[C++知识库]C++ stl 经典题目

练习3.1
写一个读取文本文件的程序，将文件中的每个单字存人map。map的key便是刚才所说的单字，map的value则是该单字在文本文件中的出现次数。再定义一份由“排除字眼”组成的有如a、an、or、the、 and和but之类的单字。将某单字放人map之前，先确定该单字并不在“排除字集”中。一旦文本文件读取完毕，请显示一份单字清单，并显示各单字的出现次数。你甚至可以再加以扩展，在显示单字之前，允许用户查询某个单字是否出现于文本文件中。
扩展已实现：
代码如下：

#include <map>
#include <set>
#include <string>
#include <iostream>
#include <fstream>
using namespace std;
void initialize_exclusion_set( set<string>& );
void process_file( map<string,int>&, const set<string>&, ifstream& );
void user_query( const map<string,int>& );
void display_word_count( const map<string,int>&, ofstream& );


int main()
{
    ifstream ifile( "C:\\Users\\liukun\\Desktop\\old.txt" );
    ofstream ofile( "C:\\Users\\liukun\\Desktop\\new.txt" );
    if ( ! ifile || ! ofile )
    {
        cerr << "Unable to open file -- bailing out!\n";
        return -1;
    }
    set<string> exclude_set;
    initialize_exclusion_set( exclude_set );
    map<string,int> word_count;
    process_file( word_count, exclude_set, ifile );
    user_query( word_count );
    display_word_count( word_count, ofile );

}

void initialize_exclusion_set( set<string> &exs )
{
    static string _excluded_words[25] =
    {
        "the","and","but","that","then","are","been",
        "can","a","could","did","for", "of",
        "had","have","him","his","her","its","is",
        "were","which","when","with","would"
    };
    exs.insert( _excluded_words, _excluded_words+25 );
}

void process_file( map<string,int> &word_count,
                   const set<string> &exclude_set, ifstream &ifile )
{
    string word;
    while ( ifile >> word )
    {
        if ( exclude_set.count( word ))
            continue;
        word_count[ word ]++;
    }
}

void user_query( const map<string,int> &word_map )
{
    string search_word;
    cout << "Please enter a word to search: q to quit";
    cin >> search_word;
    while ( search_word.size() && search_word != "q" )
    {
        map<string,int>::const_iterator it;
        if (( it = word_map.find( search_word ))
                != word_map.end() )
            cout << "Found! " << it->first
                 << " occurs " << it->second
                 << " times.\n";
        else
            cout << search_word
                 << " was not found in text.\n";
        cout << "\nAnother search? (q to quit) ";
        cin >> search_word;
    }
}

void display_word_count( const map<string,int> &word_map, ofstream &os )
{
    map<string,int>::const_iterator
    iter = word_map.begin(),
    end_it = word_map.end();
    while ( iter != end_it )
    {
        os << iter->first << " ( "
           << iter->second << " )" << endl;
        ++iter;
    }
    os << endl;
}

记住要先创建好文件！（new.txt可以不用）,old.txt文件里面不填任何东西都没事，当然这里面写点东西才更贴近题目本意。