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Programming-Idioms

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  • Java

Idiom #223 for else loop

Loop through list items checking a condition. Do something else if no matches are found.

A typical use case is looping through a series of containers looking for one that matches a condition. If found, an item is inserted; otherwise, a new container is created.

These are mostly used as an inner nested loop, and in a location where refactoring inner logic into a separate function reduces clarity.

boolean b = false;
for (int i = 0; i<items.length; i++) {
	if (items[i] == something) {
		doSomething();
		b = true;
	}
}
if (!b)
	doSomethingElse();

Java does not have a for else loop natively. Therefore we must use a boolean value to do something if no matches are found.
import static java.lang.System.out;
 a: {
     b: {
            int i, n = items.size();
            for (i = 0; i < n; ++i)
                if (f(items.get(i)))
                    break b;
            out.println("not found");
            break a;
        }
        out.println("found");
    }
Predicate<Item> condition;
items.stream()
	.filter(condition)
	.findFirst()
	.ifPresentOrElse(
		item -> doSomething(i),
		() -> doSomethingElse());

This requires at least Java 8, and uses a stream instead of a loop to get to the same result.
(if (seq (filter odd? my-col))
  "contains odds"
  "no odds found")

(seq (filter pred-fn col)) returns nil when there are no matches, and nil is falsy.

New implementation...
< >
Ruien