Interpretation and simulation implementation of important library functions in c language-commonly used string library functions

Summary of commonly used string library functions

Function usage and realization

The purpose and realization of char* strcat( char *s1, char *s2)

#include<stdio.h>
#include<string.h>

int main()
{
    
    
	char arr1[20] ="abcd";
	char arr2[20] ="efdg";
	char* pr = NULL;
	pr = strcat(arr1, arr2); // 返回一个char*指针
	printf( "%s\n",pr);
	puts(pr);
	return 0;
}

note:

1. arr1 must specify the size, otherwise the space will be used illegally
2. Both printing methods are available

Simulation implementation

#include<stdio.h>
#include<string.h>

char* My_strcat(char* p, char* r)
{
    
    
	int i = 0;
	while (*p != '\0')
	{
    
    
		p++;
		i++;
	}
	int j = 0;
	char tmp = 0;
	while (*r != '\0')
	{
    
    
		*p = *r;
		p++;
		r++;
		i++;    // 计数器i 得知指针p移动的距离 在退回去
	}

	return p-i;
}


int main()
{
    
    
	char arr1[20] = "ab cd";  // arr 容量不可少
	char arr2[20] = "ef0ko";
	
	char* pr=My_strcat(arr1,arr2);
	printf(pr);
	return 0;
}

The purpose and realization of char* strchr( char *s1, int ch)

Note: What is returned is the address of the string after ch

#include<stdio.h>
#include<string.h>


int main()
{
    
    
	char arr[20] = "abcdef";
	int ch = 'b';
	char* pr = strchr(arr, ch);
	printf("%p\n", *pr);
	printf(pr);
	return 0;
}

What is returned is the address after the
simulation implementation:

char* My_strchr(char* p, int k)
{
    
    
	while (*p != '\0')
	{
    
    
		if (*p == k)
		{
    
    
			return p;
		}
		p++;
	}
	return NULL;
}

int main()
{
    
    
	char arr[20] = "abcdef";
	int ch = 'd';
	char * pr = My_strchr(arr, ch);
	printf(pr);
	return 0;
}

The purpose and realization of char* strcmp( char *s1, char *s2)

#include<stdio.h>
#include<string.h>

int main()
{
    
    
	char arr[20] = "adc";
	char arr2[20] = {
    
     0 };

	scanf("%s", arr2);
	if (!(strcmp(arr, arr2)))
	{
    
    
		printf("oppen");
	}
	else
	{
    
    
		printf("no");
	}
	return 0;
}

实现密码开锁 以及字符串比较

Simulation
method one

int My_strcmp(char* p, char* r)
{
    
    
	int i = 0;
	int j = 0;
	int b = 0;
	assert((*p != NULL) && (*r != NULL));
	while (*(p + i) == '\0')
	{
    
    
		i++;
	}
	while (*(p + i) == '\0')
	{
    
    
		j++;
	}
	if (i != j)
	{
    
    
		return i - j; // 不相等返回差值
	}
	else
	{
    
    
		for (int a = 0; a < i; a++)
		{
    
    
				if (*(p + a) != *(r + a))
				{
    
    
					return 0; // 不相等返回0
				}
			
		}
		return 1; // 相等返回1
	}
}

int main()
{
    
    
	char arr[20] = "abc";
	char arr2[20] = "abcd";
	if ((My_strcmp(arr, arr2))==1)
	{
    
    
		printf("oppen");
	}
	else
	{
    
    
		printf("no");
	}
	return 0;
}

Method Two

#include<stdio.h>
#include<string.h>
#include<assert.h>

int My_strcmp(const char* p, const char* r)
{
    
    
	assert ((*p != NULL) && (*r != NULL));
	while (*p == *r)
	{
    
    
		if (*p == '\0')
		{
    
    
			return 0;
		}
		p++;
		r++;
	}
	return *p - *r;
}
int main()
{
    
    
	char arr[20] = "abc";
	char arr2[20] = "abcd";
	if ((My_strcmp(arr, arr2))==0)
	{
    
    
		printf("oppen");
	}
	else
	{
    
    
		printf("no");
	}
	return 0;
}

The purpose and realization of char* strcpy( char *s1, char *s2)

Copy a string

#include<stdio.h>
#include<string.h>
#include<assert.h>

int main()
{
    
    
	char arr1[20] = "abcd";
	char arr2[20] = {
    
     0 };
	 char*p=strcpy(arr2, arr1);
	//My_strcpy(arr2, arr1);
	printf(arr2);
	printf("\n");
	printf(p);
	return 0;
}

note

1. Copy the back to the front.
2. Pay attention to the allocation of memory, otherwise it will be regarded as illegal memory access
3. What is returned is the first address of the copied string.

Simulation implementation

#include<stdio.h>
#include<string.h>
#include<assert.h>

char* My_strcpy(char* r, char* p)
{
    
    
	int c = 0;
	while (*p != '\0')
	{
    
    
		*r = *p;
			p++;
		r++;
		c++;
	}
	return r - c;
}

int main()
{
    
    
	char arr1[20] = "abcd";
	char arr2[20] = {
    
     0 };
	 char*p=My_strcpy(arr2, arr1);
	//My_strcpy(arr2, arr1);
	printf(arr2);
	printf("\n");
	printf(p);
	return 0;
}

The purpose and realization of unsigned strlen (char*s)

First of all, the difference between strlen and sizeof I have written a special blog, poke sizeof and strlen here

Simulate strlen

method 1

#include<stdio.h>
#include<string.h>
#include<assert.h>

int My_strlen(char* p)
{
    
    
	int i = 0;
	while (*p != '\0')
	{
    
    
		p++;
		i++;
	}
 	return i;
}

int main()
{
    
    
	char arr[] = "abcd";
	int len = My_strlen(arr);
	printf("%d", len);
	return 0;
}

Method 2 recursion

#include<stdio.h>
#include<string.h>
#include<assert.h>


int My_strlen(char* p)
{
    
    
	if (*p == '\0')
	{
    
    
		return 0;   // 出口
	}
	return 1+My_strlen(p+1);
}





int main()
{
    
    
	char arr[] = "abcd";
	int len = My_strlen(arr);
	printf("%d", len);
	return 0;
}

The purpose and realization of char* strstr( char *s1, char *s2)

#include<stdio.h>
#include<string.h>
#include<assert.h>



int main()
{
    
    
	char arr1[20] = "abcdefg";
	char arr2[20] = "def";
	char* p = strstr(arr1, arr2);
	printf(p);
	return 0;
}

Simulation implementation

#include<stdio.h>
#include<string.h>
#include<assert.h>
char* My_strstr(char* p, char* r)
{
    
    
	int i = 0;
	int k = 0;
	while (*p != '\0')
	{
    
    
		if (*p == *r)
		{
    
    
			r++;
			i++;
			p++;
			
		}
		else if (*r == '\0')
		{
    
    
			return p - k;
		}
		else 
		{
    
    
			p++;
			k++;
		}
	}
	if (*p == '\0')
	{
    
    
		return p - i;
	}
	return NULL;
}

int main()
{
    
    
	char arr1[20] = "abcdefg";
	char arr2[20] = "bcdefg";
	char* p = My_strstr(arr1, arr2);
	printf(p);

	if (p == NULL)
	{
    
    
		printf("heh");
	}
	return 0;
}