c++ 迭代器++和+1_C ++中的迭代器简介

c++ 迭代器++和+1

In this article, we’ll learn about Iterators in C++ in detail. While working with data elements, we often come across situations where we need to traverse through every element of the data structure. In order to serve the purpose, C++ provides us with Iterators.

在本文中，我们将详细了解C ++中的迭代器。在使用数据元素时，我们经常遇到需要遍历数据结构的每个元素的情况。为了达到目的， C ++为我们提供了Iterators 。

Iterators are basically objects which help programmers access the data items from the containers. It points at a particular index position(memory address) of the Standard Template Library container and enables to access the data item at that position.

迭代器基本上是可以帮助程序员访问容器中数据项的objects 。它指向标准模板库容器的特定索引位置 （内存地址），并允许访问该位置的数据项。

Syntax:

句法：


container-name<data-type>::iterator iterator-name;

Example:

例：


list<int>::iterator A;

Thus, iterators help us move and traverse through the items of the containers.

因此，迭代器可帮助我们在容器中移动和遍历。

C ++中的迭代器类型 (Types of Iterators in C++)

At an initial stage, Iterators and Pointers may feel similar to you in terms of functionality. But by the end of this tutorial, you will get to know the differences between an Iterator and a Pointer in C++.

在初始阶段，迭代器和指针在功能上可能与您相似。但是，在本教程结束时，您将了解C ++中的Iterator和Pointer之间的区别。

Iterators work in a different manner than pointers. The following are the type of iterators that serve different functionality altogether:

迭代器的工作方式与指针不同。以下是完全提供不同功能的迭代器的类型：

Input Iterator输入迭代器
Output Iterator输出迭代器
Forward Iterator正向迭代器
Bi-directional Iterator双向迭代器
Random-Access Iterator随机访问迭代器

1.输入迭代器 (1. Input Iterator)

An Input Iterator is a basic type of iterator which serves the sole purpose of traversing the elements of a container.

输入迭代器是迭代器的基本类型，其唯一目的是traversing容器的元素。

It helps us move through the elements of the container but it does not provide us with the access to modify the data values of the container.

它可以帮助我们遍历容器的元素，但不能为我们提供修改容器的数据值的访问权限 。

Following is the list of operators which can be used along with Input Iterators:

以下是可与输入迭代器一起使用的运算符的列表：

Dereference Operator (*)解引用运算符（*）
Increment Operator (++)增量运算符（++）
Equal to Operator (==)等于运算符（==）
Not Equal to Operator (!=)不等于运算符（！=）

Example:

例：


#include <iostream>
#include <list>
using namespace std;
int main()
{ list<int> li= { 10, 20, 42, 50, 75 }; list<int>::iterator x; cout<<"Traversing elements of the list.."<<endl;for (x = li.begin(); x != li.end(); x++) { cout << (*x) << " "; } return 0;
}

In the above snippet of code, we have used a list as a container and declared ‘x‘ as the iterator to that list ‘li‘. The begin() and end() functions are used to point to the first and last position of the container. The iterator traverses the list in the forward (ascending) direction.

在上面的代码片段中，我们将列表用作容器，并声明“ x ”作为该列表“ li ”的迭代器。 begin()和end()函数用于指向容器的第一个和最后一个位置。迭代器以向前（升序）的方式遍历列表。

Output:

输出：


Traversing elements of the list..
10 20 42 50 75

We will get to learn more about these operations in the coming section of this article.

我们将在本文的后续部分中了解有关这些操作的更多信息。

2.输出迭代器 (2. Output Iterator)

The Output Iterators are complementary to the Input Iterators i.e. opposite to each other in terms of functionality.

输出迭代器与输入迭代器是complementary ，即在功能上彼此opposite 。

They can be used to assign and manipulate the values of the data items of the container. But, they cannot be used to access or traverse through the elements of a container.

