sort()和stable_sort()都對container做sort的動作,但對於相等的值,sort()和stable_sort()處理的方式不一樣,stable_sort()會保證不更改原先的順序,但sort()則不保證,有可能更改順序,但也有可能不改,這樣講還是很籠統,若用SQL來解釋,就一目暸然了。
在SQL中,我們常有以下寫法
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SELECT *
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FROM [customers]
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ORDER BY [age],[name]
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我們想先依年齡排序,若年齡相同,則依姓名的字母排序
在STL我們該怎麼寫呢?
我們必須先對姓名排序,使用sort(),
然後再對年齡排序,使用stable_sort(),如此當年齡相同時,因為之前已經用姓名排序過了,stable_sort()將依照原先的排序不與改變,這樣剛好就對姓名排序了,而達到ORDER BY [age],[name]的要求。
sort()比stable_sort()速度快,若沒有stable的需求,應該考慮先使用sort()。
以下範例想先依字串長度排序,若長度相同,則依字母順序排序。
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/*
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(C) OOMusou 2006 http://oomusou.cnblogs.com
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Filename : GenericAlgo_stable_sort.cpp
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Compiler : Visual C++ 8.0 / ISO C++
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Description : Demo how to use stable_sort() algorithm
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Release : 12/10/2006
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*/
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#include <iostream>
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#include <algorithm>
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#include <string>
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#include <sstream>
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#include <vector>
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using namespace std;
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bool sortRule(const string&, const string&);
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int main() {
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string s = "to be or not to be is a question";
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// Transfer string to stringstream
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istringstream ss(s);
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// Copy stringstream to vector
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vector<string> svec;
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copy(istream_iterator<string>(ss), istream_iterator<string>(), back_inserter(svec));
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// Sort vector by alphabetic order for unique() algorithm
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sort(svec.begin(), svec.end());
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// Unique vector
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vector<string>::iterator iter = unique(svec.begin(),svec.end());
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svec.erase(iter, svec.end());
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// Stable sort vector by size
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stable_sort(svec.begin(), svec.end(), sortRule);
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// Copy vector to cout
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copy(svec.begin(), svec.end(), ostream_iterator<string>(cout, "\n"));
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return 0;
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}
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bool sortRule(const string& s1, const string& s2) {
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return s1.size() < s2.size();
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}
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執行結果
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a
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be
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is
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or
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to
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not
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question
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