如何使用Kotlin将列表转换成地图

Question

I'm trying to construct a map from a list. My goal is to compare two lists and found differences between thoses two lists. Then, I want to construct a map, in order to know in which index I found differences.

I did it in Java, not in a great way I believe, but it's working.

//I compare the two values for a given index, if value are the same, I set null in my result list
List<String> result = IntStream.range(0, list1.size()).boxed()
                .map(i -> list1.get(i) != list2.get(i) ? (list1.get(i)  + " != "+ list2.get(i)) : null)
                .collect(Collectors.toList());

//I filter all the null values, in order to retrieve only the differences with their index
Map<Integer, String> mapResult =
            IntStream.range(0, result.size())
            .boxed().filter(i-> null != result.get(i))
            .collect(Collectors.toMap(i -> i,result::get));

It's not optimal, but it's working. If you have suggestions regarding thoses lines of codes, I will gladly take it.

I tried two replicate this kind of behavior in Kotlin, but I didn't succeed to use the map() constructor. (I'm still learning Kotlin, I'm not very familiar with it).

Thank you for your help.

Answer 1

You may use zip function in collections to join two elements. The withIndex() function helps to turn a list into a list of pairs of an element index and value. The full solution may be as follows

    val list1 = listOf("a", "b", "c")
    val list2 = listOf("a", "B", "c")

    val diff : Map<Int, String> = list1.withIndex()
        .zip(list2) { (idx,a), b -> if (a != b) idx to "$a != $b" else null}
        .filterNotNull().toMap()

Note that the zip function iterates while there are elements in both lists, it will skip a possible leftover from any of the lists. It can be fixed by adding empty elements with the following function:

fun <T> List<T>.addNulls(element: T, toSize: Int) : List<T> {
    val elementsToAdd = (toSize - size)
    return if (elementsToAdd > 0) {
        this + List(elementsToAdd) { element }
    } else {
        this
    }
}

and call the function on both lists before using the zip function