How to count amount of unique elements in an array of structs?

Question

I have a structure:

struct wordItem
{
   string word;
   int count;
};

I'm reading in a text file with many different words and storing them into an array.

    ifstream inputFile("data.txt");
    if(inputFile.is_open())
    {
         while(getline(inputFile, data, ' '))
         {
         wordItemList[i].word = data;
         i++;
         }
    }

My question is what is the best way to count each time a word appears in the array. For example if my data.txt file was

the fox jumped over the fence

I want to be able to store how many times each word appears within the struct within the "int count;"

You need a data structure to map a word to a counter. In C++ we have std::map. If this is homework you should tell us what your level of progress is, maybe a std::map is not yet an option to you. — vz0
– vz0, Commented Sep 5, 2016 at 20:55
std::map<std::string, int> or std::unordered_map<std::string, int>. — Cornstalks
– Cornstalks, Commented Sep 5, 2016 at 20:57
Concur with Cornstalks. Something like this. Apologies in advance for any syntax errors. Change every unordered_map to map if you want ordering). Yes, it really is that simple. In short, you don't really need that structure; the map will hold the count for your as the mapped-to value. — WhozCraig
– WhozCraig, Commented Sep 5, 2016 at 21:06

user31264 · Accepted Answer · 2016-09-05 21:08:17Z

1

ifstream inputFile("data.txt");
if(!inputFile.is_open()) {
    cerr << "Can't open data.txt\n";
    exit(0);
}

map<string, int> freq;
while(getline(inputFile, word, ' '))
    ++freq[word];

answered Sep 5, 2016 at 21:08

user31264

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paul-g · Accepted Answer · 2016-09-05 21:11:40Z

1

Use an std::multiset or std::unordered_multiset. The performance depends a bit on your data set so some tuning is required to find the best one in practice. Something like this would work (adapt with your file reading code):

#include <iostream>
#include <unordered_set>

int main() {

    std::unordered_multiset<string> dict;

    for (auto&& s : {"word1", "word2", "word1"}) {
       dict.insert(s);
    }

    std::cout << dict.count("word1") << std::endl; // prints 2
    return 0;
}

Depending on the data set & size, you could also use a more optimised data structure for storing & comparing strings, such as a trie, but this is not available in the standard, or boost and most of the times is a bit of an overkill IMHO (although you can find some implementations).

edited Sep 5, 2016 at 21:11

answered Sep 5, 2016 at 20:58

paul-g

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