The C++ Function using STL to Check Duplicate Elements/Characters in Array/Vector/String

cplusplus

Let’s say we want to implement a C++ function based on STL containers to check if a given string contains duplicate characters, or a given vector/array contains duplicate elements. Luckily we can use the unordered set (or set which maintains order), that we can construct a set based on the vector/array/string, then we just need to compare the sizes of both set and the original container – if they are equal, it means all unique elements or duplicates otherwise.

bool hasDuplicateCharacters(string arr) {
	unordered_set<char> st(begin(arr), end(arr));
	return st.size() != arr.size();
}

Example:

cout << (hasDuplicateCharacters("abcde") ? "true" : "false"); // false
cout << (hasDuplicateCharacters("abcdea") ? "true" : "false"); // true

We can use the C++ generic that allows us to pass in most data types – using the templates.

template <typename T>
bool hasDuplicateItems(vector<T> arr) {
	unordered_set<T> st(begin(arr), end(arr));
	return st.size() != arr.size();
}

Example:

cout << (hasDuplicateItems({ 1, 2, 3, 4 }) ? "true" : "false");  // true
cout << (hasDuplicateItems({ 1, 2, 3, 4, 1 }) ? "true" : "false"); // false

This is quite similar approach to checking the duplicates in Javascript – which is also based on the Set.

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