C++ Iterators
C++ Iterators
Iterators are used to access and iterate through elements of data structures (vectors, sets, etc.), by "pointing" to them.
It is called an "iterator" because "iterating" is the technical term for looping.
To iterate through a vector, look at the following example:
Example
// Create a vector called cars that will store strings
vector<string> cars = {"Volvo", "BMW",
"Ford", "Mazda"};
// Create a vector iterator
called it
vector<string>::iterator it;
// Loop through the vector with the
iterator
for (it = cars.begin(); it != cars.end(); ++it) {
cout << *it <<
"\n";
}
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Example explained
- First we create a vector of strings to store the names of different car manufactures.
- Then we create a "vector iterator" called
it, that we will use to loop through the vector. - Next, we use a
forloop to loop through the vector with the iterator. The iterator (it) points to the first element in the vector (cars.begin()) and the loop continues as along asitis not equal tocars.end(). - The increment operator (
++it) moves the iterator to the next element in the vector. - The dereference operator (
*it) accesses the element the iterator points to.
Note: The type of the iterator
must match the type of the data structure it should iterate through (string in
our example)
What is begin() and end()?
begin() and end() are
member functions that belong to data structures, like vectors and lists. They
do not belong to the iterator itself. Instead, they are used with iterators to
access and iterate through the elements of these data structures.
begin()returns an iterator that points to the first element of the data structure.end()returns an iterator that points to one position after the last element.
To understand how they work, let's continue to use vectors as an example:
vector<string> cars
= {"Volvo", "BMW", "Ford", "Mazda"};
vector<string>::iterator it;
Begin Examples
begin() points to the first element in the
vector (index 0, which is "Volvo"):
To point to the second element (BMW), you can write
cars.begin() + 1:
And of course, that also means you can point to the third element with
cars.begin() + 2:
End Example
end() points to one position after the last
element in the vector (meaning it doesn't point to an actual element, but rather
indicates that this is the end of the vector).
So, to use end() to point to
the last element in the cars vector (Mazda), you can use cars.end() - 1:
Why do we say "point"?
Iterators are like "pointers" in that they "point" to elements in a data structure rather than returning values from them. They refer to a specific position, providing a way to access and modify the value when needed, without making a copy of it. For example:
Example
// Point to the first element in the vector
it = cars.begin();
//
Modify the value of the first element
*it = "Tesla";
// Volvo is now
Tesla
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The auto Keyword
In C++ 11 and later versions, you can use the auto keyword instead of
explicitly declaring and specifying the type of the iterator.
The auto keyword allows the compiler to
automatically determine the correct data type, which simplifies the code and
makes it more readable:
Instead of this:
vector<string>::iterator it = cars.begin();
In the example above, the compiler knows the type of it based on the return type of cars.begin(), which is vector<string>::iterator.
The auto keyword works in for loops as well:
for (auto it = cars.begin(); it != cars.end(); ++it) {
cout << *it
<< "\n";
}
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For-Each Loop vs. Iterators
You can use a for-each loop to just loop through elements of a data structure, like this:
Example
// Create a vector called cars that will store strings
vector<string> cars = {"Volvo", "BMW",
"Ford", "Mazda"};
// Print vector elements
for (string car : cars) {
cout << car << "\n";
}
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However, when you need to add or remove elements during iteration, iterate in reverse, or skip elements, you should use iterators:
Example
// Create a vector called cars that will store strings
vector<string> cars
= {"Volvo", "BMW", "Ford", "Mazda"};
// Loop through vector elements
for (auto it = cars.begin(); it != cars.end(); ) {
if (*it == "BMW")
{
it = cars.erase(it); // Remove the BMW element
} else {
++it;
}
}
// Print vector elements
for (const
string& car : cars) {
cout << car << "\n";
}
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Iterate in Reverse
To iterate in reverse order, you can use rbegin() and rend() instead of begin() and end():
Example
// Iterate in reverse order
for (auto it = cars.rbegin(); it !=
cars.rend(); ++it) {
cout << *it << "\n";
}
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Iterate Through other Data Structures
Iterators are great for code reusability since you can use the same syntax for iterating through vectors, lists, deques, sets and maps:
List Example
// Create a list called cars that will store strings
list<string> cars =
{"Volvo", "BMW", "Ford", "Mazda"};
// Loop through the list with an
iterator
for (auto it = cars.begin(); it != cars.end(); ++it) {
cout << *it << "\n";
}
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Deque Example
// Create a deque called cars that will store strings
deque<string> cars =
{"Volvo", "BMW", "Ford", "Mazda"};
// Loop through the deque with an
iterator
for (auto it = cars.begin(); it != cars.end(); ++it) {
cout << *it << "\n";
}
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Set Example
// Create a set called cars that will store strings
set<string> cars =
{"Volvo", "BMW", "Ford", "Mazda"};
// Loop through the set with an
iterator
for (auto it = cars.begin(); it != cars.end(); ++it) {
cout << *it << "\n";
}
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Map Example
// Create a map that will store strings and integers
map<string, int>
people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
// Loop through
the map with an iterator
for (auto it = people.begin(); it != people.end();
++it) {
cout << it->first << " is: " << it->second << "\n";
}
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Iterator Support
The examples above shows how to iterate through different data structures that support iterators (vector, list, deque, map and set support iterators, while stacks and queues do not).
Algorithms
Another important feature of iterators is that they are used with different
algorithm functions, such as sort() and find()
(found in the <algorithm> library), to sort and
search for elements in a data structure.
For example, the sort() function takes
iterators (typically returned by begin()
and end()) as parameters to sort elements in a data structure
from the beginning to the end.
In this example, the elements are sorted alphabetically since they are strings:
Example
#include <iostream>
#include <vector>
#include <algorithm>
// Include the <algorithm> library
using namespace std;
int main() {
// Create a vector called cars
that will store strings
vector<string> cars = {"Volvo", "BMW",
"Ford", "Mazda"};
// Sort cars in alphabetical order
sort(cars.begin(), cars.end());
// Print cars in alphabetical order
for (string car : cars) {
cout << car << "\n";
}
return 0;
}
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And in this example, the elements are sorted numerically since they are integers:
Example
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main() {
// Create a vector called numbers that will store integers
vector<int> numbers = {1, 7, 3, 5, 9, 2};
// Sort numbers
numerically
sort(numbers.begin(), numbers.end());
for (int num : numbers) {
cout << num << "\n";
}
return 0;
}
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To reverse the order, simply use rbegin() and rend() instead of begin() and end():
Example
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main() {
// Create a vector called numbers that will store integers
vector<int> numbers = {1, 7, 3, 5, 9, 2};
// Sort numbers
numerically in reverse order
sort(numbers.rbegin(), numbers.rend());
for (int num : numbers) {
cout << num << "\n";
}
return 0;
}
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