new float[x.length];
        foreach (i,elem; x) { arr[i] = fn(elem);}
        return arr;
    }

}


float times_two(float x) {return 2*x;}


void main() {

    import std.stdio;

    Mapper map2x = new Mapper(&times_two);
    writeln(map2x([1., 2., 3.]));
}
```

I'm not sure what programming languages you are used to, so let 
me just show you the idiomatic D way ;)

```d
float times_two(float x) {return 2*x;}

// if you would rather make sure the result is an array
float[] times_two_array(float[] arr) {
   import std.algorithm; // for map
   import std.array; // for array
   return arr
      .map!times_two // map your function
      .array; // convert to an array
}

void main() {
   import std.stdio;
   import std.algorithm; // for map
   alias map2x = map!times_two;
   writeln(map2x([1., 2., 3.]));
   float[] arr2 = times_two_array([1., 2., 3.]);
   writeln(arr2);
}
```

-Steve