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POFirst N values of a Map<K, V> sorted by value
 

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    <p>I have a list of Strings. I want to evaluate each string based on a function that returns a double. Then I want the first 5 strings, based on their calculated values. If there are fewer than 5, I want all of them (in order). Let's say the strings are chemical compounds and the function computes the mass. The function is computationally expensive; I need to evaluate it once per string. (I'm just making up data here, though.)</p> <pre><code>H2O =&gt; 18.5 C12H11O22 =&gt; 109.1 HeNe =&gt; 32.0 H2SO4 =&gt; 54.37 HCl =&gt; 19.11 4FeO3 =&gt; 82.39 Xe6 =&gt; 281.9 </code></pre> <p>The program should return the first five strings arranged in order by their respective values. For this sample data: <code>H20, HCl, HeNe, H2SO4, 4FeO3</code>. Actually, I don't really care about the order; I just need the five lowest in any order.</p> <p>I thought about how I'd do this in Perl. It's just a few lines:</p> <pre><code>foreach $s (@str) { $strmap{$s} = f($s); } @sorted = sort { $strmap{$a} &lt;=&gt; $strmap{$b} } keys %strmap; return @sorted[0, 4] </code></pre> <p>But I need to do it in Java. And it's driving me crazy.</p> <p>First I tried populating a <code>HashMap&lt;String, Double&gt;</code>, then using <code>Collections.sort</code> with a custom comparator, just like the Perl version. But scoping on the Comparator prevented it from referring to the HashMap to look up the values.</p> <p>Then I tried a <code>TreeMap&lt;String, Double&gt;</code>, but it only sorts by key and no amount of coercing could get it to order the entries by value.</p> <p>So I tried a <code>TreeMap&lt;Double, String&gt;</code>. It will discard entries with the same Double. However, the likelihood of having Strings that map to the same Double is low, so I pressed forward. Adding the entries to the TreeMap is no problem, but I ran into issues trying to extract the values from it.</p> <p>TreeMap supplies a method called <code>subMap</code>, but its parameters are the keys that delimit the subset. I don't know what they are; I just want the first five of them. So I tried using the <code>values</code> method to get all the values out of the TreeMap, hoping they'd be in order. Then I can just get the first ten.</p> <pre><code>ArrayList&lt;String&gt; strs = (ArrayList&lt;String&gt;)(treemap.values()); return new ArrayList&lt;String&gt;(strs.subList(0, 5)); </code></pre> <p>Nope. Runtime error: cannot cast TreeMap$Values to ArrayList.</p> <pre><code>List&lt;String&gt; strs = (List&lt;String&gt;)(treemap.values()); return new ArrayList&lt;String&gt;(strs.subList(0, 5)); </code></pre> <p>Same. Runtime error trying to do the cast. OK, let's just assign to a Collection...</p> <pre><code>Collection&lt;String&gt; strs = treemap.values(); return new ArrayList&lt;String&gt;(strs.subList(0, 5)); </code></pre> <p>Sorry, <code>subList</code> isn't a method of Collection.</p> <pre><code>Collection&lt;String&gt; strs = treemap.values(); ArrayList&lt;String&gt; a = new ArrayList&lt;String&gt;(strs); return new ArrayList&lt;String&gt;(a.subList(0, 5)); </code></pre> <p>Finally, something that works! But two extra data structures just to get the first five elements? And I'm not too wild about using Double as the key for TreeMap.</p> <p>Is there a better solution?</p>
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    1. USBarry Brown
    1. USBarry Brown
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    1. COCould you please provide some samples to understand the question better
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      1. USasifsid88
    2. COSample data? Or samples code of the things I tried?
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    3. COBy sample data I mean given a set of inputs what is the expected output
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      1. USasifsid88
 

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