Char array parsing

1、 Statement

Here is an example （=> Express Expression equivalence ）：

char a[20] = "abcd";
char b[] = "abcd";      // => char b[5] = "abcd";
 
const char c[] = "abcd";
char *d = "abcd";       // => const char d[] = "abcd";
const char *e = "abcd"; // => const char e[] = "abcd";
 
char *f = a + 1;        //  The pointer  f  Point to  a[1]
const char *g = b;      //  Fixed pointer  g  Point to  b[0]
char *h = f;            //  The pointer  h  And  f  Point to the same address 

2、 Characters and character string

2.1 char character

char Is the character , It's also 0~127 Of unsigned integers . Usually you can use one char The expression is called ASCII code .

String is based on NULL Consecutive addresses at the end . Escape characters are often used in daily life '\0' Express .

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2.2 char [] It's an array

Here we talk about arrays , It is necessary to know the relationship with the pointer .

Basic knowledge ： The array name itself is a pointer , Point to the starting element of the array .

The compiler is processing something like a[i] Of expression when , Convert this expression to *(a + i) In the form of , Then calculate the pointing address of the expression .（ This is also C/C++ Subscript from 0 The reason for the beginning , Represents the offset from the header address .）

thus , Can explain ：

• Use scanf("%s", str) sentence , There is no need to add an address character to a string , because str Itself represents str[] The address of ;

• Use sort(a + 1, a + n + 1) sentence ,a + 1 and a[1] It is the same after compilation , A fellow a[1] The address of .

2.3 char * And char [] The difference between

char * It's essentially a pointer ;char [] It's essentially an array .

special ：

char *a       = "abcd";     // (1)
char a[20] = "abcd";     // (2)

The string that appears in the source code （ use "" What's included ） All strings Constant .(1) A pointer to a string constant "abcd" First character ;(2) The string constant is copied into the character array . obviously The former cannot be modified , The latter can be modified .

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3、 <cstring> library

It is recommended to use the shortcut keys together Control + F.

3.1 String manipulation #include <cstring>

strcpy(p, p1) Copy string

strncpy(p, p1, n) Copies the specified length string

strcat(p, p1) Additional string

strncat(p, p1, n) Attach the specified length string

strlen(p) Take the string length

strcmp(p, p1) Compare strings

strcasecmp Ignore case comparison strings

strncmp(p, p1, n) Compare a specified length string

strchr(p, c) Find the specified character in a string

strrchr(p, c) Looking backward in a string

strstr(p, p1) Find string

strpbrk(p, p1) Take all the characters of the target string as a set , Find any element of the collection in the current string

strspn(p, p1) Take all the characters of the target string as a set , Find the offset of any element that does not belong to the collection in the current string

strcspn(p, p1) Take all the characters of the target string as a set , Find the offset of any element that belongs to the collection in the current string

• The string handler with the specified length fills the zero terminator after the processed string

3.2 String to numeric conversion #include <cstdlib>

strtod(p, ppend) From a string p Medium conversion double Type value , And store the subsequent string pointer to ppend Point to the char* Type storage .

strtol(p, ppend, base) From a string p Medium conversion long Type integer value ,base Explicitly set the converted integer base , Set to 0 To determine the base number used according to a specific format ,0x, 0X Prefix to be interpreted as an integer in hexadecimal format ,0 Prefix to be interpreted as an octal format integer

atoi(p) String conversion to int integer

atof(p) String conversion to double Sign points

atol(p) String conversion to long integer

3.3 Character checking #include <cctype>

isalpha() Check for alphabetic characters

isupper() Check whether it is an uppercase character

islower() Check for lowercase characters

isdigit() Check if it's a number

isxdigit() Check whether it is a valid character represented by hexadecimal digits

isspace() Check whether it is a space type character

iscntrl() Check whether it is a control character

ispunct() Check whether it is punctuation

isalnum() Check for letters and numbers

isprint() Check if it is a printable character

isgraph() Check whether it is a graphic character , Equivalent to isalnum() | ispunct()

