C++ : How to find duplicates in a vector ?
In this article we will discuss how to find duplicate elements in vector and their repetition count.
Suppose we have a vector of strings i.e.
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// Vector of strings std::vector<std::string> vecOfStings{ "at" , "hello", "hi", "there", "where", "now", "is", \ "that" , "hi" , "where", "at", "no", "yes", "at"}; |
Let’s find duplicate elements from this list and their duplication count. For example in above vector duplicate strings and their duplication count is as follows,
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at :: 3 hi :: 2 where :: 2 |
Let’s see how to do that,
Finding duplicates in a vector
Steps are :
- Create a map of <string , int> type to store the frequency count of each string in vector.
- Iterate over all the elements in vector try to insert it in map as key with value as 1.
- If string already exists in map then increment its value by 1.
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// Create a map to store the frequency of each element in vector std::map<std::string, int> countMap; // Iterate over the vector and store the frequency of each element in map for (auto & elem : vecOfStings) { auto result = countMap.insert(std::pair<std::string, int>(elem, 1)); if (result.second == false) result.first->second++; } |
- Now iterate over the map and print items whose value is greater than 1 i.e.
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// Iterate over the map for (auto & elem : countMap) { // If frequency count is greater than 1 then its a duplicate element if (elem.second > 1) { std::cout << elem.first << " :: " << elem.second << std::endl; } } |
It will print duplicate elements in vector and their duplication count i.e.
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at :: 3 hi :: 2 where :: 2 |
Generic function to find duplicates in vector :
Create a Generic function to get the duplicate elements and their duplication count i.e.
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/* * Generic function to find duplicates elements in vector. * It adds the duplicate elements and their duplication count in given map countMap */ template <typename T> void findDuplicates(std::vector<T> & vecOfElements, std::map<T, int> & countMap) { // Iterate over the vector and store the frequency of each element in map for (auto & elem : vecOfElements) { auto result = countMap.insert(std::pair<std::string, int>(elem, 1)); if (result.second == false) result.first->second++; } // Remove the elements from Map which has 1 frequency count for (auto it = countMap.begin() ; it != countMap.end() ;) { if (it->second == 1) it = countMap.erase(it); else it++; } } |
Let’s use this generic function to find duplicate elements in vector i.e.
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std::map<std::string, int> duplicateElements; // Get the duplicate elements in vector findDuplicates(vecOfStings, duplicateElements); |
Now Iterate over this map to print the duplicate elements with count i.e.
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for (auto & elem : duplicateElements) std::cout << elem.first << " :: " << elem.second << std::endl; |
Output:
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at :: 3 hi :: 2 where :: 2 |
Complete example is as follows,
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#include <iostream> #include <vector> #include <map> #include <algorithm> #include <functional> #include <string> // Print the contents of vector template <typename T> void print(T & vecOfElements, std::string delimeter = " , ") { for(auto elem : vecOfElements) std::cout<<elem<<delimeter; std::cout << std::endl; } /* * Generic function to find duplicates elements in vector. * It adds the duplicate elements and their duplication count in given map countMap */ template <typename T> void findDuplicates(std::vector<T> & vecOfElements, std::map<T, int> & countMap) { // Iterate over the vector and store the frequency of each element in map for (auto & elem : vecOfElements) { auto result = countMap.insert(std::pair<std::string, int>(elem, 1)); if (result.second == false) result.first->second++; } // Remove the elements from Map which has 1 frequency count for (auto it = countMap.begin() ; it != countMap.end() ;) { if (it->second == 1) it = countMap.erase(it); else it++; } } int main() { // Vector of strings std::vector<std::string> vecOfStings{ "at" , "hello", "hi", "there", "where", "now", "is", \ "that" , "hi" , "where", "at", "no", "yes", "at"}; print(vecOfStings); // Create a map to store the frequency of each element in vector std::map<std::string, int> countMap; // Iterate over the vector and store the frequency of each element in map for (auto & elem : vecOfStings) { auto result = countMap.insert(std::pair<std::string, int>(elem, 1)); if (result.second == false) result.first->second++; } std::cout << "Duplicate elements and their duplication count " << std::endl; // Iterate over the map for (auto & elem : countMap) { // If frequency count is greater than 1 then its a duplicate element if (elem.second > 1) { std::cout << elem.first << " :: " << elem.second << std::endl; } } /* * Finding duplicates in vector using generic function */ std::map<std::string, int> duplicateElements; // Get the duplicate elements in vector findDuplicates(vecOfStings, duplicateElements); std::cout << "Duplicate elements and their duplication count " << std::endl; for (auto & elem : duplicateElements) std::cout << elem.first << " :: " << elem.second << std::endl; return 0; } |
Output:
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at , hello , hi , there , where , now , is , that , hi , where , at , no , yes , at , Duplicate elements and their duplication count at :: 3 hi :: 2 where :: 2 Duplicate elements and their duplication count at :: 3 hi :: 2 where :: 2 |
To compile the example use following command,
g++ –std=c++11 example.cpp
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