它们可用于分配和操作容器数据项的值。但是，它们不能用于访问或遍历容器的元素 。

Following is the list of operators which can be used along with Output Iterators:

以下是可与输出迭代器一起使用的运算符的列表：

Dereference Operator (* , ->)解引用运算符（*，->）
Increment Operator (++)增量运算符（++）
Equal to Operator (==)等于运算符（==）
Not Equal to Operator (!=)不等于运算符（！=）

Example:

例：


#include <iostream>
#include <list>
using namespace std;
int main()
{ list<int> li= { 10, 20, 42, 50, 75 }; list<int>::iterator x; cout<<"Traversing elements of the list.."<<endl;for (x = li.begin(); x != li.end(); x++) {
//using output iterator to change or assign new values to all the
// elements of the list*x = 4;} for (x = li.begin(); x != li.end(); x++) {
//using input iterator to traverse the listcout << (*x) << " "; } return 0;
}

As seen above, we have assigned value = 4 to all the elements of the container using output iterator.

如上所示，我们已使用输出迭代器将value = 4分配给了容器的所有元素。

Output:

输出：


Traversing elements of the list..
4 4 4 4 4

3.双向迭代器 (3. Bi-directional Iterator)

Bi-directional Iterator is quite similar to an Input Iterator. The only difference is that a bi-directional iterator can traverse the elements in both the directions i.e. forward and backward(reverse) direction.

双向迭代器与输入迭代器非常相似。唯一的区别是，双向迭代器可以在两个方向（即forward and backward(reverse) direction.遍历元素forward and backward(reverse) direction.

Following is the list of operators which can be used along with Input Iterators:

以下是可与输入迭代器一起使用的运算符的列表：

Dereference Operator (* , ->)解引用运算符（*，->）
Increment Operator (++)增量运算符（++）
Equal to Operator (==)等于运算符（==）
Not Equal to Operator (!=)不等于运算符（！=）
Decrement Operator (–)减量运算符（–）

Example:

例：


#include <iostream>
#include <list>
using namespace std;
int main()
{ list<int> li= { 10, 20, 42, 50, 75 }; list<int>::iterator x; cout<<"Traversing elements of the list in forward direction.."<<endl;for (x = li.begin(); x != li.end(); x++) {
//using input iterator to traverse the listcout << (*x) << " "; } cout<<endl;cout<<"Traversing elements of the list in backward direction.."<<endl;for (x=li.end();x!=li.begin();--x) { if (x != li.end()) { cout << (*x) << " "; } } cout << (*x); return 0;
}

Output:

输出：


Traversing elements of the list in forward direction..
10 20 42 50 75
Traversing elements of the list in backward direction..
75 50 42 20 10

4.转发迭代器 (4. Forward Iterator)

Forward Iterator can be termed as a combination of Input and Output Iterator respectively. It allows us to modify the values of the data items of the containers as well as traverse through the elements of the container.

正向迭代器可以分别称为输入和输出迭代器的组合。它允许我们modify容器的数据项的值以及traverse容器的元素。

But, unlike Bi-directional Iterator, the Forward Iterator does not permit the traversal of elements in a reverse or backward direction.

但是，与双向迭代器不同， 前向迭代器不允许反向或向后遍历元素 。

Following is the list of operators which can be used along with Input Iterators:

以下是可与输入迭代器一起使用的运算符的列表：

Dereference Operator (* , ->)解引用运算符（*，->）
Increment Operator (++)增量运算符（++）
Equal to Operator (==)等于运算符（==）
Not Equal to Operator (!=)不等于运算符（！=）

Example:

例：


#include <iostream>
#include <list>
using namespace std;
int main()
{ list<int> li= { 10, 20, 42, 50, 75 }; list<int>::iterator x; cout<<"Traversing elements of the list.."<<endl;for (x = li.begin(); x != li.end(); ++x) { cout << (*x) << " "; //using input iterator to traverse the list} for (x = li.begin(); x != li.end(); x++) {
// using output iterator to change or assign new values to all the
// elements of the list*x = 4; } cout<<endl;cout<<"Traversing elements of the list after assigning a new value to the list.."<<endl;for (x = li.begin(); x != li.end(); ++x) { cout << (*x) << " "; //using input iterator to traverse the list} return 0;
}

Output:

输出：


Traversing elements of the list..
10 20 42 50 75
Traversing elements of the list after assigning a new value to the list..
4 4 4 4 4

5.随机访问迭代器 (5. Random-Access Iterator)

Random-Access Iterators enable the users to access the data items at any random index/position in the container. Moreover, it contains all the properties of a Bi-directional container.

