6. Functions of C language, why functions are needed, how functions are defined, and the classification of functions

1. Why function is needed

         Avoid repetitive operations

       It is conducive to the modularization of the program

2. What is a function              

         logically ： A separate block of code that performs a specific function

         Physically ： Able to receive data , Be able to process the received data , And return the processing result

         summary ： Function is a tool , It is designed to solve a large number of similar problems

3. How to define a function  

         The return value of the function Function name （ A list of formal parameters of a function ）

        {

                The body of the function

        }    

1. Function definition The essence It is a specific method to describe in detail why a function can realize a specific function
2. return expression ; The meaning of
   1. Terminate called function , Returns the value of the expression to the calling function
   2. If the expression is empty , Then terminate the function , Do not return any value to the called function
   3. break Is used to terminate loops and switch Of ,return Is used to terminate the function

      void test_return()
      {
          for (int i = 0; i < 5; i++)
          {
              printf("the number of circle is %d", i);
              return;
          }
          printf("hello world");
      }
      

      The result of this code will not appear helloworld, And it will only appear once the number of circle is 0

This is because in the first execution for loop ,return This function has been terminated .

        3. The type of the return value of a function is also called the type of the function , So if The return value type before the function name and return expression ; If the type of expression in is different , be The final function return value type The return value type before the function name shall prevail .

int funa()

{

    return 10.5; // Because the return value type of the function is int  So the final f The return is 10 instead of 10.5;

}

4. Classification of functions

        There are parametric functions and Nonparametric functions

       There are return value functions and No return function

       Library function and User defined functions

       Ordinary function and The main function （main function ）

              A program must have and can only have one main function

              The main function can call ordinary functions , Ordinary functions cannot call the main function

              Ordinary functions can call each other

              The main function is the entry and exit of the program