从文件中查找单词

输出所有给定单词所在的行号和该行的内容。

用一个类来完成此功能TextQuery

#ifndef TEXTQUERY_H
#define TEXTQUERY_H

#include<iostream>
#include<fstream>
#include<sstream>
#include<string>
#include<vector>
#include<map>
#include<set>

using namespace std;

class TextQuery{
public:
    //typedef to make declarations easier
    typedef std::vector<std::string>::size_type line_no;
    /*interface:
     *read_file builds internal data structures for the given file
     *run_query finds the given word and returns  set of ines on which it appears
     *test_line returns a requested line from the input file
     */
    void read_file(std::ifstream &is)
    {
    store_file(is);
    build_map();
    }
    std::set<line_no> run_query(const std::string&) const;
    std::string text_line(line_no) const;
private:
    //utility functions used by read_file
    void store_file(std::ifstream&);        //store input file
    void build_map();                //associated each word with a set of line numbers
    //remember the whole input file
    std::vector<std::string> lines_of_text;
    //map word to set of the lines on which it occurs
    std::map<std::string,std::set<line_no> > word_map;
};
//read input file:store each line as element in lines_of_text
void TextQuery::store_file(std::ifstream &is)
{
    string textline;
    while(getline(is,textline))
    lines_of_text.push_back(textline);
}
//finds whitespace-seperated words in the input vector
//and puts the word in word_map along with the line number
void TextQuery::build_map()
{
    //process each line from the input vector
    for(line_no line_num=0;line_num!=lines_of_text.size();++line_num)
    {
    //we'll use line to read the text a word at a time
    istringstream line(lines_of_text[line_num]);
    std::string word;
    while(line>>word)
        //add this line number to the set
        //subscript will add to to map if it's not already there
        word_map[word].insert(line_num);
    }
}
set<TextQuery::line_no> TextQuery::run_query(const string &query_word) const
{
    //Note:must use find and not subscript the map directly
    //to avoid adding words to word_map!
    map<string,set<line_no> >::const_iterator loc=word_map.find(query_word);
    if(loc==word_map.end())
    return set<line_no>();            //not found,return empty set
    else
    //fetch and return set of line numbers for this word
    return loc->second;
}
string TextQuery::text_line(line_no line) const
{
    if(line<lines_of_text.size())
    return lines_of_text[line];
    throw;
}
#endif

主函数

#include"TextQuery.h"

void print_results(const set<TextQuery::line_no>&,const string&,const TextQuery&);
ifstream & open_file(ifstream &in,const string &file);
//program takes single argument spesifying the file to query
int main(int argc,char **argv)
{
    //open the file from which user will query words
    ifstream infile;
    if(argc<2||!open_file(infile,argv[1]))
    {
    cerr<<"No input file!"<<endl;
    return -1;
    }
    TextQuery tq;
    tq.read_file(infile);   //builds query map;
    //iterate with the user:prompt for a word to find and print result
    //loop indefinitely;the loop exits is inside the while
    while(true)
    {
    cout<<"enter word to look for,or q to quit: ";
    string s;
    cin>>s;
    //stop if hit eof on input or a 'q' is enterd
    if(!cin||s=="q")
        break;
    //get the set of line numbers on which this word appears
    set<TextQuery::line_no> loc=tq.run_query(s);
    //print count and all occurrences,if any
    print_results(loc,s,tq);
    }
    return 0;
}
//return plural version of word if ctr isn't 1
string make_plural(size_t ctr,const string &word,const string &ending)
{
    return (ctr==1)?word:word+ending;
}
ifstream & open_file(ifstream &in,const string &file)
{
    in.close();        //close in case it was already open
    in.clear();        //clear any existing errors
    //if the open fails,the stream will be in an invalid state
    in.open(file.c_str());   //open the file we were given
    return in;
}
void print_results(const set<TextQuery::line_no>& loc,const string& sought,const TextQuery& file)
{
    //if the word was found,then print count and all occurrences
    typedef set<TextQuery::line_no> line_nums;
    line_nums::size_type size=loc.size();
    cout<<"\n"<<sought<<" occurs "<<size<<" "<<make_plural(size,"time","s")<<endl;  //if size=1 then print "time",or not print "times"
    //print each line in which the word appeard
    line_nums::const_iterator it=loc.begin();
    for(;it!=loc.end();++it)
    {
    cout<<"\t(line "
        //don't confound user with text lines starting at 0
        <<(*it)+1<<")"<<file.text_line(*it)<<endl;
    }
}