Collection of practical string functions

>> Find the string length

strlen

String to ‘\0’ As an end sign ,strlen Function returns in a string ‘\0’ The number of characters that appear before （ It doesn't contain ‘\0’ )
The string that the argument points to must be in ‘\0’ end
Note that the return value of the function is size_t, It's unsigned

Classic examples （ Fallible ）：

#include <stdio.h>
int main()
{
    
   const char*str1 = "abcdef";
   const char*str2 = "bbb";
   if(strlen(str2)-strlen(str1)>0)
   {
    
     printf("str2>str1\n");
   } 
   else
   {
    
     printf("srt1>str2\n");
   }
 return 0; 
}

because strlen The return value is size_t type , therefore strlen(str2)-strlen(str1) It's also size_t type , Must be greater than zero

strlen Simulation Implementation of
List three ways
The way 1：

// Counter mode 
int my_strlen(const char * str) 
{
     
  int count = 0; 
  while(*str) 
  {
     
    count++; 
    str++; 
  } 
  return count; 
}

The way 2：

// Cannot create temporary variable counter 
int my_strlen(const char * str) 
{
     
  if(*str == '\0') 
  return 0; 
  else 
  return 1+my_strlen(str+1); 
}

The way 3：

// The pointer - The way of the pointer 
int my_strlen(char *s) 
{
     
  char *p = s; 
  while(*p != ‘\0’ ) 
  p++; 
  return p-s; 
}

>> Unlimited length string function

strcpy （ Copy string ）

The source string must be in ‘\0’ end
In the source string ‘\0’ Copy to target space
The target space has to be variable , So the source string cannot be copied into the constant string
The target space has to be large enough , To ensure that the source string can be stored

strcpy Simulation Implementation of ：

char *my_strcpy(char *dest, const char*src) 
{
     
  char *ret = dest; 
  assert(dest != NULL); 

  assert(src != NULL); 
 
  while((*dest++ = *src++)) 
  {
     
     ; 
  } 
  return ret; 
}

strcat （ Append string ）

The source string must be in ’\0’ end
The target space has to be large enough , It can accommodate the source string
In the source string ‘\0’ Copy to target space
The target space must be modifiable

String cannot be used behind itself strcat Additional , Because first put the... Of the target string ’\0’ Cover , There is no in the string ’\0’, There will be mistakes

strcat Simulation Implementation of ：

char *my_strcat(char *dest, const char*src) 
{
     
   char *ret = dest; 
   assert(dest != NULL); 
   assert(src != NULL); 
   while(*dest) 
   {
     
     dest++; 
   } 
   while((*dest++ = *src++)) 
   {
     
      ; 
   } 
 return ret; 
}

strcmp （ Compare strings ）

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 less than the second string , Then return less than 0 The number of

strcmp Simulation Implementation of ：
clever , Worthy of taste

int my_strcmp (const char * src, const char * dst) 
{
      
  assert(src != NULL); 
  assert(dest != NULL); 
  while( *src == *dst) 
  {
    
   if(*str=='\0')
     return 0;
    ++src;
    ++dst;
  }
 return( *scr-*dst); 
}

>> String function with limited length

The use method is similar to the string function with unlimited length , Just specify a length , Some special requirements can be achieved , It is also safer

strncpy （ Copy the specified characters to the target space ）

If the length of the source string is less than count, After copying the source string , Add... After the target 0, until count individual

strncpy Implementation method in the Library ：

strncat （ Append the specified characters to the target space ）

After addition , Add one more ’\0’
If the number of characters in the source string is less than count, After appending the source string , Just one more ’\0’

strncat Implementation method in the Library ：

strncmp （ Compare the specified number of characters ）

strncmp The implementation in the library is somewhat complicated ：

>> String search

strstr ( Find the string in the string )

strstr The implementation of the ：

char *my_strstr(const char* str1, const char* str2 ) 
{
     
   assert(str1); 
   assert(str2); 
  
   char *cp = NULL;   // Point to str1 China and str2 The starting position of the comparison 
   char *s1 = NULL;   // Point to str1 Characters being compared in 
   char *s2 = NULL;   // Point to str Characters being compared in 
 
   if(*str2 == '\0') 
     return((char *)str1);   // This is handled in the library function 
 
   while(*cp) 
   {
     
     s1 = cp; 
     s2 = str2; 
     while(*s1 && *s2 && (*s1 == *s2)) 
     {
     
       s1++; 
       s2++; 
     } 
     if(*substr == '\0') 
     return cp; 
     cp++; 
   } 
}

>> String segmentation

strtok ( Divide the string according to the specified character )

strDelimit Points to a collection of defined delimiters
The first parameter specifies a string , It contains 0 One or more specified delimiters
strtok Function found str Next separator in , Replace with \0 , Return a pointer to this position . （ notes ：strtok Function changes the string being manipulated , So it's using strtok The string cut by the function is usually a temporary copy and can be modified .）
strtok The first argument of the function is not NULL , Function will find str The first separator in the ,strtok Function will hold its position in the string
strtok The first argument to the function is NULL , The function will start at the same position in the string that is saved , Find next separator
If there are no more tags in the string , Then return to NULL The pointer

Examples of use ： use @ and . Split mailbox "[email protected]"

#include <stdio.h>
int main()
{
    
	char *p = "[email protected]";
	const char* sep = "[email protected]";   // Specify the separator as '.' and '@'
	char arr[30];
	char *str = NULL;
	strcpy(arr, p);// Make a copy of the data , Handle arr Contents of array 
	for (str = strtok(arr, sep); str != NULL; str = strtok(NULL, sep))
	{
    
		printf("%s\n", str);
	}
}

Running results ：

>> Error message report

strerror ( Return the error information corresponding to the error code )

When there is an error in the program , Error labels are stored in variables errno（ Standard error number , It's defined in errno.h in ), hold errno Pass to strerror function , Will return an error message

For example, open a file that does not exist ：

#include <stdio.h>
#include <string.h> 
#include <errno.h>
int main()
{
    
	FILE * pFile;
	pFile = fopen("unexist.txt", "r");
	if (pFile == NULL)
		printf("%s\n", strerror(errno));
	//errno: Last error number 
	return 0;
}

Running results ：