Part II - C language improvement_ 13. Recursive function

13.1  Basic concept of recursive function

C Support the implementation of recursive functions through the runtime stack . Recursive functions are functions that call themselves directly or indirectly .

13.2 Ordinary function call

void funB(int b)
{
	printf("b = %d\n", b);
}

void funA(int a)
{
	funB(a - 1);
	printf("a = %d\n", a);
}

int main(void)
{
	funA(2);
    printf("main\n");
	return 0;
}

Output results

b = 1
a = 2
main

The calling process of the function is as follows ：

 

13.3 Recursive function calls

void fun(int a)
{
	if (a == 1)
    {
		printf("a = %d\n", a);
		return;                        // Interrupt function is very important 
	}
	fun(a - 1);
	printf("a = %d\n", a);
}

int main()
{	
	fun(2);
	printf("main\n");
	return 0;
}

Output results

a = 1
a = 2
main

The calling process of the function is as follows ：

 

13.4 Recursive implementation of string inversion

int reverse1(char *str)
{
	if (str == NULL)
	{
		return -1;
	}

	if (*str == '\0')           //  Function recursive call end condition 
	{
		return 0;
	}
	reverse1(str + 1);
	printf("%c", *str);
	return 0;
}

char buf[1024] = { 0 };         // Global variables 
int reverse2(char *str)
{
	if (str == NULL) 
	{
		return -1;
	}

	if ( *str == '\0' )        //  Function recursive call end condition 
	{
		return 0;
	}

	reverse2(str + 1);
	strncat(buf, str, 1);
	return 0;
}

int reverse3(char *str, char *dst)
{
	if (str == NULL || dst == NULL) 
	{
		return -1;
	}

	if (*str == '\0') //  Function recursive call end condition 
	{
		return 0;
	}
	reverse3(str + 1);
	strncat(dst, str, 1);
	return 0;
}