Summary - Iterators
- C++ course
- Intermediate
- Lesson 64/116
- 5 min
Combine begin/end and arithmetic to write a search function.
Summary - Iterators
In this group you learned what iterators are, the begin/end range, the five categories, range-based for and arithmetic. Now we combine these to write a search function.
Search starts with std::find: auto it = std::find(v.begin(), v.end(), 30);. To convert the found iterator to an index use std::distance(v.begin(), it). If the element is not found the iterator equals v.end(); we distinguish this case with if (it != v.end()).
Practice Find and print the index
For v = {10, 20, 30, 40}, search for 30 with std::find. Compute its index with std::distance(v.begin(), it) and print it with std::cout.
Starter code
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> v = {10, 20, 30, 40};
// Search 30 with std::find, print index with std::distance
return 0;
}Sample solution
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> v = {10, 20, 30, 40};
auto it = std::find(v.begin(), v.end(), 30);
std::cout << std::distance(v.begin(), it);
return 0;
}Expected output
2
Practice Add a found message
Take the previous step's std::find and std::distance calls. After the index, print "found" if it != v.end(), otherwise "missing". Put the index and the message on separate lines.
Starter code
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> v = {10, 20, 30, 40};
auto it = std::find(v.begin(), v.end(), 30);
std::cout << std::distance(v.begin(), it);
// print "found" on success, otherwise print "missing"
return 0;
}Sample solution
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> v = {10, 20, 30, 40};
auto it = std::find(v.begin(), v.end(), 30);
std::cout << std::distance(v.begin(), it);
if (it != v.end()) {
std::cout << "\nfound";
} else {
std::cout << "\nmissing";
}
return 0;
}Expected output
2 found
Iterators recap
Iterators are the foundation of the STL. The begin/end half-open range is shared by all containers and algorithms. The five categories (input, output, forward, bidirectional, random access) are set by the container type.
Range-based for uses iterators behind the scenes. Random access iterators support +n, -n and []. With std::find and std::distance you can search and find indices. In the next group you will study STL algorithms in detail.
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What converts an iterator to an index?
auto it = std::find(v.begin(), v.end(), 30);- std::count
- std::distance Correct answer
- std::sort
- std::copy
Explanation: std::distance(begin, it) gives the index.
2 Complete the std::find call
auto it = std::____(v.begin(), v.end(), 30);auto it = std::find(v.begin(), v.end(), 30);Explanation: std::find performs a value search.