Detailed explanation of string function and string function with unlimited length

Preface

author ： Little snail rushes forward

quotes ： I can accept failure , But I can't accept giving up

  If you think the blogger's article is good , Please give the thumbs-up , Collection , Follow and support bloggers . If you find any problems, you are welcome * Please make corrections in the comments section .

  This blog mainly introduces some string function implementations and points for attention .

  Find the string length

strlen function

Definition

  The ginseng

  Pass string In the past or The first address of the string .

This function is used to find the length of the string , encounter '\0' Just stop counting , The return type is size_t（unsigned int ）,

The header file is <string.h>, Next we will continue to introduce how to use .

strlen Use examples ：

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

int main()
{
	char arr []= "abcdef";
	int ret = strlen(arr);
	printf("ret = %d\n", ret);
	return 0;
}

It is also possible to directly transmit strings here .

  We know strlen Use of functions , In order to let us better understand strlen function . Next, I will use three ways to simulate the implementation strlen function .

1 The way of the counter

// Counter implementation 
size_t my_strlen(const char* str)
{
	assert(str);// The assertion string is not an empty string 
	int count = 0;// Record the number of characters 
	while (*str != '\0')
	{
		count++;
		str++;
	}
	return count;
}

2 Recursive implementation

size_t my_strlen_1(const char* str)
{
	assert(str);// The assertion string is not an empty string 
	if (*str != '\0')
	{
		return 1 + my_strlen_1(str + 1);
	}
	else
	{
		return 0;
	}
}

3 The pointer - Pointer implementation

// The pointer - Pointer implementation 
int my_strlen(const char* str)
{
	assert(str);// The assertion string is not an empty string 
	char* start = str;// take arr The address to start 
	char* end = str;//arr Address to end 
	while (*end != '\0') 
	{
		// If *end It's not equal to '\0' 
		end++;// The pointer ++ 
	}
	return end - start;// The pointer - The pointer is equal to the length  
}

  We know strlen How to calculate the string , What should we pay attention to when using this function ？

Be careful ：

String is based on '\0' As a sign of the end ,strlen Is a statistical string '\0' Previous characters .

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

The return type of the string is size_t type .

Unlimited length string function

strcpy String copy function

strcat String append function

strcmp String comparison function

 strcpy String copy function

Definition

  Parameters

strcpy（  Target string , The source string ）;

strcpy Is a string copy function , It can copy the source string into the target string ,

The return type is char*( The return is the address at the beginning of the target string ),

The header file is <string.h>.

strcpy Use examples ： 

int main()
{
	char arr1[20] = "abcdefg" ;
	char arr2[] = "ping";
	strcpy(arr1, arr2);// Copy string 
	printf("%s\n", arr1);
	return 0;
}

  What we should pay attention to here is strcpy Not just arr2 Copy the characters in , Also put '\0' Copy it .

  Simulation Implementation strcpy

char* my_strcpy(char* dest, const char* source)
{
	assert(dest && source);// Assertion 
	char* ret = dest;// Save the first address of the target string 
	while (*dest++ = *source++)
	{
		;
	}
	return ret;
}

strcpy Function note ：

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 .

strcat String append function

Definition

  Parameters

 strcat（  Target string , The source string ）;

strcat Is a string appending function , To be able to The characters of the source string are appended to the destination string .

The return type is char*( The return is the address at the beginning of the target string ),

The header file is <string.h>.

 strcat Use examples ： 

int main()
{
	char arr1[20] = "allow";
	char arr2[] = "act";
    strcat(arr1, arr2);// Append string 
	printf("%s\n", arr1);
	return 0;
}

 strcat Simulation Implementation

char* my_strcat(char* dest, const char* scoure)
{
	assert(dest && scoure);// Assertion 
	char* ret = dest;
	// Find the destination string '\0'
	while (*dest!=NULL)// Don't use it here (*dest++) This will cause skipping '\0'
	{
		dest++;
	}
	// Append string 
	while (*dest++ = *scoure++)
	{
		;
	}
	return ret;
}

Be careful ：strcat function

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

The target space must be large enough , It can hold the contents of the source string .

The target space must be modifiable .

You can't append strings to yourself , The program will crash .

strcmp String comparison function

Definition

  Parameters

 strcmp(string1,string2);

Return value

 

strcmp Is a string comparison function , This function starts with the elements of two strings , Compare （ The essence of comparison is letters ascii The size of the code value ）.

The header file is <string.h>.

 strcmp Use examples ： 

#define  _CRT_SECURE_NO_WARNINGS

#include<stdio.h>
#include<string.h>
\
int main()
{
	char* p1 = "abcdef";
	char* p2 = "afe";
	int ret = strcmp(p1, p2);
	if (ret > 0)
	{
		printf(" Constant string p1 Greater than constant string p2\n");
	}
	else if (ret < 0)
	{
		printf(" Constant string p1 Less than constant string p2\n");
	}
	else
	{
		printf(" Constant string p1 Equal to constant string p2\n");
	}
	return 0;
}

Many people here may have questions , How is a string p1< character string p2, Mingming string p1 Yes 6 Characters and strings p2 Only 3 Characters .

Here we have to distinguish strcmp The function does not compare the number of characters in a string , With p1 and p2 give an example , Compare first p1 Medium a and p2 Medium a It is found that the two are equal , Just continue to compare ,b and f Size , Obviously f It's better than b Big , therefore strcmp Just return to a less than 0 Number of numbers .

We continue simulation First strcmp function Deepen the understanding

int my_strcmp(const char* str1, const char* str2)
{
	assert(str1 && str2);// Assertion 
	while (*str1== *str2)
	{
		// Judge str1 and str2 Whether it is equal or not 
		if (*str1 == '\0')// When str1==str2
		{
			return 0;
		}
		str1++;
		str2++;
	}
	//str1 And str2 It's not equal , Compare the size 
	return (*str1 - *str2);
}

strcmp function

Be careful ：

Understand well strcmp Comparison method , It is convenient for us to use it correctly strcmp function .

If you like, please point a compliment and go ！