By danyalzia, last updated October 19, 2019

There is a time during the usage of a c++ class **std::vector,** **std::list** and others, when you need to find an element in the container. For instance, you want to create a collection of numbers (let’s suppose you have stored them in **std::vector**) and you have some function that checks for the given element in a container. Now you might need to find out whether some number exists in the **std::vector** class.

Finding an element in a container or array is quite a common task in programming, yet some programmers don’t know the number of ways it can be done. **<algorithm>** header provides several useful functions for manipulating containers, including a function **std::find** for getting the iterator of a given range. Through it, you can easily find whether an element in a container exists or not. It takes three parameters: *First iterator, Last iterator and the element that you want to find in a container.*

**Read also: **How to optimize a sort algorithm

C++ vector or std::vector find example:

int main() { std::vector<int> numbers; // this can be std::list, std::deque etc. numbers.push_back(10); numbers.push_back(20); auto it = std::find(numbers.begin(), numbers.end(), 10); if (it != numbers.end()) { // Found } else { // Not Found } }

If your vector is ordered (if not, then do so with **std::sort**), then you can use **std::binary_search** which yields O(log n) worst-case performance, which is way more efficient than the first approach. For sorting elements in a **std::vector**, you should use **std::sort** and NOT **std::qsort**, because the latter is quite expensive for **std::vector**. Note that if you want to find an element in a container at several times in your program, then you are better off using **std::list**, because it doesn’t need to be sorted and thus searching is more efficient. Following is its usage:

int main() { std::vector<int> numbers; numbers.push_back(10); numbers.push_back(20); std::sort(numbers.begin(), numbers.end()); // we need to sort it, as we are using std::vector auto it = std::binary_search(numbers.begin(), numbers.end(), 10); if (it) { // Found } else { // Not Found } }

As you can see, how easy it is to find an element through **std::find** function. Another easy way to find an element in a container is by using **std::find_if** alongside with the **lambda** feature of **C++11**. It finds an element in a container with a given condition; if you want to get the first odd/even number in your container that holds numbers, you can define the function for it and then pass it in its parameter. It takes three parameters: *First iterator, Last iterator and the function that defines the condition.* Following is an example of its usage:

bool IsOdd(int i) { return ((i%2)==1); } int main() { std::vector<int> numbers; numbers.push_back(10); numbers.push_back(20); numbers.push_back(25); auto it = std::find_if(numbers.begin(), numbers.end(), IsOdd); // this can be also written using lambda: std::find_if(numbers.begin(),numbers.end(),[](int i){return ((i%2)==1); }); std::cout << "The first odd value is " << *it << '\n'; }

Now you can decide, which way is better suited for your case.

Is there any other way that you know, through which an element in a container can be found? Please let me know in the comments section below!

Finding an element in a container (std::vector, std::list, etc.) was last modified: October 19th, 2019 by danyalzia

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