Re: for :each style question
On 12/01/2011 12:32 AM, Roedy Green wrote:
In a for:each loop, sometimes you want to treat the first and or last
element specially.
The obvious way to handle is to revert to a for int i= loop and check
for special values of i.
You can keep the for:each style if you have a boolean first= true that
you set false to detect the first.
I don't know of an equivalent way to detect the last.
The trick for detecting the last element is to defer evaluation of each
element by one iteration and handle the last one after the loop.
In the olden days I would have handled the first and last cases
outside the loop, with the loop running over the middle elements. You
can't do that with for:each.
What do you consider the best style to deal with this?
For the fun of it, here is one way to do it - although I find it quite
silly:
package clj;
import java.util.Arrays;
public final class FirstLast {
private static enum Pos {
NONE, FIRST, LATER
}
public static <T> void firstLast(final Iterable<T> items) {
Pos p = Pos.NONE;
T last = null;
for (final T item : items) {
switch (p) {
case NONE:
p = Pos.FIRST;
break;
case FIRST:
System.out.println("First: " + last);
p = Pos.LATER;
break;
case LATER:
System.out.println("Next: " + last);
break;
}
last = item;
}
if (p != Pos.NONE) {
System.out.println("Last: " + last);
}
}
public static void main(String[] args) {
System.out.println("empty");
firstLast(Arrays.asList());
System.out.println("one");
firstLast(Arrays.asList("a"));
System.out.println("two");
firstLast(Arrays.asList("a", "b"));
System.out.println("more");
firstLast(Arrays.asList("a", "b", "c", "d"));
}
}
Kind regards
robert