优化boost无序映射和集合,C++

Question

我将解析60GB的文本并在地图中进行大量的插入和查找.我刚开始使用boost :: unordered_set和boost :: unordered_map当我的程序开始填充这些容器时,它们开始变得越来越大,我想知道为这些容器预先分配内存是否是一个好主意.像mymap :: get_allocator().allocate(N); ？

或者我应该让他们自己分配并找出成长因素？代码看起来像这样

boost::unordered_map <string,long> words_vs_frequency, wordpair_vs_frequency;   
boost::unordered_map <string,float> word_vs_probability, wordpair_vs_probability,
           wordpair_vs_MI;                  
//... ... ...                                   

N = words_vs_frequency.size();
long   y =0; float MIWij =0.0f, maxMI=-999999.0f;
for (boost::unordered_map <string,long>::iterator i=wordpair_vs_frequency.begin(); 
                     i!=wordpair_vs_frequency.end(); ++i){
if (i->second >= BIGRAM_OCCURANCE_THRESHOLD)
    {
    y++;
    Wij = i->first;
    WordPairToWords(Wij, Wi,Wj);
    MIWij =  log ( wordpair_vs_probability[Wij] /
             (word_vs_probability[Wi] * word_vs_probability[Wj]) 
            );

    // keeping only the pairs which MI value greater than 
    if (MIWij > MUTUAL_INFORMATION_THRESHOLD)
        wordpair_vs_MI[ Wij ] = MIWij;
    if(MIWij > maxMI )
        maxMI = MIWij; 
    }

   }

提前致谢

Answer 1

根据该文件,都unordered_set和unordered_map有一个方法

void rehash(size_type n);

重新生成哈希表,使其至少包含n桶.(听起来它对reserve()STL容器有什么作用).