STL实践与分析

--map类型（下）

六、查找并读取map中的元素

map容器提供了两个操作：count和find，用于检查某个键是否存在而不会插入该键：

1、使用count检查map对象中某键是否存在

因为map容器只允许一个键对应一个实例，所以，对于map对象，count成员的返回值只能是1或0。

如果返回值非0，则可以使用下标操作符来获取该键所关联的值，而不必担心这样会在map容器中插入新元素：

    int occurs = 0;
    if (word_cnt.count("Dream"))
    {
        occurs = word_cnt["Dream"];
    }
    cout << occurs << endl;

2、读取元素而不插入元素

    int occurs = 0;
    map<string,int>::iterator iter = word_cnt.find("Dream");
    if (iter != word_cnt.end())
    {
        occurs = iter -> second;
    }
    cout << occurs << endl;

//P316 习题10.16
map<string,vector<int> > str_vec;
map<string,vector<int> >::iterator iter = str_vec.find("dream");

七、从map对象中删除元素

    int remCount = 0;
    if (remCount = word_cnt.erase("Dream"))
    {
        cout << "Have removed " << remCount << " words" << endl;
    }
    else
    {
        cout << "Not found!" << endl;
    }

erase函数返回被删除元素的个数。对于map容器，该值必然是0或者1。如果返回0值，则表示该元素并不存在于map容器中。

八、map对象的迭代遍历

与其他容器一样，map同样提供了begin和end运算，以生成用于遍历整个容器的迭代器：

    cout << "First:\t\tSecond:" << endl;
    map<string,int>::iterator iter = word_cnt.begin();
    while (iter != word_cnt.end())
    {
        cout << iter -> first << "\t\t" << iter -> second << endl;
        ++ iter;
    }

或：

    cout << "First:\t\tSecond:" << endl;
    typedef map<string,int>::iterator mapIter;
    for (mapIter iter = word_cnt.begin(); iter != word_cnt.end(); ++iter)
    {
        cout << iter -> first << "\t\t" << iter -> second << endl;
    }

在使用迭代器遍历map容器时，迭代器指向的元素按照键的升序排列。

九、“单词转换”map对象

1、问题：

给出一个string对象,把它转换为另一个string对象。本程序的输入是两个文件。第一个文件包括了若干单词对,每对的第一个单词将出现在输入的字符串中,而第二个单词则是用于输出。本质上,这个文件提供的是单词转换的集合——在遇到第一个单词时,应该将之替换为第二个单词。第二个文件则提供了需要转换的文本。如果单词转换文件的内容是:

'em them
cuz because
gratz grateful
i I
nah no
pos supposed
sez said
tanx thanks
wuz was

2、测试样例

而要转换的文本是:

	nah i sez tanx cuz i wuz pos to
	not cuz i wuz gratz 

则程序将产生如下输出结果:

	no I said thanks because I was supposed to
	not because I was grateful

4、思路：

将被替换的单词作为键，而用作的替换的单词则作为其相应要替换的值。

5、程序：

#include <iostream>
#include <sstream>
#include <fstream>
#include <map>
using namespace std;

int main()
{
    ifstream inFile("input1");
    string line,firWord,secWord;
    map<string,string> convMap;

    while (getline(inFile,line))
    {
        istringstream strItem(line);
        while (strItem >> firWord >> secWord)
        {
            convMap.insert(make_pair(firWord,secWord));
        }
    }

    inFile.close();
    inFile.clear();
    inFile.open("input2");

    string word;
    while (getline(inFile,line))
    {
        istringstream strItem(line);
        while (strItem >> word)
        {
            map<string,string>::iterator iter = convMap.find(word);
            if (iter != convMap.end())
            {
                cout << iter -> second << ' ';
            }
            else
            {
                cout << word << ' ';
            }
        }
        cout << endl;
    }
}

//附上书上的原程序
int main(int argc, char **argv)
{
    map<string, string> trans_map;
    string key, value;
    if (argc != 3)
        throw runtime_error("wrong number of arguments");

    ifstream map_file;
    if (!open_file(map_file, argv[1]))
        throw runtime_error("no transformation file");

    while (map_file >> key >> value)
        trans_map.insert(make_pair(key, value));

    ifstream input;
    if (!open_file(input, argv[2]))
        throw runtime_error("no input file");
    string line;
    while (getline(input, line))
    {
        istringstream stream(line);
        string word;
        bool firstword = true;
        while (stream >> word)
        {
            map<string, string>::const_iterator map_it =
                trans_map.find(word);
            if (map_it != trans_map.end())
                word = map_it->second;
            if (firstword)
                firstword = false;
            else
                cout << " ";
            cout << word;
        }
        cout << endl;
    }
    return 0;
}

//P319 习题10.18
#include <iostream>
#include <fstream>
#include <string>
#include <map>
#include <vector>
using namespace std;

int main()
{
    typedef string Family;
    typedef vector<string> Names;
    map<Family,Names> pepMap;

    Family first;
    string second;
    ifstream inFile("input");
    while (inFile >> first >> second)
    {
        Names child;
        pair<map<Family,Names>::iterator,bool> res = pepMap.insert(make_pair(first,child));
        res.first -> second.push_back(second);
    }

    inFile.close();
    inFile.clear();
    inFile.open("test");

    string searchFamily;
    while (inFile >> searchFamily)
    {
        map<Family,Names>::iterator iter = pepMap.find(searchFamily);

        if (iter == pepMap.end())
        {
            cout << "NO this Family name" << endl;
        }
        else
        {
            cout << iter -> first << ":" << endl;
            Names::iterator itName = iter -> second.begin();
            while (itName != iter -> second.end())
            {
                cout << *itName << endl;
                ++ itName;
            }
            cout << endl;
        }
    }
}