如何將雙打列表減少為字符串？

Question

我是 java 8 和 Kotlin 中減少和折疊等功能方法的新手。我想減少 -

List<List<List<Double>>> boundingPolygon = [[[-125.48845080566404,47.94508483691371],[-124.96110705566404,42.309040799653665],[-117.13884143066404,45.04173793121063],[-118.36931018066404,48.93624688577435],[-125.48845080566404,47.94508483691371]]]; 

到表示一個由坐標連接的字符串的單個字符串 -

"-118.359053 33.931562,-118.372443 33.946939,-118.369053 33.951562,-118.337612 33.944342,-118.342012 33.944042,-118.359053 33.931562"

試圖做 -

val polygonCoordinates = boundingPolygon.first().reduce { acc, element ->
                    acc + "${element[0]} ${element[1]}"
                }

這是行不通的。

Answer 1

reduce 操作中的acc是List<Double>類型，而不是String類型。 看看 reduce function 簽名，你應該明白為什么了。 這是我建議做你想做的事情：

coords.first().joinToString(", ") { (x, y) -> "$x $y" }

我使用解構從坐標列表中提取第一個和第二個值。 所以這只適用於二維坐標。

Answer 2

與其減少它，不如將它們添加到StringBuilder中，這在您進行多項操作時非常有效（例如連接大量字符串）：

val boundingPolygon = listOf(
    listOf(
        listOf(-125.48845080566404, 47.94508483691371),
        listOf(-124.96110705566404, 42.309040799653665),
        listOf(-117.13884143066404, 45.04173793121063),
        listOf(-118.36931018066404, 48.93624688577435),
        listOf(-125.48845080566404, 47.94508483691371)
    )
)

val sb = StringBuilder()
for (nestedList in boundingPolygon) {
    for (innerNestedList in nestedList) {
        sb.append(innerNestedList.joinToString(" "))
        sb.append(',')
    }
}
if (sb.isNotEmpty()) sb.deleteCharAt(sb.lastIndex)

println(sb)
// Output: -125.48845080566404 47.94508483691371,-124.96110705566404 42.309040799653665,-117.13884143066404 45.04173793121063,-118.36931018066404 48.93624688577435,-125.48845080566404 47.94508483691371

// val stringRepresentation = sb.toString()  // for further use as String data-type

Answer 3

您可以使用flatMap代替 Reduce。 它會幫助你。

        List<List<List<Double>>> boundingPolygon = List.of(List.of(List.of(-124.96110705566404, 42.309040799653665)
            , List.of(-117.13884143066404, 45.04173793121063)
            , List.of(118.36931018066404, 48.93624688577435)
    ));
        var l = boundingPolygon.stream().flatMap(lists -> lists.stream().flatMap(doubles -> doubles.stream())).collect(Collectors.toList());
        System.out.println(l);

它將打印如下所示的 output。

---

[-124.96110705566404, 42.309040799653665, -117.13884143066404, 45.04173793121063, 118.36931018066404, 48.93624688577435]

試試上面的代碼，這會對你有所幫助。