3.4 The function prototype

Prototype 1：strcpy(char destination[], const char source[]);

function ： The string source Copy to string destination in

routine ：

#include <iostream.h>
#include <string.h>
void main(void)
{
　 char str1[10] = { "TsinghuaOK"};
　 char str2[10] = { "Computer"};
　 cout <<strcpy(str1,str2)<<endl;
}

The result of the operation is : Computer

The second string will overwrite all the contents of the first string ！

Be careful ： When defining an array , A character array 1 The string length of must be greater than or equal to the string 2 String length of . You cannot assign a string constant or a character array directly to a character array with an assignment statement . All string handling functions are contained in the header file string.h in .

Prototype 2：strncpy(char destination[], const char source[], int numchars);

function ： The string source Middle front numchars Copy characters to string destination in

routine ：

#include <iostream.h>
#include <string.h>
void main(void)
{
　 char str1[10] = { "Tsinghua "};
　 char str2[10] = { "Computer"};
　 cout <<strncpy(str1,str2,3)<<endl;
}

Running results ：Comnghua

Be careful ： character string source Middle front numchars Characters will overwrite character string destination Middle front numchars Characters ！

Prototype 3：strcat(char target[], const char source[]);

function ： The string source Received string target Behind

routine ：

#include <iostream.h>
#include <string.h>
void main(void)
{
　 char str1[] = { "Tsinghua "};
　 char str2[] = { "Computer"};
　 cout <<strcpy(str1,str2)<<endl;
}

Running results ：Tsinghua Computer

Be careful ： When defining a character array 1 The length of the character array should be considered 2 The length of , Because the length of the new string after connection is the sum of the two string lengths . After string connection , character string 1 The terminator of will be automatically removed , At the end of the ending string, keep an ending character after the new string .

Prototype 4：strncat(char target[], const char source[], int numchars);

function ： The string source Before numchars Characters to string target Behind

routine ：

#include <iostream.h>
#include <string.h>
void main(void)
{
　 char str1[] = { "Tsinghua "};
　 char str2[] = { "Computer"};
　 cout <<strncat(str1,str2,3)<<endl;
}

Running results ：Tsinghua Com

Prototype 5：int strcmp(const char firststring[], const char secondstring);

function ： Compare two strings firststring and secondstring

routine ：

#include <iostream.h>
#include <string.h>
void main(void)
{
　 char buf1[] = "aaa";
　 char buf2[] = "bbb";
　 char buf3[] = "ccc";
　 int ptr; 
　 ptr = strcmp(buf2,buf1); 
　 if(ptr > 0)
　　 cout <<"Buffer 2 is greater than buffer 1"<<endl;
　 else 
　　 cout <<"Buffer 2 is less than buffer 1"<<endl;
　 ptr = strcmp(buf2,buf3); 
　 if(ptr > 0)
　　 cout <<"Buffer 2 is greater than buffer 3"<<endl;
　 else 
　　 cout <<"Buffer 2 is less than buffer 3"<<endl;
}

The result of the operation is :

1. Buffer 2 is less than buffer 1

2. Buffer 2 is greater than buffer 3

Prototype 6：strlen(const char string[]);

function ： Statistics string string The number of characters in

routine ：

#include <iostream.h>
#include <string.h>
void main(void)
{
  char str[100]; 
  cout <<" Please enter a string :";
  cin >>str;
  cout <<"The length of the string is :"<<strlen(str)<<" individual "<<endl;
}

Running results ：The length of the string is x (x The total number of characters entered for you )

Be careful ：strlen The function calculates the actual length of a string , barring '\0' , . in addition ,strlen Function can also directly test the length of string constants , Such as ：strlen("Welcome").

7、

void *memset(void *dest, int c, size_t count); 

take dest front count Characters Set as character c, return dest Value .

