Introduction to C language II. Functions

Catalog

One . Composition of functions

1. The actual parameter （ Actual parameters ）：

2. Formal parameters （ Shape parameter ）：

Two . Function call

1. Value transfer call

2. Address call

3. Nested calls and chained access to functions

3、 ... and . Function declaration and definition  

1. Declaration of functions ：

2. Definition of function ：

Four . Recursion and iteration of functions

1. Recursion of a function

2. Iteration of function

3. Solve classical problems with function recursion

（1） The hanotta problem

（2） The problem of frog jumping on the steps

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Functions are divided into library functions and user-defined functions . In the process of development, every programmer may use , In order to support portability and improve program efficiency , therefore C A series of similar library functions are provided in the basic library of the language , Convenient for program development .

C Languages have many libraries , Each library contains many library functions .

C The library functions commonly used in the language are ：

IO function

String manipulation functions

Character manipulation functions

Memory manipulation function

Time / Date function

Mathematical functions

Other library functions

One . Composition of functions

ret_type fun_name(para1, * )

{

statement;// Statement item

}

ret_type Return type

fun_name Function name

para1     Function parameter

for example ： Implement a function that can exchange two numbers

#include <stdio.h>
// Method 1 cannot complete the task 
void Swap1(int x, int y) {
 int tmp = 0;
 tmp = x;
 x = y;
 y = tmp; 
}
// Mode 2 can be achieved 
void Swap2(int *px, int *py) {
 int tmp = 0;
 tmp = *px;
 *px = *py;
 *py = tmp; 
}

1. The actual parameter （ Actual parameters ）：

The parameters that are actually passed to the function , It's called the actual parameter .

The argument can be ： Constant 、 Variable 、 expression （ Value of expression ）、 function （ The return value of the function ） etc. .

Whatever the type of argument is , When you make a function call , They all have to have certain values , So that we can send these values to the shape

ginseng .

2. Formal parameters （ Shape parameter ）：

Formal parameters refer to the variables in brackets after the function name , Because formal parameters are only instantiated when the function is called （ Within distribution

Storage unit ）, So it's called formal parameter . Formal parameters are automatically destroyed when the function call is completed . So formal parameters are only valid in functions .

Code 1 ： A formal parameter is a copy of an argument , The address of formal parameter and argument is different , Formal parameters and arguments are independent spaces , The change of formal parameter does not affect the change of actual parameter .

Code 2 ： Pass in the address of the argument , The formal parameter copies the address of the argument , Dereference modify formal parameters , You can modify the arguments .

Two . Function call

1. Value transfer call

The formal and actual parameters of a function occupy different memory blocks , Modification of a parameter does not affect the argument .

2. Address call

Address call is a way to call a function by passing the memory address of the created variable outside the function to the function parameter .

This parameter transfer method can establish a real relationship between the function and the variables outside the function , That is, you can directly operate inside the function

As a variable outside the function .

3. Nested calls and chained access to functions

Different functions can be combined according to actual needs , That is, calling each other . Functions can be called nested , But you can't nest definitions .

for example ： Function two nested calls function one .

#include <stdio.h>

void new_line()// Function one

{

printf("hehe\n");

}

void three_line()// Function two

{

    int i = 0;

for(i=0; i<3; i++)

  {

        new_line();

  }

}

Take the return value of one function as the parameter of another function . for example ：strcat The return value of the function is strlen The parameters of the function

3、 ... and . Function declaration and definition  

1. Declaration of functions ：

1. Tell the compiler that there is a function called , What are the parameters , What is the return type . But does it exist , function

The statement does not determine .（ When declaring, the formal parameter name of the function can be omitted ）

2. The declaration of a function usually precedes the use of the function . To meet the Declare before use .

3. The declaration of the function is usually placed in the header file .

2. Definition of function ：

The definition of a function refers to the concrete implementation of a function , Explain the function realization .

Four . Recursion and iteration of functions

1. Recursion of a function

The programming skill of program calling itself is called recursion （ recursion）.

Recursion as an algorithm is widely used in programming languages . A procedure or function has direct or indirect meaning in its definition or description

Calls itself . The main way to think about recursion is ： Turn the big thing into a small one .

The condition of recursion ： There are restrictions on terminating recursion , Each recursion keeps approaching the limit , Finally reach the limiting conditions .

for example ： Realize the function of printing the bits of a number , The constraint is n For the wrong 0.

#include<stdio.h>
void print(int n) {
	if (n) {
		print(n / 10);
		printf("%d ", n % 10);
	}
}

Recursive implementation of the string length function ：

int my_strlen(char*str){
  if(*str!='\0')
  return 1+my_strlen(str+1);
  else
  return 0;
}

2. Iteration of function

Iteration is Activities that repeat the feedback process , Its purpose is usually to approach the desired goal or result . Iteration of function refers to loop implementation , Is a non recursive process .

For example, o n Factorial function implementation of ：

// Recursive implementation of functions 
int fun1(int n){
  if(n==1)return 1;
  else
  return n*fun1(n-1)
}
// Iterative implementation of function 
int fun2(int n){
  int ret,i;
  for(i=1;i<=n;i++){
    ret=ret*i;
  }
return ret;
}

3. Solve classical problems with function recursion

（1） The hanotta problem

  From the analysis , Steps 1 and 3 can be realized recursively .

// Realize the function of finding the total number of times to move the disk 
int fun(int n){
  if(n==1) return 1;
  else
  return 2*fun(n-1)+1;
}

（2） The problem of frog jumping on the steps

A frog can jump up at a time 1 Stepped steps , You can jump on it 2 level . Ask the frog to jump on one n How many jumps are there in the steps .

List function expressions ：( Similar to the Fibonacci sequence ）

int fun(int n){
  if(n==1)return 1;
  if(n==2)return 2;
  if(n>2) return fun(n-1)+fun(n-2);
}