for循环的两种格式 In C++

• for loop in C++

A for loop is a repetition control structure that allows you to efficently write a loop that needs to execute a specific number of times.

• Syntax

1. normal

   for (init; condition; increment){
       statement(s);
   }
   

   which can also expressed :

   for (initialization; condition; increase) statement;
   

   The three fileds in a for-loop are optional. They can be left empty, but in all cases the semicolon signs between them are required.

   A loop with no condition is equivalent to a loop with true as condition (i.e., an infinite loop).

   It is valid that more than a single expression as any of initialization, condition, or statement.

2. Range-based for loop

   The for-loop has another syntax, which is used exclusively with ranges:

   for (declaration : range) statement;
   

   This kind of for loop iterates over all the elements in range, where declaration declares some variable able to take the value of an element in this range. Ranges are sequences of elements, including arrays, containers, and any other type supporting the functions begin and end.

   #include <iostream>
   #include <string>
   using namespace std;
   
   int main()
   {
       string str {"hello"};
       for (char c : str)
       {
           cout << c;
       }
       cout << '\n';
   }
   

   What precedes the colon (:) in the for loop is the declaration of a char variable (the elements in a string are of type char). We then use this variable, c, in the statement block to represent the value of each of the elements in the range.

   This loop is automatic and does not require the explicit declaration of any counter variable.

   Range based loops usually also make use of type deduction for the type of the elements with auto. Typically, the range-bases loop above can also be written as :

   for (auto c : str) 
       cout << c;
   

   Here, the type of c is automatically deduced as the type of the elemets in str.

• References

1. cpluplus