std::find
- C++ course
- Intermediate
- Lesson 71/116
- 5 min
Learn to search with std::find.
std::find
std::find searches for a value in a range. With auto it = std::find(v.begin(), v.end(), 20); you search for the value 20. If found it returns an iterator to that element, otherwise it returns v.end().
The search is linear (O(n)): it checks each element from start to end. If the value is not found it == v.end(). That is why you check the result with if (it != v.end()). std::find works with every iterator category: std::vector, std::list and even raw pointers.
Checking the result
std::find returns an iterator. If the value was found the iterator points at a valid element; if not it equals v.end(). That is why we distinguish the cases with if (it != v.end()).
You read the found value with *it. You compute its index with std::distance(v.begin(), it). These two approaches are the standard way to turn an iterator into a value or an index. If the search fails *it is not used; because end() is never dereferenced.
Practice Search a value and print a message
For v = {10, 20, 30}, search for 20 with std::find. Print "found" if found, "missing" otherwise. Check the condition with it != v.end().
Starter code
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> v = {10, 20, 30};
// Search 20 with std::find, print the result
return 0;
}Sample solution
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> v = {10, 20, 30};
auto it = std::find(v.begin(), v.end(), 20);
if (it != v.end()) {
std::cout << "found";
} else {
std::cout << "missing";
}
return 0;
}Expected output
found
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the find call
auto it = std::____(v.begin(), v.end(), 20);auto it = std::find(v.begin(), v.end(), 20);Explanation: std::find searches for a value.
2 What does std::find return when the element is not found?
auto it = std::find(v.begin(), v.end(), 99);- the v.end() iterator Correct answer
- 0
- -1
- nullptr
Explanation: When not found it returns the end() iterator.
3 Match each expression to its meaning
| Term | Meaning |
|---|---|
std::find(begin, end, v) |
Value search |
it != v.end() |
Found condition |
*it |
Found element |
std::distance(begin, it) |
Index computation |