String functions that you need to know

Preface

do person ： Little snail rushes forward

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

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This blogger will continue to supplement the string function in the previous blog .

Catalog

Introduction to string functions with limited length

  String lookup function

Report error messages

Character classification function

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Introduction to string functions with limited length

What string function with limited length ？ It is a string function with unlimited length （strcmp,strcpy,strcat） These functions they all encounter '\0' Will stop the function , But there will be some unsafe situations , With strcpy Example of string copy function , When the storage space of the target string is smaller than that of the source string , The copy will still proceed , This causes the program to crash .

  therefore , In order to use the corresponding functions more safely , Strings with limited length are naturally (strncmp,strncpy,strncant). Next, I will analyze for you one by one .

strncpy function

Definition

Parameters  

strncpy(strDest,strSource,count);

Among them strDest finger Target string .

strSource finger The source string .

count Refers to the... To be copied from the source string to the target string The number of characters .

Example implementation

int main()
{
	char arr1[5] = "he";
	char arr2[10] = "hehewang";
	strncpy(arr1, arr2, 4);// Restricted copy of string 
	printf("%s\n", arr1);
	return 0;
}

  Let's go on Simulation Implementation strncpy function

char* my_strncpy(char* dest, char* source, size_t count)
{
	assert(dest && source);// Assertion 
	char* start = dest;// Save the address of the target string 
	while (count && (*dest++ = *source++) != '\0')
	{
		count--;// Each copy of a character to be copied is one less 
	}
	if (count)// if conut The number of characters is greater than the number of copies                               
		while (--count)
			*dest++ = '\0';// The excess part is set 0
	return (start);
}

  about strncpy Is a function that appears for safer use of string copying .

function ：

How many characters can be copied according to your needs .

The header file ：

<string.h>.

Return value ：

The first address of the target string is returned .

strncmp function

Definition

  Parameters

 strncmp(str1,str2,count);

Return value

We know strncmp and strcmp The function of is extremely similar , but strncmp What is more brilliant is that we can compare the characters we want .

  Illustrate with examples

int main()
{
	char* p1 = "abcdfeg";
	char* p2 = "abed";
	int ret = strncmp(p1, p2, 3);
	if (ret > 0)
	{
		printf("p1>p2\n");
	}
	else if (ret < 0)
	{
		printf("p1<p2\n");
	}
	else
	{
		printf("p1=p2");
	}
	return 0;
}

  simulation strncpy

int my_strncmp(const char* p1, const char* p2, size_t count)
{
	assert(p1 && p2);// Assertion 
	// First, judge whether they are equal 
	while ((*p1 == *p2)&&count)
	{
		count--;// Every character is judged --
		if (count==0)
		{
			return 0;//p1==p2
		}
		p1++;
		p2++;
	}
	return (*(unsigned char*)p1 - *(unsigned char*)p2);// return 
}

function

Limited comparison of string longitude

The header file

<string.h>

 strncat function

Definition

  Parameters

 strncat(dest,Soure,count);

strncat Is a limited append string .

The return value is the first address of the returned target string

The header file  <string.h>

Illustrate with examples  

#define  _CRT_SECURE_NO_WARNINGS

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

int main()
{
	char arr1[10] = "menace";
	char arr2[5] = "mehe";
	strncat(arr1, arr2, 2);
	printf("%s\n", arr1);
	return 0;
}

  Simulation Implementation

char* my_strncat(char* dest, const char* source, size_t count)
{
	assert(dest && source);// Assertion 
	// Find the destination string '\0'
	char* start = dest;
	while (*dest != '\0')
	{
		dest++;
	}
	while (count && (*dest++ = *source++))// Add 
	{
		count--;
	}
	*dest = '\0';// Put the end of the string at the end of the target string 
	return start;
}

There may be no doubt , The first address of the return target string is used above ？ What I think of for the moment is that it is easy to print strings .

  String lookup function

Now I will mainly introduce C Language string lookup function (strstr,strtok)

strstr function

Definition

  Parameters

 strstr(str1,str2);

function

strstr Function to find substring , It's a search str1 Whether there is str2 Substring of

Return value

Return to point str1 For the first time in str2 The pointer to , If the substring does not exist , Then return the null pointer .

The header file

<string.h>

Illustrate with examples

int main()
{
	char* p1 = "amcdefme";
	char* p2 = "def";
	char* ret = strstr(p1, p2);// Store the return value of the function 
	if (NULL == ret)
	{
		printf(" Substring does not exist \n");
	}
	else
	{
		printf("%s\n", ret);// Print the substring found 
	}
	return 0;
}

Notice if there is a string , The first address of the first occurrence of the substring returned by the function .

