std::for_each_C ++中的std :: for_each（）

本是古典何须时尚 2023-03-05 09:41 13阅读 0赞

std::for\_each

## std :: for\_each() **(**std::for\_each()**)** ##

**for\_each()** is a very useful function which helps to invoke a function ***fn()*** on each element in the STL container. This helps actually to write code in short and to reduce size of our codebase.

**for\_each()**是一个非常有用的函数，它有助于在STL容器中的每个元素上调用函数***fn()*** 。 这实际上有助于简短地编写代码并减小代码库的大小。

Below is the syntax for the **for\_each()**,

以下是**for\_each()**的语法，

**Syntax:**

**句法：**

for_each(
        InputIterator first, 
        InputIterator last, 
        Function fn);

Where,

哪里，

*  InputIterator first = starting of the container
    
    InputIterator first =容器的启动
 *  InputIterator last = end of the container
    
    InputIterator last =容器的末尾
 *  Function fn = function to be invoked on each element of container
    
    函数fn =在容器的每个元素上调用的函数

Below are examples of using for\_each() efficiently.

以下是有效使用for\_each()的示例。

**1) Printing all elements**

**1)打印所有元素**

For each can be used to print elements in any container. Below is the example where we have printed a vector and map to understand the usage.

对于每个可用于在任何容器中打印元素。 下面是我们打印矢量和映射以了解用法的示例。

#include <bits/stdc++.h>
    using namespace std;
    
    void myfunc1(int i)
    {
        cout << i << " ";
    }
    
    void myfunc2(pair<char, int> p)
    { //ecah element of a map is pair
        cout << p.first << "->" << p.second << endl;
    }
    
    int main()
    {
        vector<int> arr{ 3, 4, 2, 6, 5, 1 };
     
        map<char, int> mymap;
     
        mymap['a'] = 3;
        mymap['c'] = 3;
        mymap['b'] = 6;
        mymap['d'] = 4;
        mymap['e'] = 2;
        mymap['f'] = 1;
    
        cout << "Printing the vector\n";
        for_each(arr.begin(), arr.end(), myfunc1);
        cout << "\n";
     
        cout << "Printing the map\n";
        for_each(mymap.begin(), mymap.end(), myfunc2);
        cout << "\n";
        
        return 0;
    }

**Output:**

**输出：**

Printing the vector
    3 4 2 6 5 1
    Printing the map
    a->3
    b->6
    c->3
    d->4
    e->2
    f->1

**2) Replace the vowels with '\*'**

**2)用“ \*”代替元音**

In this example, we will see how to update the elements of the container. We can update by passing a reference to the elements and updates within the function. In an early article, we saw how to replace vowels using *find\_first\_of()*. In the below program we have replaced all the vowels using *for\_each()* which is more efficient and takes only *O(n)*.

在此示例中，我们将看到如何更新容器的元素。 我们可以通过传递对元素的引用和函数内的更新来进行更新。 在早期的文章中，我们看到了如何使用*find\_first\_of()*替换元音。 在下面的程序中，我们使用了*for\_each()*替换了所有的元音，这更加有效并且仅占用*O(n)* 。

#include <bits/stdc++.h>
    using namespace std;
    
    void myfunc(char& c)
    { //to update reference is passed
    
        if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'o')
            c = '*';
    }
    
    int main()
    {
        string str = "includehelp";
        
        cout << "Initially string is: " << str << endl;
        for_each(str.begin(), str.end(), myfunc);
        cout << "After updation\n";
        cout << str << endl;
        
        return 0;
    }

**Output:**

**输出：**

Initially string is: includehelp
    After updation
    *nclud*h*lp

So, the advantage of using *for\_each()* is it reduces the codebase size and makes the code product level. Also, the time complexity is *O(n)*. If there is any case, where each and every element undergoes huge processing with a large codebase you can use *for\_each()*.

因此，使用*for\_each()*的优势在于它减少了代码库的大小并提高了代码产品级别。 同样，时间复杂度为*O(n)* 。 如果有任何情况，每个元素都需要使用大型代码库进行大量处理，则可以使用*for\_each()* 。

> 翻译自: [https://www.includehelp.com/stl/std-for\_each-in-cpp.aspx][https_www.includehelp.com_stl_std-for_each-in-cpp.aspx]

std::for\_each

[https_www.includehelp.com_stl_std-for_each-in-cpp.aspx]: https://www.includehelp.com/stl/std-for_each-in-cpp.aspx