distinct function usage distance function c usage tutorial

std::unique removes adjacent duplicate elements in the container and moves them to the end, returning an iterator pointing to the first duplicate element. std::distance calculates the distance between two iterators, that is, the number of elements they point to. These two functions are useful for optimizing code and improving efficiency, but there are also some pitfalls to be paid attention to, such as: std::unique only deals with adjacent duplicate elements. std::distance is less efficient when dealing with non-random access iterators. By mastering these features and best practices, you can fully utilize the power of these two functions.

Exploring the secret of std::unique and std::distance : C container's weapon

Are you often troubled by repeating elements or the need to calculate the iterator distance when working with C containers? This article will dive into the two powerful standard library functions, std::unique and std::distance , to take you through their charm in optimizing code, improving efficiency, and to reveal some potential pitfalls and best practices. After reading this article, you will be able to use these two functions proficiently to write more efficient and elegant C code.

Basic knowledge: Iterators and algorithms

Before we go into the deeper explanation of std::unique and std::distance , we need to review the concept of C iterator. An iterator is a generic pointer to access container elements. It allows us to operate various containers in a unified way (such as std::vector , std::list , std::deque , etc.). Standard library algorithms, such as std::unique , rely on iterators to manipulate elements in containers.

std::unique : a tool for removing weight

Instead of directly removing duplicate elements, std::unique function moves adjacent duplicate elements in the container to the end of the container and returns an iterator pointing to the position of the first duplicate element. This sounds a bit confusing, but if you understand how it works, you will find it very practical.

Let's look at an example:

 <code class="c  ">#include <iostream> #include <algorithm> #include <vector> int main() { std::vector<int> numbers = {1, 1, 2, 2, 3, 4, 4, 5}; auto it = std::unique(numbers.begin(), numbers.end()); // it现在指向第一个重复元素的位置，也就是第二个'2' numbers.erase(it, numbers.end()); // 移除重复元素for (int num : numbers) { std::cout </int></vector></algorithm></iostream></code>

This code first uses std::unique to move the repeating elements to the end, and then uses the erase method to remove these elements. Note that std::unique itself does not change the size of the container, it just rearranges the elements.

std::distance : Iterator distance calculator

std::distance function calculates the distance between two iterators, that is, the number of elements they point to. This function is very useful in many algorithms, such as calculating the length of a subsequence, or determining the position of an element in a container.

 <code class="c  ">#include <iostream> #include <algorithm> #include <vector> #include <iterator> int main() { std::vector<int> numbers = {1, 2, 3, 4, 5}; auto it1 = numbers.begin() 1; auto it2 = numbers.end() - 1; std::cout </int></iterator></vector></algorithm></iostream></code>

This code calculates the distance between iterators it1 and it2 , and the result is 3.

Traps and optimization

When using std::unique , you need to be careful that it only deals with adjacent duplicate elements . If your duplicate elements are not adjacent, you need to sort the container first (for example using std::sort ).

std::distance is very efficient when dealing with random access iterators (such as std::vector 's iterators) because the difference can be calculated directly. But for other types of iterators (such as the iterators of std::list ), it requires linear time complexity and is therefore less efficient. In performance-critical code, you should try to avoid using std::distance on non-random access iterators.

Best Practices

• Before using std::unique , consider whether you need to sort the container first.
• For performance-sensitive applications, choose the appropriate container type and algorithm to avoid unnecessary iterator traversal.
• Write clear and readable code and add necessary comments for easy understanding and maintenance.

All in all, std::unique and std::distance are very useful tools in the C standard library, and mastering them can help you write more efficient and elegant code. Only by remembering their characteristics and potential pitfalls can they fully exert their power. I hope this article can help you better understand and use these two functions.

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