DLang Front Page Code Example

[bearophile via Digitalmars-d-learn]

Nicholas Londey:

> So I tried rewriting the example on the dlang.org home page and 
> came up with the following.
>
> import std.algorithm, std.exception, std.stdio;
>
> double average(T)(T range)
> {
>   enforce(!range.empty, "No inputs");
>
>   auto totals = range.map!(a => tuple(1.0, 
> cast(double)(a))).reduce!((a, b) => tuple(a[0] + b[0], a[1] + 
> b[1]));
>   return totals[1] / totals[0];
> }
>
> void main()
> {
>   writeln("Average line length: ", stdin.byLine.map!(a => 
> a.length).average);
> }

This is a little better, but you can also use a reduce with two
functions:


import std.stdio, std.algorithm, std.exception, std.range;

real average(R)(R range) if (isInputRange!R) {
      enforce(!range.empty, "No inputs");

      immutable totals = range
                         .map!(x => tuple(real(x), 1L))
                         .reduce!((a, b) => tuple(a[0] + b[0], a[1]
+ b[1]));
      return totals[0] / real(totals[1]);
}

void main() {
      writeln("Average line length: ",
              stdin.byLine.map!walkLength.average);
}


But I think the Dsite has preferred the raw loop to show a
simpler D code that can be understood by not


> - Average or mean did not seem to already exist in algorithm or 
> numeric.

It should be added.


> - I could not think of an easy way to add component wise 
> binaryOp to a Tuple.

In theory you can define a new Tuple struct with "alias this" on
a Tuple, plus binaryOp +, but I don't know how well this could
work.


> - Tried using static array instead of tuple but could not work 
> out how to crate a static array as the result of a lambda.

You need a slightly different syntax:

void main() {
      import std.stdio;

      auto myLambda = (int x) { int[2] a = x; return a; };
      myLambda(10).writeln;
}


But in your code it's not a good idea to keep the length in a
double value, and currently using the built-in array plus
operation on a fixed size array of length 2 is not efficient
because the D compilers perform a run-time call for that, and
they don't replace the tiny array with two inlined sums:
https://issues.dlang.org/show_bug.cgi?id=4240


> - Using dynamic array caused a compile issue and presumably 
> would have had terrible heap garbage.

What compile issue? Real issues should go in Bugzilla.

Bye,
bearophile