随机访问迭代器使用户可以在容器中的任何随机索引/位置访问数据项。而且，它包含双向容器的所有属性。

Further, Random-Access Iterators have access to pointer subtraction and addition for the sake of random access to the data items.

此外，为了随机访问数据项，随机访问迭代器可以访问指针减法和加法 。

C ++中迭代器执行的操作 (Operations performed by Iterators in C++)

The following are some of the basic and most frequently used functions to manipulate data items through iterators:

以下是一些通过迭代器操作数据项的基本功能和最常用功能：

begin(): This method returns the starting/beginning index of the data items in the container.begin() ：此方法返回容器中数据项的开始/开始索引。
end(): It returns the ending index of the data items in the container.end() ：返回容器中数据项的结束索引。
prev(int): Returns a new iterator that points to the data item which is present ahead of the position mentioned in the function parameter.prev(int) ：返回一个新的迭代器，该迭代器指向位于函数参数中提到的位置之前的数据项。
next(int): It returns a new iterator that points to the data item which is present after the position mentioned in the function parameter.next(int) ：它返回一个新的迭代器，该迭代器指向函数参数中提到的位置之后的数据项。
inserter(container, int): This function adds/inserts a value at the specified index or position in the container.inserter(container, int) ：此函数在容器中的指定索引或位置添加/插入一个值。
advance(int): It increments by the number specified in the parameter and points to the data element specified at that index.advance(int) ：以参数中指定的数字递增，并指向该索引处指定的数据元素。

Example:

例：


#include <iostream>
#include <list>
using namespace std;
int main()
{ list<int> li= {10, 20, 42, 50, 75}; list<int>::iterator x; cout<<"Traversing elements of the list in forward direction.."<<endl;for (x = li.begin(); x != li.end(); x++) { cout << (*x) << " "; //using input iterator to traverse the list} cout<<endl;list<int>::iterator ee = li.begin(); advance(ee, 3); cout << "The element pointed by the iterator after using advance(): "; cout << *ee << " ";
cout<<endl;
list<int>::iterator A = li.begin();
list<int>::iterator B = li.end(); auto aa = next(A, 2); auto  bb = prev(B, 2); cout << "Element pointed using next() is : "; cout << *aa << " "; cout << endl; cout << "Element pointed using prev()  is : "; cout << *bb << " "; cout << endl; return 0;
}

Output:

输出：


Traversing elements of the list in forward direction..
10 20 42 50 75
The element pointed by the iterator after using advance(): 50
Element pointed using next() is : 42
Element pointed using prev()  is : 50

C ++中迭代器的优点 (Advantages of Iterators in C++)

Easy for programming: With the help of iterators, we don’t need to keep an account of the varying size of the container and we can thus easily traverse through the container using begin() and end() methods.Easy for programming ：借助迭代器，我们无需考虑容器大小的变化，因此可以使用begin（）和end（）方法轻松地遍历容器。
High level of re-usability of code.高度re-usability of code 。
Dynamically add or remove data elements from the container.从容器Dynamically添加或删除数据元素。

C ++中的指针与迭代器 (Pointers vs Iterators in C++)

结论 (Conclusion)

Thus, in this article, we have understood the need, working and benefits of Iterators in C++ STL.

因此，在本文中，我们了解了C ++ STL中Iterators的需求，工作和好处。

参考资料 (References)

Iterator in C++ – Documentation

C ++中的迭代器–文档

翻译自: https://www.journaldev.com/37011/iterators-in-c-plus-plus

c++ 迭代器++和+1