On Monday, 21 September 2015 at 15:38:40 UTC, French Football
wrote:
Going through a book on coding in D,
http://ddili.org/ders/d.en/foreach.html , I find the following
very useful feature:
When two names are specified in the names section [with a plain
array], they represent an automatic counter and the value of
the element, respectively:
foreach (i, element; array) {
writeln(i, ": ", element);
}
I understand that foreach is built on top of a for loop... I'm
just wondering why I can't access the automatic counter from a
doubly linked list, or an associative array, or some range?
It's pretty common for me to have to rewrite foreach loops to
be for loops when I get to the bottom and realize I need to
know where in the sequence I am...
That's because this isn't really a counter. It makes more sens if
you think of it as:
foreach (key, value ; array) {
...
}
In the case of an array, the key to access directly a value is
its index, and the array is read in its natural order so the
'key' part acts as a counter. If array is an associative array
instead of an array, then you get the key and value as well.
If you want a counter, you want to look at std.range.enumerate()
which takes a range and returns a tuple (counter, element). The
following example demonstrates the two usages with associative
arrays:
void main(string[] args)
{
import std.stdio;
import std.range;
bool[int] aa = [0:true, 1:true, 2:false];
writeln("Get keys and values from an AA");
foreach (key, value ; aa) {
writeln(key, ": ", value);
}
writeln;
writeln("Get a counter and the key of an AA");
foreach (count, key ; aa.byKey.enumerate) {
writeln("count: ", count, " key: ", key, " value: ",
aa[key]);
}
}
Note that contrary to languages such as PHP, D's associative
arrays are unordered so you can't use this counter as an index.