Simulation Implementation strstr

char* my_strstr(const char* p1, const char* p2)
{
	assert(p1 && p2);// Assertion 
	char* s1 = NULL;
	char* s2 = NULL;// Prepare two char* Null pointer of type 
	char* pt = (char*)p1;// Store the starting address of the found substring 
	// Judge p2 Null pointer or not 
	if (*p2 == '\0')
	{
		return (char*)p1;// Substring must exist 
	}
	// Start looking for substrings 
	while (*pt)
	{
		s1 = pt;// Point to the location to find 
		s2 = (char*)p2;// Point to the substring you want 
		while (*s1 && *s2 && (*s1 == *s2))// Analyze whether the strings are equal one by one , So as to infer whether the substring exists 
		{
			s1++;
			s2++;
		}
		if ('\0' == *s2)// Description found substring 
		{
			return pt;// Returns the first address of the first occurrence of a substring 
		}
		pt++;// If you don't find it, keep looking down 
	}
	return (char*)NULL;// Did not find 
}

This simulation implementation is still classic , Interested partners can try .

strtok function

Definition

  Parameters

strtok(strToken,strDelimit)

strDelimit The parameter is a string , Defines the set of characters used as separators

The first parameter specifies a string , It contains 0 One or more by strDelimi A mark separated by one or more separators in a string .

strtok function find strToken The next mark in , And use it \0 ending , Returns a pointer to the tag .（ notes ： strtok Function changes the string being manipulated , So it's using strtok The string segmented by function is usually the content of temporary copy And it can be modified .）

strtok Functional The first parameter is not NULL, Function will find strToken The first mark in ,strtok Function will hold its position in the string .

strtok Functional The first parameter is zero NULL , The function will start at the same position in the string that is saved , Find next tag .

If there are no more tags in the string , Then return to NULL The pointer .

 strtok Illustrate with examples

#define  _CRT_SECURE_NO_WARNINGS

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

int main()
{
	char arr[] = "[email protected]*100";// character string 
	char p[] = "[email protected]*";// Separator 
	char* ret = NULL;
	for (ret = strtok(arr, p);ret != NULL;ret = strtok(NULL, p))
	{
		printf("%s\n",ret);// Print split strings 
	}
	return 0;
}

Report error messages

Now I will continue to share with you , Report error message function strerror function

Definition

  Parameters

 strerror function

Can get system error messages （strerror） Or print the error message provided by the user （_strerror）.

Return value ：

strerror and _strerror Returns a pointer to the error message string . Yes strerror or _strerror Subsequent calls to may overwrite the string .

The header file ：

<string.h>

Illustrate with examples  

char* str = strerror(errno)

//errno Is a global error code variable

// When C Language library functions in the process of execution , Something went wrong , The corresponding error code , Assign values to the errno.

#define  _CRT_SECURE_NO_WARNINGS

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

int main()
{
	FILE* pf = fopen("test.txt", "r");// Find files 
	if (pf == NULL)
	{
		printf("%s\n", strerror(errno));
	}
	else
	{
		printf("open file success\n");
	}
	return 0;
}

 

  Be careful ：

We borrow it here fopen function Functions that can open files , Among them Parameters 1（ Documents ）, Parameters 2（ It refers to the way to open the file ）

FILE* It's a pointer type .

Here, when the system cannot read this file , adopt strerror(errno) We can show the error message to us .

about strerror Function summary ：

strerror() The error in the process of code execution , It's reported .

strerror() The parameter is 1,2,3,4,5....... This type of error code , We can use errno Global static variables receive .

Character classification function

stay c There are many character classification functions in language , Now let's use charts to show ：

function	If his parameters meet the following conditions, it returns true
iscntrl	Any control character
isspace	Blank character ： Space ‘ ’, Change the page ‘\f’, Line break '\n', enter ‘\r’, tabs '\t' Or vertical tabs '\v'
sdigit	Decimal number 0~9
isxdigit	Hexadecimal number , Include all decimal digits , Lowercase letters a~f, Capital A~F
islower	Lowercase letters a~z
isupper	Capital A~Z
isalpha	Letter a~z or A~Z
isalnum	Letters or numbers ,a~z,A~Z,0~9
ispunct	Punctuation , Any graphic character that is not a number or letter （ Printable ）
isgraph	Any graphic character
isprint	Any printable character , Including graphic characters and white space characters

Next, we will explain some character classification functions

islower function Determine whether the character is lowercase

Definition  

Parameters

  Illustrate with examples

int main()
{
	char ret = 0;
	scanf("%c", &ret);
	if (islower(ret) != '\0')
	{
		printf(" Lowercase letters \n");
	}
	else
	{
		printf(" Not lowercase \n");
	}
	return 0;
}

  Now I won't continue to introduce you , If you are interested , You can start to study .

summary

In this blog , It mainly introduces ,strncpy,strncmp,strncat These restricted string functions ; And character lookup function strstr( Find substrings ),strtok( Find a string separated by a delimiter ); And Report error message function strerror and Character classification function .