In programming, sometimes there is a need to perform some operation more than once or (say) n number of times. Loops come into use when we need to repeatedly execute a block of statements.

在编程中，有时需要多次执行某些操作，例如n次。当我们需要重复执行一个语句块时，就会使用循环。

4 types of loops:

① Entry Controlled loops: while loop, for loop

② Exit Controlled Loops:

③ Range-based for loop

④ For_each loop

可以理解后两种循环是前两种循环的语法糖，编程语法制定语法规则，确定如何抽象，编程语言的编译器实现抽象的编译，程序员按规则写代码。

1 Entry Controlled loops

In this type of loop, the test condition is tested before entering the loop body. For Loop and While Loop is entry-controlled loops.

在这种类型的循环中，在进入循环体之前测试测试条件。For循环和While循环是入口控制循环。

1.1 for loop

#include <stdio.h>
 
int main()
{
    int i=0;
     
    for (i = 1; i <= 10; i++)
    {
        printf( "Hello World\n");   
    }
 
    return 0;
}

1.2 while loop

#include <stdio.h>
 
int main()
{
    // initialization expression
    int i = 1;
    // test expression
    while (i < 6)
    {
        printf( "Hello World\n");   

        // update expression
        i++;
    }
 
    return 0;
}

2 Exit Controlled Loops:

In this type of loop the test condition is tested or evaluated at the end of the loop body. Therefore, the loop body will execute at least once, irrespective of whether the test condition is true or false. the do-while loop is exit controlled loop.

在这种类型的循环中，在循环体的末端测试或评估测试条件。因此，无论测试条件是真还是假，循环体将至少执行一次。do while循环是出口控制循环。

#include <stdio.h>
 
int main()
{
    int i = 2; // Initialization expression
 
    do
    {
        // loop body
        printf( "Hello World\n");   
 
        // update expression
        i++;
 
    }  while (i < 1);   // test expression
 
    return 0;
}

3 Range-based for loop

Range-based for loop in C++ is added since C++ 11. It executes a for loop over a range. Used as a more readable equivalent to the traditional for loop operating over a range of values, such as all elements in a container.

C++中基于范围的for循环是从C++11开始添加的。它在一个范围内执行for循环。用作在一系列值（例如容器中的所有元素）上进行操作的传统for循环的可读性更强的等价物。

syntax:

for ( range_declaration : range_expression ) 
    loop_statement

Parameters :
range_declaration : 
a declaration of a named variable, whose type is the 
type of the element of the sequence represented by 
range_expression, or a reference to that type.
Often uses the auto specifier for automatic type 
deduction.

range_expression : 
any expression that represents a suitable sequence 
or a braced-init-list.

loop_statement : 
any statement, typically a compound statement, which
is the body of the loop.

code demo:

#include <iostream>
#include <vector>
#include <map>
int main() 
{
    // Iterating over whole array
    std::vector<int> v = {0, 1, 2, 3, 4, 5};
    for (auto i : v)
        std::cout << i << ' ';
      
    std::cout << '\n';
      
    // the initializer may be a braced-init-list
    for (int n : {0, 1, 2, 3, 4, 5})
        std::cout << n << ' ';
      
    std::cout << '\n';
   
    // Iterating over array
    int a[] = {0, 1, 2, 3, 4, 5};     
    for (int n : a)
        std::cout << n << ' ';
      
    std::cout << '\n';
      
    // Just running a loop for every array
    // element
    for (int n : a)  
        std::cout << "In loop" << ' ';
      
    std::cout << '\n';
      
    // Printing string characters
    std::string str = "Geeks";
    for (char c : str) 
        std::cout << c << ' ';
          
    std::cout << '\n';
  
    // Printing keys and values of a map
    std::map <int, int> MAP({{1, 1}, {2, 2}, {3, 3}});
    for (auto i : MAP)
        std::cout << '{' << i.first << ", " 
                  << i.second << "}\n";
}

4 for_each loop

This loop is defined in the header file “algorithm”: #include<algorithm>, and hence has to be included for successful operation of this loop.

该循环在头文件“算法”中定义：#include＜algorithm＞，因此必须包含该循环才能成功运行。

It is versatile, i.e. Can work with any container.

它是多功能的，即可以与任何容器一起工作。

It reduces chances of errors one can commit using generic for loop

它减少了使用泛型for循环犯错的机会

It makes code more readable

它使代码更具可读性

for_each loops improve overall performance of code

for_ each循环提高了代码的整体性能

syntax:

for_each (InputIterator start_iter, InputIterator last_iter, Function fnc)

start_iter : The beginning position 
from where function operations has to be executed.
last_iter : The ending position 
till where function has to be executed.
fnc/obj_fnc : The 3rd argument is a function or 
an object function which operation would be applied to each element. 

code demo:

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
 
// helper function 1
void printx2(int a)
{
    cout << a * 2 << " ";
}
 
// helper function 2
// object type function
struct Class2
{
    void operator() (int a)
    {
        cout << a * 3 << " ";
    }
} ob1;
 
int main()
{
    // initializing array
    int arr[5] = { 1, 5, 2, 4, 3 };
    cout << "Using Arrays:" << endl;
     
    // printing array using for_each
    // using function
    cout << "Multiple of 2 of elements are : ";
    for_each(arr, arr + 5, printx2);
     
    cout << endl;
     
    // printing array using for_each
    // using object function
    cout << "Multiple of 3 of elements are : ";
    for_each(arr, arr + 5, ob1);
     
    cout << endl;
     
    // initializing vector
    vector<int> arr1 = { 4, 5, 8, 3, 1 };
    cout << "Using Vectors:" << endl;
     
    // printing array using for_each
    // using function
    cout << "Multiple of 2 of elements are : ";
    for_each(arr1.begin(), arr1.end(), printx2);
     
    cout << endl;
     
    // printing array using for_each
    // using object function
    cout << "Multiple of 3 of elements are : ";
    for_each(arr1.begin(), arr1.end(), ob1);
     
    cout << endl;
}

Invalid arguments may leads to Undefined behavior.

无效参数可能导致未定义的行为。

For_each can not work with pointers of an array (An array pointer do not know its size, for_each loops will not work with arrays without knowing the size of an array).

For_ each不能处理数组指针（数组指针不知道其大小，For_each循环在不知道数组大小的情况下不能处理数组）。

ref

https://www.geeksforgeeks.org/loops-in-c-and-cpp

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