Exercises on recursion in C language

of Recursive knowledge points stay ：

(2 Bar message ) C Recursion in language , The difference between recursion and iteration , Recursion and loops _Sky* Shang's blog -CSDN Blog https://blog.csdn.net/m0_64075307/article/details/122862692?spm=1001.2014.3001.5501

subject 1：

Write a function reverse_string(char * string)（ Recursive implementation ）

Realization ： Invert the characters in the parameter string , Not in reverse order .

requirement ： No Be able to use C String manipulation functions in the function library .

such as :

char arr[] = "abcdef";


 In reverse order, the contents of the array become ：fedcba

Code implementation ：（ Non recursive methods ）

#include <stdio.h>
#include <string.h>
int main()
{
	char arr[] = "abcdef";
	int sz = strlen(arr); // Use strlen Function requires the introduction of header files 
	char *right = arr + (sz - 1); // Because of the particularity of array subscripts , So you have to subtract 1
	char *left = arr;   // Note that the type of pointer is char, instead of int
	while (left < right)
	{
		char change = *left;
		*left = *right;
		*right = change;
			left++;  // If the pointer wants to ++, You need to create a pointer variable for it first 
		right--;
	}
	printf("%s\n", arr);
	return 0;
}

  Be careful ：

                 Use strlen Function requires the introduction of header files

    char *right = arr + (sz - 1); // Because of the particularity of array subscripts , So you have to subtract 1

    char *left = arr;   // Note that the type of pointer is char, instead of int

     left++;  // If the pointer wants to ++ or --, You need to create a pointer variable for it first

Code implementation （ Recursive method ）

#include <stdio.h>
#include <string.h>
void  exchange(char *str)
{
	int sz = strlen(str); // Notice that formal parameters are used in parentheses , Not arguments 
	char change = *str;
	*str = *(str + sz - 1);// The order of the two sides of the equal sign cannot be reversed 
	*(str + sz - 1) = '\0';
	if (strlen(str + 1) >= 2)// At this point, the condition of recursion is the number of strings 
	{
		exchange(str + 1);
	}
	*(str + sz - 1) = change; // Because exchange requires two steps 
}
int main()
{
	char arr[] = "abcdef";
	exchange(arr);
	printf("%s\n", arr);
	return 0;
}

    int sz = strlen(str);// Notice that formal parameters are used in parentheses , Not arguments
    
    *str = *(str + sz - 1);// The order of the two sides of the equal sign cannot be reversed
    
    if (strlen(str + 1) >= 2)// At this point, the condition of recursion is the number of strings

    *(str + sz - 1) = change; // Because exchange requires two steps

subject 2：

Calculate the sum of each of a number ,

Write a recursive function DigitSum(n), Enter a non negative integer , Returns the sum of the numbers that make up it

for example , call DigitSum(1729), You should go back to 1+7+2+9, Its sum is 19

Input ：1729, Output ：19

Code implementation ：

#include <stdio.h>

int digitsum(unsigned int scan)
{
	int first = scan % 10;
	if (scan > 9)  // Note that the condition here is the number of digits 
	{
		return first + digitsum(scan / 10);
	}
	else
	{
		return scan;
	}
}

int main()
{
	unsigned int data = 0;
	scanf_s("%d", &data);//scanf_s Function %d It's best not to add line breaks 
	int sum = digitsum(data);
	printf("%d\n", sum);
	return 0;
}

subject 3：

Write a function to implement n Of k Power , Use recursion to implement

#include <stdio.h>

double calculation(int base,int n)  // Note that double type 
{
	if (n > 0)
	{
		return base * calculation(base, n - 1);
	}
	else if (n == 0)
	{
		return 1;
	}
	else if (n < 0)  // Don't forget that the exponent can be negative 
	{
		return 1.0 / calculation(base, -n);
	}
}

int main()
{
	int base = 0; 
    int n = 0;  //n Is an integer 
	scanf_s("%d %d", &base, &n);
	double sum = calculation(base, n);  // The result must be of decimal type 
	printf("%lf\n", sum);  
	return 0;
}