Easier way to count identical strings in a vector? - c++

Question

I am trying to get an output of the number of all the identical strings in a vector as part of a much larger program. After a lot of research I have managed to put something together that works but it seems messy and I was wondering if there was a better way to do it.
#include <vector>
#include <string>
#include <map>
#include <algorithm>
#include <iostream>
using namespace std;
void setMap(string i);
void addMap(string i);
map<string, int> myMap;
int main()
{
vector<string> myVector;
string myArray[6]={"foo","foo","bar","roo","foo","bar"};
for (int i=0; i<6; i++)
{
myVector.push_back(myArray[i]);
}
for_each (myVector.begin(), myVector.end(), setMap);
for_each (myVector.begin(), myVector.end(), addMap);
for (map<string, int, less< string >>::const_iterator iter = myMap.begin();
iter != myMap.end(); ++iter )
cout <<iter->first<<'\t'<<iter->second<<endl;
return 0;
}
void setMap(string i)
{
myMap[i]=0;
}
void addMap(string i)
{
myMap[i]++;
}
This code works fine and gives me the output I was after but I'm not that keen on having to add 2 extra functions to make it work or having to make the map global. Any hints would be gratefully received.

Answer 1

Well the simplest way to not have the extra functions and not have the map as global would be to not use for_each.
for_each (myVector.begin(), myVector.end(), setMap);
for_each (myVector.begin(), myVector.end(), addMap);
becomes
map<string, int> myMap;
for (vector<string>::iterator i = myVector.begin(); i != myVector.end(); ++i)
myMap[*i]=0;
for (vector<string>::iterator i = myVector.begin(); i != myVector.end(); ++i)
++myMap[*i];
Once you done that you could also remove the first loop
map<string, int> myMap;
for (vector<string>::iterator i = myVector.begin(); i != myVector.end(); ++i)
++myMap[*i];
since the map values will be initialised to zero anyway.
What made you think you had to use for_each anyway?

Answer 2

Your setMap function is unnecessary.
Consider what this function does, should the map's key not be present.
void addMap(string i)
{
myMap[i]++;
}
The expression myMap[i] will add a new key to your map.
Since the value type is int, this new value will be int(), which is guaranteed to be 0.

Answer 3

What about this? Encapsulate the counting mechanism in a separate function for reusability.
// Iterator pair based interface
template <class Iterator>
std::map<typename Iterator::value_type,int>
count(Iterator begin, Iterator end) {
std::map<typename Iterator::value_type,int> counts;
for (Iterator i = begin; i != end; ++i)
counts[*i]++;
return counts;
}
// Sequence interface
template <class Sequence>
inline std::map<typename Sequence::value_type,int>
count(Sequence seq) {
return count(seq.begin(), seq.end());
}
Then simply use it like this:
// C++11
for (const auto & c : count(myVector))
cout << c->first << '\t' << c->second << endl;
// C++03
std::map<string,int> counts = count(myVector);
for (std::map<string,int>::const_iterator c = counts.begin(), e = counts.end(); c != e; ++c)
cout << c->first << '\t' << c->second << endl;
Simple demo

Answer 4

Under C++11, you can do this:
#include <string>
#include <unordered_map>
#include <iostream>
int main() {
std::string myArray[6] = {"foo","foo","bar","roo","foo","bar"};
std::unordered_map<std::string, size_t> m;
for (const auto& s : myArray)
++m[s];
for (const auto& p : m)
std::cout << p.first << "\t" << p.second << std::endl;
}
This prints:
foo 3
bar 2
roo 1
This works because m[s] will automatically insert s into m if not already there.
Using std::unordered_map (a hashtable) is likely to be cheaper than std::map (a balanced tree).
You can do something very similar under C++03, except the "for each" loops shown above would be replaced by the regular "for" loops.

Answer 5

#include <iostream>
#include <string>
#include <vector>
#include <iterator>
#include <map>
using namespace std;
int main (int argc, char * const argv[]) {
string myArray[]={"foo","foo","bar","roo","foo","bar"};
int arr_length = 6;
vector<string> myVector(myArray, myArray + arr_length);
//Print contents of vector:
copy(myVector.begin(),
myVector.end(),
ostream_iterator<string>(cout, " ")
);
cout << endl;
map<string, int> myMap;
vector<string>::iterator pos;
for (pos=myVector.begin(); pos<myVector.end(); ++pos)
{
myMap[*pos] += 1;
}
map<string, int>::iterator mapPos;
for (mapPos=myMap.begin(); mapPos != myMap.end(); ++mapPos) {
cout << "word: " << mapPos->first << "\t"
<< "count: " << mapPos->second << endl;
}
return 0;
}
--output:--
foo foo bar roo foo bar
word: bar count: 2
word: foo count: 3
word: roo count: 1