我有一個(gè)數(shù)組列表的數(shù)組列表，它存儲(chǔ)點(diǎn)之間的距離。在對(duì) arraylist 排序后，我需要保留距離的原始索引，因?yàn)槲倚枰诖a的后期找到 K 最近鄰。我實(shí)現(xiàn)的類沒有輸出正確的距離索引，而只是輸出了數(shù)組列表的索引我嘗試實(shí)現(xiàn)一個(gè) 2D 可比類，但出現(xiàn)了一些錯(cuò)誤。public staticArrayList<ArrayList<Double>>distance(ArrayList<sample_points> points) { ArrayList<ArrayList<Double>> distArray = new ArrayList<ArrayList<Double>>(points.size()); double dist = 0; List<Element> elements = new ArrayList<Element>(); for(int i = 0; i<points.size()-1; i++) { ArrayList<Double> distances = new ArrayList<Double>(); for(int j=i+1; j<points.size(); j++){ // do your calculations here dist = Math.sqrt(Math.pow(points.get(i).getX() - points.get(j).getX(), 2) + Math.pow(points.get(i).getY() - points.get(j).getY(), 2)); distances.add(dist);// add the distance to the current distances list } distArray.add(distances);// } System.out.print("Distances: "distArray); System.out.println(); for(int i = 0; i < distArray.size(); i++) { for (int j = 0; j < distArray.get(i).size(); j++) { elements.add(new Element(i, distArray.get(i).get(j))); } } Collections.sort(elements); for(int i = 0; i < elements.size(); i++) { System.out.println("Dist "+ elements.get(i).distance + " " + "Index " + elements.get(i).index+" "); }}這是我的可比類：class Element implements Comparable<Element>{ public final int index; public final double distance; public Element(int index, double distance){ this.index = index; this.distance = distance; } @Override public int compareTo(Element e){ return Double.valueOf(this.distance).compareTo(Double.valueOf(e.distance)); }}預(yù)期成績(jī)：Distances: [[2.8284271247461903, 5.830951894845301, 7.280109889280518], [3.1622776601683795, 5.0], [2.23606797749979]]Dist 2.23606797749979 Index 0 Dist 2.8284271247461903 Index 0 Dist 3.1622776601683795 Index 0 Dist 5.0 Index 1 Dist 5.830951894845301 Index 1 Dist 7.280109889280518 Index 2
1 回答
慕桂英546537
TA貢獻(xiàn)1848條經(jīng)驗(yàn) 獲得超10個(gè)贊
在審查您的代碼時(shí)，我認(rèn)為錯(cuò)誤出在您的假設(shè)中，并且輸出本身實(shí)際上是正確的。您看到的索引實(shí)際上是您在此處創(chuàng)建的列表嵌套數(shù)組列表列表中外部列表的索引：
for (int i=0; i<points.size()-1; i++) {
ArrayList<Double> distances = new ArrayList<Double>();
for (int j=i+1; j < points.size(); j++) {
// do your calculations here
dist = Math.sqrt(Math.pow(points.get(i).getX() - points.get(j).getX(), 2)
+ Math.pow(points.get(i).getY() - points.get(j).getY(), 2));
distances.add(dist); // add the distance to the current distances list
}
distArray.add(distances); // ***** the index of items added here *****
}
如果添加這些調(diào)試行：
for (int i=0; i < points.size()-1; i++) {
ArrayList<Double> distances = new ArrayList<Double>();
for (int j=i+1; j<points.size(); j++) {
// do your calculations here
dist = Math.sqrt(Math.pow(points.get(i).getX() - points.get(j).getX(), 2)
+ Math.pow(points.get(i).getY() - points.get(j).getY(), 2));
distances.add(dist);// add the distance to the current distances list
}
distArray.add(distances); // ***** the index of items added here *****
// ******* add this ********
System.out.println(distArray.indexOf(distances));
System.out.println(distances);
System.out.println();
}
你會(huì)發(fā)現(xiàn)這是正確的。您需要改變創(chuàng)建指數(shù)值的方式或改變您的假設(shè)。
此外，您的元素索引字段是不可變的，并且是在對(duì)距離 ArrayList 中的任何項(xiàng)目進(jìn)行排序之前創(chuàng)建的，因此該字段不能表示距離的排序順序。
您在評(píng)論中聲明：
如果我有一個(gè)距離的二維數(shù)組列表，即 [ [d1,d2,d3], [d4,d5], [d6] ]。指數(shù)將如下d1=0、d2=1、d3=2、d4=0、d5=1和d6=0。所以我需要在對(duì)每個(gè) arraylist 進(jìn)行排序后獲取原始索引。
然后用來j創(chuàng)建索引，不i
for(int i = 0; i < distArray.size(); i++) {
for (int j = 0; j < distArray.get(i).size(); j++) {
// elements.add(new Element(i, distArray.get(i).get(j)));
elements.add(new Element(j, distArray.get(i).get(j))); // ***** note change *****
}
}
旁注：將來，創(chuàng)建一個(gè)最小的可運(yùn)行程序來演示問題，我們可以輕松編譯和運(yùn)行。對(duì)于這個(gè)問題，我必須自己創(chuàng)建一個(gè)：
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class Foo01 {
public static void main(String[] args) {
List<SamplePoints> points = new ArrayList<>();
int maxXY = 100;
int max = 4;
for (int i = 0; i < max; i++) {
int x = (int) (maxXY * Math.random());
int y = (int) (maxXY * Math.random());
points.add(new SamplePoints(x, y));
}
distance(points);
}
private static class SamplePoints {
private int x;
private int y;
public SamplePoints(int x, int y) {
this.x = x;
this.y = y;
}
public int getX() {
return x;
}
public int getY() {
return y;
}
}
private static class Element implements Comparable<Element> {
public final int index;
public final double distance;
public Element(int index, double distance) {
this.index = index;
this.distance = distance;
}
@Override
public int compareTo(Element e) {
return Double.valueOf(this.distance).compareTo(Double.valueOf(e.distance));
}
}
public static void distance(List<SamplePoints> points) {
ArrayList<ArrayList<Double>> distArray = new ArrayList<ArrayList<Double>>(points.size());
double dist = 0;
List<Element> elements = new ArrayList<Element>();
for (int i = 0; i < points.size() - 1; i++) {
ArrayList<Double> distances = new ArrayList<Double>();
for (int j = i + 1; j < points.size(); j++) {
// do your calculations here
dist = Math.sqrt(Math.pow(points.get(i).getX() - points.get(j).getX(), 2)
+ Math.pow(points.get(i).getY() - points.get(j).getY(), 2));
distances.add(dist);// add the distance to the current distances
// list
}
distArray.add(distances);//
System.out.println(distArray.indexOf(distances));
System.out.println(distances);
System.out.println();
}
System.out.print("Distances: " + distArray);
System.out.println();
for (int i = 0; i < distArray.size(); i++) {
for (int j = 0; j < distArray.get(i).size(); j++) {
elements.add(new Element(i, distArray.get(i).get(j)));
}
}
Collections.sort(elements);
for (int i = 0; i < elements.size(); i++) {
System.out.println("Dist " + elements.get(i).distance + " " + "Index "
+ elements.get(i).index + " ");
}
}
}
但在未來，您將希望這樣做以幫助您的問題更容易回答。
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