8、

void *memmove(void *dest, const void *src, size_t count); 

from src Copy count Byte characters to dest. If src and dest Overlap , The function automatically handles . return dest Value .

9、

void *memcpy(void *dest, const void *src, size_t count); 

from src Copy count Byte characters to dest. And memmove Function as , Just can't handle src and dest Overlap . return dest Value .

10、

void *memchr(const void *buf, int c, size_t count); 

stay buf front count Find first occurrence character in bytes c The location of . Characters found c Or have searched count Bytes , Find and stop . Returned if the operation is successful buf First time in c Position pointer of , Otherwise return to NULL.

11、

void *_memccpy(void *dest, const void *src, int c, size_t count); 

from src Copy 0 Characters of one or more bytes to dest. When character c Be copied or count When characters are copied , Replication stopped .

If the characters c Be copied , Function returns a pointer to the next character position after this character . Otherwise return to NULL.

12、

int memcmp(const void *buf1, const void *buf2, size_t count); 

Compare buf1 and buf2 front count Byte size .

Return value < 0, Express buf1 Less than buf2;

The return value is 0, Express buf1 be equal to buf2;

Return value > 0, Express buf1 Greater than buf2.

13、

int memicmp(const void *buf1, const void *buf2, size_t count); 

Compare buf1 and buf2 front count Bytes . And memcmp The difference is , It's case insensitive .

The return value is the same as above .

14、

char *strrev(char *string); 

The string string The order of the characters in is reversed . NULL The ending position remains unchanged . Returns a pointer to the adjusted string .

strrev The function is not C++ Standard functions , stay gcc There is no..., defined in the compiler . An alternative （ Yes BUG）：

char* strrev(char* s) {
    /* h Point to s The head of  */
    char* h = s;
    char* t = s;
    char ch;
 
    /* t Point to s Tail of  */
    while (*t++);
    t--;/*  And t++ offset  */
    t--;/*  Jump back over the Terminator '\0' */
 
    /*  When h and t When not coincident , Swap the characters they point to  */
    while (h < t) {
        ch = *h;
        *h++ = *t;    /* h Move to the tail  */
        *t-- = ch;    /* t Move to the head  */
    }
    return (s);
}

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15、

char *_strupr(char *string); 

take string Replace all lower case letters with corresponding upper case letters , Other characters remain unchanged . Returns a pointer to the adjusted string .

16、

char *_strlwr(char *string); 

take string Replace all upper case letters with corresponding lower case letters , Other characters remain unchanged . Returns a pointer to the adjusted string .

17、

char *strchr(const char *string, int c); 

Find word strand string The first place in , NULL The terminator is also included in the lookup . Return a pointer , Pointing character c In string string The first place in , If not found , Then return to NULL.

18、

char *strrchr(const char *string, int c); 

 strrchr() Function lookup character The position of the first occurrence starting from the end in the specified string , If it works , All characters from this position to the end of the string are returned , If you fail , Then return to false. In contrast to this strstr() function , It finds the position in the string where the specified character first appears .

Example

1 2 3

<?php     echo strrchr('Hello world!','world'); ?>

Output ： world!

1 2 3

<?php     echo strrchr('Helloworld.women!','.'); ?>

Output ： women！

19、strstr()

Return a pointer , Pointing character c In string string The last place in , If not found , Then return to NULL.

char *strstr(const char *string, const char *strSearch); 

In string string Search for strSearch Substring . Returns the substring strSearch stay string Pointer to the first occurrence position in . If no substring is found strSearch, Then return to NULL. If the substring strSearch For the empty string , The function returns string value .

Example ：

1 2 3

char str[]="1234xyz"; char *str1=strstr(str,"34"); cout << str1 << endl;

It is shown that : 34xyz

20、

char *strdup(const char *strSource); 

When the function is running, it will call itself malloc The function is copy strSource String allocates storage space , And then strSource Copy to the allocated space . Pay attention to releasing the allocated space in time .

