英文:
analysing the proportion of names in a set of names. my output is incorrect. ("John") should return 0.25. and mary should be 0.5
问题
以下是翻译好的内容：
public class Problem{private int count;private final HashMap<String, Integer> counts;public Problem() {this.counts = new HashMap<>();}public void addName(String name){Integer nameCount = counts.get(name);if (nameCount == null){nameCount = 1;counts.put(name, nameCount);nameCount++;}nameCount++;count++;}// 这个函数应该计算比例public double nameProportion(String name){return counts.get(name) / (double) count;}public static void main(String[] args){Problem namesCount = new Problem();namesCount.addName("James");namesCount.addName("John");namesCount.addName("Mary");namesCount.addName("Mary");System.out.println("约翰的比例: " + namesCount.nameProportion("John"));System.out.println("玛丽的比例: " + namesCount.nameProportion("Mary"));}}
目前输出结果是不正确的，由于某种原因没有将两个玛丽视为同一人。
英文:
my output is currently ("john") returns 0.25 and ("mary") also returns 0.25
public class Problem{private int count;private final HashMap<String, Integer> counts;public Problem() {this.counts = new HashMap<>();}public void addName(String name){Integer nameCount = counts.get(name);if (nameCount == null){nameCount = 1;counts.put(name, nameCount);nameCount++;}nameCount++;count++;}
this function should calculate the proportion
public double nameProportion(String name){return counts.get(name) / (double) count;}public static void main(String[] args){Problem namesCount = new Problem();namesCount.addName("James");namesCount.addName("John");namesCount.addName("Mary");namesCount.addName("Mary");System.out.println("Fraction of Johns: " + namesCount.nameProportion("John"));System.out.println("Fraction of Marys: " + namesCount.nameProportion("Mary"));}}
right now the output is incorrect, for some reason its not seeing the two marys as the same
答案1
得分: 0
原始类型和原始包装类型在执行nameCount++操作时并不是不可变的，它不会改变哈希映射中键的值。它会创建另一个整数对象，并将nameCount + 1的值放入该对象中，因此在您的代码中，哈希映射中所有键的值都是1。
英文:
Primitive and primitive wrapper are not immutable when you do nameCount++, it doesn't change the value of key in hashmap. It create another Integer object and put the value of nameCount +1 into that object, so the value of all key in hashmap in your code is 1.
答案2
得分: 0
这一切都在你的函数 Problem#addName 中。实际上，你是在函数的末尾增加了 nameCount，但是当函数到达块的末尾时，增加后的值就丢失了（nameCount 是一个局部变量，不是成员变量）。
解决方法是，如果 nameCount 不为 null，就将其递增，并将其放入 counts 映射中，示例如下：
public void addName(String name) {Integer nameCount = counts.get(name);if (nameCount == null) {counts.put(name, 1);} else {counts.put(name, ++nameCount);}count++;}
如果为 null，则只需将 1 放入映射中，对应于相应的 name。
递增 count 是可以的，因为那是一个成员变量，并且在创建的 namesCount 引用到 Problem 对象中得以保存。
英文:
It's all in your function Problem#addName. You are actually incrementing your nameCount at the end of the function, but the incremented value is lost when the function reaches the end of the block (nameCount is a local variable, not a member variable).
The solution to this is to increment nameCount if it is not null and put that into the counts map, like so for example:
public void addName(String name) {Integer nameCount = counts.get(name);if (nameCount == null) {counts.put(name, 1);} else {counts.put(name, ++nameCount);}count++;}
If it is null, then simply a 1 is put into the map for the corresponding name.
Incrementing the count is fine, because that is a member variable and is saved within the created namesCount reference to the Problem object.
答案3
得分: 0
你如果名称已经存在于Map中，则不会对其进行更新。需要注意的是，Integer对象是不可变的；如果你不想不断丢弃先前的对象，可以使用AtomicInteger来代替。 <sup>演示</sup>
public void addName(String name) {Integer nameCount = counts.get(name);if (nameCount == null) {nameCount = 1;counts.put(name, nameCount);} else {counts.put(name, nameCount + 1);}count++;}
你可以使用Map#getOrDefault来简化这个过程。 <sup>演示</sup>
public void addName(String name) {counts.put(name, counts.getOrDefault(name, 0) + 1);count++;}
英文:
You never update the Map if the name already exists in it. It is important to note that Integer objects are immutable; you can use AtomicInteger instead if you do not want to keep discarding previous objects. <sup>Demo</sup>
public void addName(String name) {Integer nameCount = counts.get(name);if (nameCount == null) {nameCount = 1;counts.put(name, nameCount);} else {counts.put(name, nameCount + 1);}count++;}
This can be simplified using Map#getOrDefault. <sup>Demo</sup>
public void addName(String name) {counts.put(name, counts.getOrDefault(name, 0) + 1);count++;}
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