catalogue
Nested call and chained access of functions
Declaration and definition of functions
What is a function?
Wikipedia definition of function: subroutine
Classification of functions: library functions, custom functions
Library function
Library functions are learned mainly by consulting documents to understand the meaning and how to use library functions
Here we recommend a website for checking library functions: http://www.cplusplus.com
Log in to the website and try to learn the following two common library functions!
1. strcpy usage:
#include <stdio.h>
#include <string.h>
int main ()
{
char str1[]="Sample string";
char str2[40];
char str3[40];
strcpy (str2,str1);
strcpy (str3,"copy successful");
printf ("str1: %s\nstr2: %s\nstr3: %s\n",str1,str2,str3);
return 0;
}2.memset usage (memory setting function)
void * memset ( void * ptr, int value, size_t num );
#include <stdio.h>
#include <string.h>
int main ()
{
char str[] = "almost every programmer should know memset!";
memset (str,'-',6);
puts (str);
return 0;
}Note: the header file corresponding to #include must be included when using library functions
Custom function
ret_type fun_name(para1, * )
{
statement;//Statement item
}
ret_type Return type of function
fun_name Function name
para1 Function parameters
{}And its content function bodyFor example, writing a function can find the maximum value of two integers:
#include <stdio.h>
int get_max(int x, int y)
{
int z = 0;
if (x > y)
z = x;
else
z = y;
return z; //Return maximum (return z)
}
int main()
{
int a = 10;
int b = 20;
//Function call
int max = get_max(a,b);
printf("max = %d\n",max);
} Write a function to exchange the contents of two integers:
#include <stdio.h>
//The return type of the function is void, which means that the function does not return any value and does not need to return
//Function definition part
void Swap(int* pa,int* pb) //pa and pb are formal parameters - formal parameters
{
int z = 0;
z = *pa;
*pa = *pb;
*pb = z;
} //Pass address through pointer
int main()
{
int a = 10;
int b = 20;
//Write a function - swap the values of two integer variables
//Function call part
printf("Before exchange: a=%d b=%d"，a，b)；
Swap2(&a,&b); //&a. & B is the actual parameter - actual parameter
printf("After exchange: a=%d b=%d"，a，b)；
return 0;
}From the difference between the above two examples, we can find the difference between passing values and passing addresses: after the Swap function passes parameters, we hope to exchange a and b outside the function inside the function (the external x and y will be exchanged in this process). In this way, the relationship between the inside and outside of the function will be established. Only when the address is passed, the inside of the function can be found through the address.
Parameters of function
Formal parameters and arguments: according to the comments in the above example, the parameters passed by the function when calling the function are called arguments, and the x and y written in the function definition are called formal parameters.
Actual parameter (actual parameter):
The parameters actually passed to the function are called arguments. Arguments can be constants, variables, expressions, functions, etc. No matter what type of arguments are, they must have definite values when making a function call in order to pass these values to the formal parameters.
Storage unit), so it is called formal parameter. Formal parameters are automatically destroyed after the function call is completed. Therefore, formal parameters are only valid in functions.
After the formal parameter is instantiated, it is actually equivalent to a temporary copy of the argument.
Function call
Nested call and chained access of functions
- Nested Call
#include <stdio.h>
void new_line()
{
printf("hehe\n");
}
void three_line()
{
int i = 0;
for(i=0; i<3; i++)
{
new_line();
}
}
int main()
{
three_line();
return 0;
}Note here: the previous function can call the following function, that is, new_ The three defined later can be called in the line function_ Line function
2. Chain access
//Normal call
int len = strlen("abc");
printf("%d\n",len);
//Chain access
printf("%d\n",strlen("abc"));
//The printing results of the above two cases are the sameDeclaration and definition of functions
- Function declaration:
1. Tell the compiler what a function is called, what the parameters are and what the return type is. But whether it exists or not depends on the function declaration.2. The declaration of a function usually appears before the use of the function. To meet Declaration before use . 3. The declaration of a function is usually placed in the header file
- Function definition: the definition of a function refers to the specific implementation of the function and explains the function implementation of the function.
In the vs compiler, you will find that there are c and h is a file with extended name. The difference between them is: test h declaration of placement function, test Implementation of c placement function. The specific syntax is as follows:
//Declaration of function
int Add(int x, int y);
//Implementation of Add function
#include "test.h"
int Add(int x, int y)
{
return x+y;
}Function recursion
Let's take the solution of Fibonacci sequence as an example. Finally, we output count, which is a very large value. That is to say, we have made many calculations in the solution process, but these calculations are actually a lot of repetitions, which is not as efficient as using iteration
int count = 0;//global variable
int fib(int n)
{
if(n == 3)
count++;
if (n <= 2)
return 1;
else
return fib(n - 1) + fib(n - 2);
}