Operation: Iteration

参考文章：

1) stackoverflow C++ Loop through Map

2) thispointer How to Iterate over a map in C++

使用普通的iterator

关于map的iterator，参见

使用for

map<string, int>::iterator it;

for ( it = symbolTable.begin(); it != symbolTable.end(); it++ )
{
    std::cout << it->first  // string (key)
              << ':'
              << it->second   // string's value 
              << std::endl ;
}

使用while

#include <iostream>
#include <map>
#include <string>
#include <iterator>
#include <algorithm>

int main() {

    std::map<std::string, int> mapOfWordCount;
    // Insert Element in map
    mapOfWordCount.insert(std::pair<std::string, int>("first", 1));
    mapOfWordCount.insert(std::pair<std::string, int>("second", 2));
    mapOfWordCount.insert(std::pair<std::string, int>("third", 3));
    mapOfWordCount.insert(std::pair<std::string, int>("third", 4));
    mapOfWordCount.insert(std::pair<std::string, int>("third", 5));

    // Create a map iterator and point to beginning of map
    std::map<std::string, int>::iterator it = mapOfWordCount.begin();

    // Iterate over the map using Iterator till end.
    while (it != mapOfWordCount.end())
    {
        // Accessing KEY from element pointed by it.
        std::string word = it->first;

        // Accessing VALUE from element pointed by it.
        int count = it->second;

        std::cout << word << " :: " << count << std::endl;

        // Increment the Iterator to point to next entry
        it++;
    }
    return 0;
}

Iterating over the map using C++11 range based for loop

参见Range-based for loop

#include <map>
#include <string>
#include <iterator>
#include <algorithm>

int main() {

    std::map<std::string, int> mapOfWordCount;
    // Insert Element in map
    mapOfWordCount.insert(std::pair<std::string, int>("first", 1));
    mapOfWordCount.insert(std::pair<std::string, int>("second", 2));
    mapOfWordCount.insert(std::pair<std::string, int>("third", 3));
    mapOfWordCount.insert(std::pair<std::string, int>("third", 4));
    mapOfWordCount.insert(std::pair<std::string, int>("third", 5));

    // Create a map iterator and point to beginning of map
    std::map<std::string, int>::iterator it = mapOfWordCount.begin();

    // Iterate over the map using c++11 range based for loop
    for (std::pair<std::string, int> element : mapOfWordCount) {
        // Accessing KEY from element
        std::string word = element.first;
        // Accessing VALUE from element.
        int count = element.second;
        std::cout << word << " :: " << count << std::endl;
    }

    return 0;
}

因为map中的元素的类型是std::pair。

使用type inference

for (auto const& x : symbolTable)
{
    std::cout << x.first  // string (key)
              << ':' 
              << x.second // string's value 
              << std::endl ;
}

使用Structured binding

for( auto const& [key, val] : symbolTable )
{
    std::cout << key         // string (key)
              << ':'  
              << val        // string's value
              << std::endl ;
}

Iterating over the map using std::for_each and lambda function

#include <iostream>
#include <map>
#include <string>
#include <iterator>
#include <algorithm>

int main() {

    std::map<std::string, int> mapOfWordCount;
    // Insert Element in map
    mapOfWordCount.insert(std::pair<std::string, int>("first", 1));
    mapOfWordCount.insert(std::pair<std::string, int>("second", 2));
    mapOfWordCount.insert(std::pair<std::string, int>("third", 3));
    mapOfWordCount.insert(std::pair<std::string, int>("third", 4));
    mapOfWordCount.insert(std::pair<std::string, int>("third", 5));

    // Create a map iterator and point to beginning of map
    std::map<std::string, int>::iterator it = mapOfWordCount.begin();

    // Iterate over a map using std::for_each and Lambda function
        std::for_each(mapOfWordCount.begin(), mapOfWordCount.end(),
                [](std::pair<std::string, int> element){
                    // Accessing KEY from element
                    std::string word = element.first;
                    // Accessing VALUE from element.
                    int count = element.second;
                    std::cout<<word<<" :: "<<count<<std::endl;
        });

    return 0;
}

这种写法有些类似于函数式编程。