How to change the key value of inner map in c++? - c++

Question

I'm working on map and I have following nested map and initialized with some values:
map<string, map<int, int> > wordsMap;
map<int, int> innerMap;
map<int, int>::iterator iti;
for(int i = 2; i < argc; i++)
{
wordsMap[argv[i]].insert(pair<int, int>(0,0));
}
And after some processing I'm trying to change the content if inner map, I use following code:
while(some_condition)
{
i = 0
for( it = wordsMap.begin() ; it != wordsMap.end(); it++)
{
innerMap = it->second;
int cnt = count(words.begin(), words.end(), it->first);
if(cnt != 0){
wordsMap[it->first][i] = cnt;
}
}
i++;
}
In the above scenario, How to change the value of first key (i.e. "0") and its value used while initialization of the inner map with another key-value pair?

Answer 1

You can't change the key of an element in an std::map. Doing so would break ordering.
Instead, you must insert a new element in the map with the key you want, and delete the previous element from the map.

Answer 2

I'm not sure if I understand your intend. I assume you want to save,
<KEY : file_name, VALUE : <KEY : line, VALUE : words count>>
And you don't want to save second map if there is no words.
So, I wrote below code.
If you want to not present second map, just keep empty map through not inserting key-value.
Additionally, sincestd::map is an associative container, which means it is saved sorted based on Key value, you should try to avoid change the key value after saving it.
#include "stdafx.h"
#include <algorithm>
#include <string>
#include <iostream>
#include <vector>
#include <map>
using namespace std;
typedef std::map<int, int> WORDS_COUNT_MAP_T; //for line, words count
typedef std::map<string, WORDS_COUNT_MAP_T> FILE_WORDS_COUNT_MAP_T; //for file name, WORDS_COUNT_MAP_T
int _tmain(int argc, _TCHAR* argv[])
{
FILE_WORDS_COUNT_MAP_T file_words_count_map;
//Input dummy data for test
//init file names
std::vector<string> file_names;
file_names.push_back("first");
file_names.push_back("second");
file_names.push_back("third");
//get and set words count in each file
for_each(file_names.begin(), file_names.end(), [&](const string& file_name)
{
//Just for test
WORDS_COUNT_MAP_T words_count_map;
if(file_name == "second")
{
//not find words, so nothing to do
}
else
{
words_count_map[0] = 10;
words_count_map[1] = 20;
}
file_words_count_map.insert(FILE_WORDS_COUNT_MAP_T::value_type(file_name, words_count_map));
});
//print
for_each (file_words_count_map.begin(), file_words_count_map.end(), [&](FILE_WORDS_COUNT_MAP_T::value_type& file_words_map)
{
cout << "file name : " << file_words_map.first << endl;
WORDS_COUNT_MAP_T words_count_map = file_words_map.second;
for_each (words_count_map.begin(), words_count_map.end(), [](WORDS_COUNT_MAP_T::value_type& words_map)
{
cout << "line : " << words_map.first << ", count : " << words_map.second << endl;
});
cout << "----" << endl;
});
getchar();
return 0;
}
This code will print like below,