How to make multiconditional for loop in kotlin

问题:

In Java:

for(int j = 0; j < 6 && j < ((int)abc[j] & 0xff); j++) { 
  // ...
}

How we can make this loop in Kotlin?

回答1:

I'd suggest to use a more functional approach like

(0..5).takeWhile {
    it < (abc[it].toInt() and 0xff) // or `as Int` if array is not numeric
}.forEach {
    // do something with `it`
}

回答2:

If you don't mind creating a new ArrayList instance, it can be done like this:

(0..5).takeWhile { it < (abc[it] as Int and 0xff) }
        .forEach {
            // ...
        }

回答3:

Note: the .takeWhile { ... }.forEach { ... } approach suggested in some answers is not equivalent to the Java for loop. The range is first processed with .takeWhile { ... } and only then the prefix it returned is iterated over. The problem is that the execution of the .forEach { ... } body won't affect the condition of .takeWhile { ... }, which has already finished execution by the time the body gets called on the first item.

^{(see this runnable demo that shows how the behavior is different)}

To fix this, you can use Sequence<T>. In constrast with eager evaluation over Iterable<T>, it won't process the whole set of items with .takeWhile { ... } and will only check them one by one when .forEach { ... } is up to process a next item. See: the difference between Iterable<T> and Sequence<T>.

To use the Sequence<T> and achieve the behavior that is equivalent to the Java loop, convert the range .toSequence():

(0..5).asSequence()
    .takeWhile { it < (abc[it].toInt() and 0xff) }
    .forEach {
        // Use `it` instead of `j`
    }

Alternatively, just use the while loop:

var j = 0
while (j < 6 && j < (abc[j] as Int and 0xff)) {
    // do something
    j++
}

回答4:

This is how the kotlin version will look like.

var j = 0
while (j < 6 && j < (abc[j] as Int and 0xff)) {
    // do something
    j++
}

You can convert Java to Kotlin online here. Try Kotlin. Also if you are using IntelliJ, here is a link to help you convert from Java to Kotlin. IntelliJ Java to Kotlin.

回答5:

This is the output of the intellij plugin for conversion:

 var j = 0
 while (j < 6 && j < abc[j] as Int and 0xff) {
      j++
      // ...
 }

回答6:

I would move the j < ((int)abc[j] & 0xff) part into an if-test inside the loop. You could then do this:

for (j in 0..5) {
    if (j < (abc[j].toInt() and 0xff)) {
      // Do stuff here
    } else break
}