Recursion in C

C recursion

Recursion refers to the method of using the function itself in the definition of the function.

For example:
Once upon a time, there was a mountain, and there was a temple in the mountain. In the temple, there was an old monk who was telling stories to the young monk! What is the story? "Once upon a time there was a mountain, and there was a temple in the mountain. In the temple, there was an old monk who was telling a story to the young monk! What is the story?" Once upon a time, there was a mountain, and there was a temple in the mountain. Tell the little monk a story! What is the story?...'"

The syntax format is as follows:

void recursion()
{
   statements;
   ... ... ...
   recursion(); /* 函数调用自身 */
   ... ... ...
}
 
int main()
{
   recursion();
}

The C language supports recursion, that is, a function can call itself. But when using recursion, programmers need to pay attention to define a condition for exiting from the function, otherwise it will enter an infinite loop.

Recursive functions play a vital role in solving many mathematical problems, such as calculating the factorial of a number, generating the Fibonacci sequence, and so on.

Fibonacci sequence

The Fibonacci sequence, also known as the golden section sequence, refers to such a sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

Mathematical recursion representation: F(0)=0, F(1)=1, F(n)=F(n - 1)+F(n - 2) (n ≥ 2, n ∈ N)

Core code breakthrough point: F(0)=0, F(1)=1, F(n)=F(n - 1)+F(n - 2) (n ≥ 2, n ∈ N)

Question: Find the nth Fibonacci sequence

Input: Enter an integer n, representing the nth item of the Fibonacci sequence (0≤n≤20)

Output: output an integer representing the value of the nth item of the Fibonacci sequence
. Example:

Input: 3

Output: 2

Recursive implementation:

#include <stdio.h>
int fibonacci(int n) 
{  
   if(n <= 2)  
     {  return 1;  }  
else  
     {      return fibonacci(n - 1) + fibonacci(n - 2);     } 
}
 int main()
{  int n;//第n项  
   scanf("%d", &n); 
   printf("%d\n", fibonacci(n));//第n项斐波那契数列的值  
   return 0; 
}

Naturally, it can also be realized by means of loops, etc., so I won’t make any more statements here.

Find the factorial of n

The factorial of a positive integer is the product of all positive integers less than or equal to that number, and the factorial of 0 is 1. The factorial of a natural number n is written as n!.

Core code breakthrough point:

greater or equal to:

The factorial representation of any natural number n greater than or equal to 1:

n!=1×2×3×…×(n-1)×n

or

n!=n×(n-1)!

Factorial of 0:

0！=1。

Question: Find the factorial of n

Input: Enter an integer n (0≤n≤10)

Output: Output a number representing the factorial of n

Example:

Input: 5

Output: 120

#include<stdio.h>
int Fact(int n)//递归函数  
{     
    if( n <= 1)     
       {    return 1;     }  
    return n * Fact(n - 1);
}
int main()//主函数  
{     
    int n;//阶乘数      
    scanf("%d", &n);     
    printf("%d\n", Fact(n));     
    return 0; 
}

prints an integer with each bit

Problem: Output each bit of an integer

Input: Enter an integer a (size is within the range of int data)

Output: output each digit separated by spaces

Example:

Input: 13579

Output: 1 3 5 7 9

#include<stdio.h>
void Prt(int a)//递归函数
{     
     if(a > 9)         
        Prt(a / 10);   //单个语句将{}省略     
     printf("%d ", a % 10); 
} 
int main()//主函数  
{     
    int a ;
​    scanf("%d",&a);
​    Prt(a); ​   
    printf("\n"); ​   
    return 0; 
}