如何从现有地图获取给定双精度值的最接近值的键< String double> [英] how to get key of the most nearest value of a given double value from an existing map <String double>

问题描述

因为我需要使用 BiMap 获取double值的关键值。

  BiMap< String，Double> mapObj = HashBiMap.create（）; 
 mapObj.put（a1，3.58654）; 
 mapObj.put（a2，4.1567）; 
 mapObj.put（a3，4.2546）; 
  

对于像4.0156这样的特殊值，我必须得到键值a2 ..也就是说， p>

  Double value = 4.0156; 
 mapObj.inverse（）。get（value）= a2 
  

我尝试了很多方法，它始终为空，因为没有完全匹配。请任何人都帮助我...如果我选择了错误的方式，请更正它，因为我是Java新手。

首先：您可能想要：

 地图< String，Double> mapObj = HashMap<>（）; 
 mapObj.put（a1，3.58654）; 
 mapObj.put（a2，4.1567）; 
 mapObj.put（a3，4.2546）; 
 mapObj.put（a4，4.1567）; //重复值
  

然后您需要一个具有最接近值的反向查找。

为此，最好让所有条目按值排序。

 列表< Map.Entry< String，Double>>这不能是一个Set，因为这个值会出现多次。 entries = new ArrayList<>（mapObj.entrySet（））; 
比较器< Map.Entry< String，Double>> cmp =（1hs，rhs） - > 
 Double.compare（lhs.getValue（），rhs.getValue（））; 
 Collections.sort（entries，cmp）; 
  

我知道Java中没有包含这种结构的数据结构。虽然可能有。
为了不丢失信息我使用Map.Entry，键值对。这需要一个比较值的值。
为简单起见，我在这里借用了Java 8语法。

现在搜索：

  Map.Entry< String，Double>最近（double值）{
 int index = Collections.binarySearch（entries，cmp）; 
 if（index< 0）{//未找到
 index = -index + 1; //插入位置
 Map.Entry< String，Double> before = index！= 0？ entries.get（i  -  1）：null; 
 Map.Entry< String，Double> after = index< entries.size（）？ 
 entries.get（i）：null; 
 if（before == null&&== after null）{
 return null; 
} else if（before == null）{
 return after; 
} else if（after == null）{
 return before; 
} 
返回值--VESTVALUE（）< after.getValue（） - 值？之前：之后; 
} 
返回entries.get（index）; 
 
 $ / code> 

要在增量内查找值的子列表，需要使用索引。

现在每个搜索的费用都是N，这是可以接受的。

Since I need to get the key values of double value I' m using BiMap.

BiMap<String,Double>mapObj = HashBiMap.create();
mapObj.put("a1",3.58654);
mapObj.put("a2",4.1567);
mapObj.put("a3",4.2546);

For a particular value like 4.0156 I must get the key value a2.. that is if,

Double value=4.0156; 
mapObj.inverse().get(value)=a2 

I tried many ways but its getting always null, since there is no exact match. please anyone help me... if I have chosen a wrong way please correct it because I'm new in Java.

解决方案

First: you probably want to have:

Map<String, Double> mapObj = HashMap<>();
mapObj.put("a1", 3.58654);
mapObj.put("a2", 4.1567);
mapObj.put("a3", 4.2546);
mapObj.put("a4", 4.1567); // Repeated value

And then you want a reverse lookup with nearest value.

For that it would be nice to have all entries sorted by value. This cannot be a Set because of a value occuring multiple times.

List<Map.Entry<String, Double>> entries = new ArrayList<>(mapObj.entrySet());
Comparator<Map.Entry<String, Double>> cmp = (lhs, rhs) ->
    Double.compare(lhs.getValue(), rhs.getValue());
Collections.sort(entries, cmp);

I know of no data structure in Java that combines this. Though there probably is. To not lose information I use the Map.Entry, key-value pair. That needs a Comparator on the values. For shortness, I have borrowed from Java 8 syntax here.

Now search:

Map.Entry<String, Double> nearest(double value) {
    int index = Collections.binarySearch(entries, cmp);
    if (index < 0) { // Not found
        index = -index + 1; // The insertion position
        Map.Entry<String, Double> before = index != 0 ? entries.get(i - 1) : null;
        Map.Entry<String, Double> after = index < entries.size() ?
                entries.get(i) : null;
        if (before == null && after == null) {
            return null;
        } else if (before == null) {
            return after;
        } else if (after == null) {
            return before;
        }
        return value - before.getValue() < after.getValue() - value ? before : after;
    }
    return entries.get(index);
}

To find a sub list of values inside a delta, one would need to use the index.

Now every search costs ²log N, which is acceptable.

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