我如何计算 Arraylist 中的每个唯一项？

Question

我需要计算 Arraylist 中的每个唯一字符。我已经将每个字符分开。

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.util.ArrayList;

public class Aschenputtel {
    public static void main(String args[]) {
        ArrayList <String> txtLowCase = new ArrayList <String> ();
        ArrayList <Character> car = new ArrayList <Character> ();
        
        File datei = new File ("C:/Users/Thomas/Downloads/Aschenputtel.txt");
        Scanner scan = null;
        try {
                scan = new Scanner (datei);
        } catch (FileNotFoundException e) {
            System.out.println("File not found.");
        }
        while (scan.hasNext()) {
            String temp = scan.next().replace("„", "„").replace("“", "“").toLowerCase();
            txtLowCase.add(temp);
            for(int i = 0; i < temp.length(); i++) {
                car.add(temp.charAt(i));
            }
        }
        System.out.println(car);
    }
}

那是我当前的代码。 car 目前给出了每个字符，但结果应该是这样的：a = 16, b = 7, c = 24,....有没有好的方法来做到这一点？

Answer 1

一旦你有了你的character ，你就可以在你的 for 循环中做一些类似的事情：

... 
Map<Character, Integer> map2=new HashMap<Character, Integer>();
     for (int i = 0; i < temp.length(); i++) {
        map2.put(temp.charAt(i), map2.getOrDefault(temp.charAt(i), 0)+1);
   }
    System.out.println(map2);
...

Answer 2

您已经了解了算法的第一部分，即数据处理：

1. 处理数据，从文本文件到 ArrayList 个字符， car 。
2. 计算列表中的字符。

您想要将每个字符关联到一个计数。 一个 HashMap 就可以了。

这是第二部分的方法，有一些解释：

/*
This will return something that looks like:
{ a: 16, b: 7, c: 24, ... }
*/
HashMap<Character, Int> getCharacterCount(ArrayList<Character> charList) {
  // for each character we associate an int, its count.
  HashMap<Character, Int> counter = new Hashmap<>();

  for (Character car: charList) {   
    // if the map doesn't contain our character, we've never seen it before.          
    int currentCount = counter.containsKey(car) ? counter.get(car) : 0;
    // increment this character's current count
    counter.put(car, currentCount + 1);
  }

  return counter;
}

Answer 3

如果您需要计算汽车列表中的字符数，您可以使用集合 stream 和聚合操作groupingby结合下游counting 。

此操作将产生一个Map ，其键是列表的字符，它们的对应值是它们在列表中的出现。

示例代码

List<Character> list = new ArrayList<>(List.of('a', 'a', 'a', 'b', 'c', 'b', 'b', 'b', 'a', 'c'));
Map<Character, Long> mapRes = list.stream().collect(Collectors.groupingBy(c -> c, Collectors.counting()));
System.out.println(mapRes);

你的代码

public class Aschenputtel {
    public static void main(String args[]) {
        ArrayList<String> txtLowCase = new ArrayList<String>();
        ArrayList<Character> car = new ArrayList<Character>();

        File datei = new File("C:/Users/Thomas/Downloads/Aschenputtel.txt");
        Scanner scan = null;
        try {
            scan = new Scanner(datei);
        } catch (FileNotFoundException e) {
            System.out.println("File not found.");
        }
        while (scan.hasNext()) {
            String temp = scan.next().replace("„", "„").replace("“", "“").toLowerCase();
            txtLowCase.add(temp);
            for (int i = 0; i < temp.length(); i++) {
                car.add(temp.charAt(i));
            }
        }

        Map<Character, Long> mapRes = car.stream().collect(Collectors.groupingBy(c -> c, Collectors.counting()));
        System.out.println(mapRes);
    }
}

Answer 4

这是最简单的方法，在初始化时将数组列表传递给 HashSet。

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.util.ArrayList;

public class Aschenputtel {
    public static void main(String args[]) {
        ArrayList <String> txtLowCase = new ArrayList <String> ();
        ArrayList <Character> car = new ArrayList <Character> ();
        
        File datei = new File ("C:/Users/Thomas/Downloads/Aschenputtel.txt");
        Scanner scan = null;
        try {
                scan = new Scanner (datei);
        } catch (FileNotFoundException e) {
            System.out.println("File not found.");
        }
        while (scan.hasNext()) {
            String temp = scan.next().replace("„", "„").replace("“", "“").toLowerCase();
            txtLowCase.add(temp);
            for(int i = 0; i < temp.length(); i++) {
                car.add(temp.charAt(i));
            }
        }
        HashSet<String> hset = new HashSet<String>(ArrList);  // Only adds an element if it doesn't exists prior.
        System.out.println("ArrayList Unique Values is : " + hset);  // All the unique values.
        System.out.println("ArrayList Total Coutn Of Unique Values is : " + hset.size());  // Count of all unique items.
    }
}

Answer 5

如果你想有更多的控制/可定制性，你也可以这样做：

 private static void calcCount(List<Character> chars) {
    chars.sort(Comparator.naturalOrder());

    Character prevChar = null;
    int currentCount = 0;
    for (Character aChar : chars) {
      if (aChar != prevChar) {
        if (prevChar != null) {
          System.out.print(currentCount + " ");
          currentCount = 0;
        }
        System.out.print(aChar + ":");
        prevChar = aChar;
      }
      currentCount++;
    }

    System.out.print(currentCount);
  }