Composite map: take data from another Map - c++

Question

I need to take the occurrences of words taken from a file, using map<string,int>, and then I need to copy them to a map<int,
vector<string>, cmpDec >, and print them in decreasing order.
I tried to take word frequencies from a file to a map<string, int> and then I'm trying to copy it to a map<int,
vector<string> > with no results
I have declared 2 maps:
map<string, int> text;
map<int, vector<string>, cmpDec> freq;
I take the text from a file in the first map with the word frequencies:
while (rf >> words) {
text[words]++;
}
Now I have to put the frequencies in the second map (required), where I need to have first int, for num of word frequencies, vector with the words for each freq, and the compare for decreasing frequencies.
Now i'm trying to put the datas in the second map from the first in these ways:
map<string, int>::iterator iter_map1 = text.begin();
map<int, vector<string>>::iterator iter = freq.begin();
vector<string>::iterator iter_v;
for (; iter_map1 != text.end(); ++iter_map1) {
iter->first.insert(make_pair(iter_map1->second, iter->second.push_back(iter_map1->first)));
}
It gives 2 errors on the iter->second.... line:
...\BagOfWords.cpp|56|error: request for member 'insert' in 'iter.std::_Rb_tree_iterator<_Tp>::operator-><std::pair<const int, std::vector<std::__cxx11::basic_string<char> > > >()->std::pair<const int, std::vector<std::__cxx11::basic_string<char> > >::first', which is of non-class type 'const int'|
and
...\BagOfWords.cpp|56|error: invalid use of void expression|
What am I doing wrong? Is there an easier way to take words (and their frequencies) from a file and put them on the second map without passing from the first?

Answer 1

With C++17 you can do structured binding, which helps a lot when iterating through a map.
#include <map>
#include <vector>
#include <string>
#include <iostream>
using WordCounts = std::map<std::string, int>;
using FrequencyOfWords = std::map<int, std::vector<std::string>, std::greater<int>>;
int main()
{
WordCounts word_counts;
FrequencyOfWords words_freq;
std::vector<std::string> words = {"test", "hello", "test", "hello", "word"};
for(const auto& word : words)
word_counts[word]++;
for(const auto& [word, count] : word_counts)
words_freq[count].push_back(word);
for (const auto& [freq, words] : words_freq)
{
std::cout << "freq " << freq << " words";
for (auto const& word: words)
std::cout << " " << word;
std::cout << '\n';
}
}

Answer 2

I don't think that you can do this in one pass as you don't know the word counts upfront.
First, a couple of recommendations. Use typedef (or using for C++ 11 or later). This will save you some typing and also ensure that your types are correct. In your code freq and iter don't have the same underlying container type (they differ in the comparison used).
Secondly, try to use the standard library as much as possible. You don't show cmpDec but I guess that it is a comparator based on greater-than rather than the default less-than. I would prefer to see std::greater<int> rather than a custom comparator.
For your errors, in the line
iter->first.insert(...
iter is at the start of freq and you are trying to insert to first which is int.
This should probably be something like
freq[iter_map1->second].push_back(iter_map1->first);
Breaking that down
freq[iter_map1->second] This uses the int word count from text to lookup an entry in freq. If there is no entry an empty one will be inserted to freq.
.push_back(iter_map1->first) This inserts the string from text to the vector that was found or created in the previous step
Here is a full example of what I think you are trying to achieve.
#include <map>
#include <vector>
#include <string>
#include <functional>
#include <fstream>
#include <iostream>
using std::map;
using std::vector;
using std::string;
using std::greater;
using std::ifstream;
using std::cout;
using WordCounts = map<string, int>;
using FrequencyOfWords = map<int, vector<string>, greater<int>>;
int main()
{
WordCounts text;
FrequencyOfWords freq;
ifstream rf("so26.cpp");
string words;
while (rf >> words)
{
text[words]++;
}
WordCounts::const_iterator iter_map1 = text.begin();
for (; iter_map1 != text.end(); ++iter_map1)
{
freq[iter_map1->second].push_back(iter_map1->first);
}
for (auto const& e: freq)
{
cout << "freq " << e.first << " words";
for (auto const& w: e.second)
{
cout << " " << w;
}
cout << "\n";
}
}

Answer 3

Perhaps I misunderstood the question, but I reckon the following does what you want (I prefer unordered maps, as they are faster and you don't seem to need the ordering)
std::unordered_map<std::string,int> word_counts;
std::string word;
while(input >> word)
word_counts[word]++;
std::unordered_map<int,std::vector<std::string>> words_by_freq;
for(const auto& counted : word_counts)
words_by_freq[counted::second].push_back(counted::first);