Analyzing several common string library functions in C language

List of articles

• One 、 Introduce strlen、strcpy、strcpy、strcmp、strstr.
• Two 、 Simulation and implementation of library functions
• summary

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Tips ： The following is the main body of this article , The following cases can be used for reference

One 、 Introduce typical string library functions

1、 Function to find the length of string strlen

        size_t strlen( const char* str);

The string has '\0' As an end sign ,strlen Function returns

In the string '\0' The number of characters that appear before （ It doesn't contain '\0').

Be careful ： The string that the argument points to must be in '\0' end .

            The return value of the function is size_t, It's a sign free .

Usage examples ：

#include<stdio.h>
#include<string.h>
int main()
{

	char arr[] = "ajhbduhr";
	int i = strlen(arr);
	printf("%d ", i);
}

2、 String copy function strcpy   

        char*strcpy(char*destination, const char* source);

notes ： The source string must be in '\0' end .

        In the source string '\0' Copy to target space .

        The target space has to be large enough , To ensure that the source string can be stored .

        The target space has to be variable .

Usage examples ： Use strcpy after , take arr2 Will be copied to arr1 in

#include<stdio.h>
#include<string.h>
int main()
{
	char arr1[100] = {0};
	char arr2[] = "abc";
	strcpy(arr1,arr2);
	printf("%s ", arr1);
}

3、 String append function strcat                                                                                           

        char* strcat (char * destination, const char* source);

  notes ： The source string must be in '\0' end                                                                                           

         The target space has to be large enough , To ensure that the source string can be stored .

Usage examples ： Use strcat after ,arr1 You can add arr2 The content of 、

#include<stdio.h>
#include<string.h>
int main()
{
	char arr1[100] = "duhedu";
	char arr2[] = "abc";
	strcat(arr1,arr2);
	printf("%s ", arr1);
}

4 String comparison function strcmp

        int strcmp ( const char* str1, const char* str2);

The standard stipulates ：

The first string is larger than the second string , Return greater than 0 The number of ;

The first string is equal to the second string , Then return to 0 ;

The first string is larger than the second string , Then return less than 0 The number of .

Usage examples ： Positive number returned 1

#include<stdio.h>
#include<string.h>
int main()
{
	char arr1[100] = "duhedu";
	char arr2[] = "abc";
	int j = strcmp(arr1,arr2);
	printf("%d", j);
}

 5、 String lookup function strstr

        char* strstr(const char* str1, const char* str2);

Its purpose is to find substrings in a string .

If it cannot be found, it returns a null pointer .

If it can be found, it will return the starting address of the substring of the string .

  Usage examples ： Return string arr1.

int main()
{
	char arr1[] = "rnuvhvvr";
	char arr2[] = "rnu";
	char* j = strstr(arr1, arr2);
	if (j == NULL)
		printf(" Can't find ");
	else
		printf("%s", j);
	return 0;
}

                                                                                         

Two 、 Simulation Implementation of string library function

1、strlen Simulation Implementation of .（ Counter method ）

#include<stdio.h>
int my_strlen(char arr[])
{
	int i = 0;
	while (arr[i] != '\0')
	{
		i++;
	}
	return i;
}
int main()
{
	char arr[] = "abcdefg";
	int i = my_strlen(arr);
	printf("%d\n", i);
	
 return 0;
}

2、strcpy Simulation Implementation of .

char* my_strcpy(char* dest, const char* src)
{
	char* ret = dest;
	assert(dest && src);
	while (*dest++ = *src++)
	{
		;
	}
	return ret;
}
int main()
{
	char arr1[] = "abcdef";
	char arr2[10] = { 0 };
	my_strcmp(arr2, arr1);
	printf("%s\n", arr2);
	return 0;
}

3、strcat Simulation Implementation

char* my_strcat(char* dest, const char* src)// because dest Is to be modified , and src It can't be modified, so add const.
//char* my_strcat(char* strDestination, const char* strSource) The function finally returns strDestination
{
	char* ret = dest;// Well preserved dest The address of the starting position of 
	assert(dest && src);//assert We can only judge whether it is NULL, It is impossible to judge whether it is a wild pointer .
	while (*dest)// When dest No '\0'
	{
		dest++;
	}
	while (*dest++ = *src++)// Additional world To hello Of '\0' Location 
	{
		;
	}
	return ret;//
}
int main()
{
	char arr1[20] = "hello";
	char arr2[] = "world";
	my_strcat(arr1, arr2);// Additional 
	printf("%s\n", arr1);
	return 0;
}

4、 strcmp Simulation Implementation

#include<assert.h>
int my_strcmp(const char* str1, const char* str2)
{
	assert(str1 && str2);
	while (*str1 == *str2)
	{
		if (*str1 == '\0')
			return 0;
		str1++;
		str2++;
	}
	if (*str1 > *str2)
		return 1;
	else
		return -1;
}
int main()
{
	char arr1[] = "rnuvhvvr";
	char arr2[] = "rnu";
	char* j = strstr(arr1, arr2);
	if (j == NULL)
		printf(" Can't find ");
	else
		printf("%s", j);
	return 0;
}

5、 strstr Simulation Implementation

char* my_strstr(char* str, char* substr)
{
	assert(str && substr);
	if (*substr == '\0') return str;
	const char* s1 = str;
	const char* s2 = substr;
	char* cur = str;
	while (*cur)
	{
		s1 = cur;
		s2 = substr;
		while (*s1 != '\0' && *s2 !='\0' && * s1 == *s2)
		{
			s1++;
			s2++;
		}
		if (*s2 == '\0')
			return cur;
		cur++;
	}
	return NULL;
}
int main()
{
	char arr1[] = "abcdefabcdef";
	char arr2[] = "bcd";
	char* ret = my_strstr(arr1, arr2);
	if (ret == NULL)
		printf(" Did not find \n");
	else
		printf("%s\n", ret);
	return 0;
}

 

summary

Tips ： Here is a summary of the article ：
for example ： That's what we're going to talk about today , This article only briefly introduces 5 The use of string functions and simple simulation implementation .