Return a pointer , Points to the space allocated for the copy string ; If space allocation fails , Then return to NULL value .

char *strcat(char *strDestination, const char *strSource); 

Put the source string strSource Add to target string strDestination Back , And add... After the new string NULL Terminator . The source string strSource The character of will overwrite the target string strDestination The following Terminator NULL. There is no overflow check in the process of copying or adding strings , Therefore, ensure that the target string space is large enough . Cannot handle the case where the source string overlaps with the target string . The function returns strDestination value .

char *strncat(char *strDestination, const char *strSource, size_t count); 

Put the source string strSource At the beginning count Characters are added to the target string strDest after . The source string strSource The character of will overwrite the target string strDestination The following Terminator NULL. If count Greater than source string length , Will be replaced by the length value of the source string count value . The new string will be automatically followed by NULL Terminator . And strcat The function is the same , This function cannot handle the case that the source string overlaps with the target string . The function returns strDestination value .

char *strcpy(char *strDestination, const char *strSource); 

Copy source string strSource To the target string strDestination The designated location , contain NULL Terminator . Cannot handle the case where the source string overlaps with the target string . The function returns strDestination value .

char *strncpy(char *strDestination, const char *strSource, size_t count); 

Put the source string strSource At the beginning count Characters are copied to the target string strDestination The designated location . If count The value is less than or equal to strSource The length of the string , It doesn't automatically add NULL Terminator in target string , and count Greater than strSource The length of the string , Will strSource use NULL The terminator is filled in count Characters , Copy to target string . Cannot handle the case where the source string overlaps with the target string . The function returns strDestination value .

char *strset(char *string, int c); 

take string All characters of the string are set to characters c, encounter NULL The terminator stops . Function returns the adjusted string The pointer .

char *strnset(char *string, int c, size_t count); 

take string The string starts count Characters set as characters c, If count Greater than string The length of the string , Will use string Length replacement of count value . Function returns the adjusted string The pointer .

size_t strspn(const char *string, const char *strCharSet); 

Find any one that is not included in strCharSet Characters in string ( String Terminator NULL With the exception of ) stay string The sequence number of the first position in the string . Returns an integer value , Specified in the string All of them are characters The length of the substring composed of characters in . If string With a not included in strCharSet The beginning of a character in , Function will return 0 value .

size_t strcspn(const char *string, const char *strCharSet); 

lookup strCharSet Any character in the string is in string The sequence number of the first position in the string , Contains the string terminator NULL.

Returns an integer value , Specified in the string In all by non characters The length of the substring composed of characters in . If string With a contained in strCharSet The beginning of a character in , Function will return 0 value .

char *strspnp(const char *string, const char *strCharSet); 

Find any one that is not included in strCharSet Characters in string ( String Terminator NULL With the exception of ) stay string The first position pointer in the string . Return a pointer , Pointing to non strCharSet Characters in string The first place in .

char *strpbrk(const char *string, const char *strCharSet); 

lookup strCharSet Any character in the string is in string The position of the first occurrence in the string , Does not contain a string Terminator NULL.

Return a pointer , Point to strCharSet Any character in string The first place in . If two string parameters do not contain the same characters , Then return to NULL value .

int strcmp(const char *string1, const char *string2); 

Compare strings string1 and string2 size .

Return value < 0, Express string1 Less than string2;

The return value is 0, Express string1 be equal to string2;

Return value > 0, Express string1 Greater than string2.

int stricmp(const char *string1, const char *string2); 

Compare strings string1 and string2 size , and strcmp Different , Compare their lowercase versions . Return value and strcmp identical .

int strcmpi(const char *string1, const char *string2); 

Equivalent to stricmp function , Just provide a backward compatible version .

int strncmp(const char *string1, const char *string2, size_t count); 

Compare strings string1 and string2 size , Just compare the front count Characters . In the process of comparison , The length of any string is less than count, be count Will be replaced by a shorter string length . In this case, if the characters before the two strings are equal , The shorter string is smaller .

Return value < 0, Express string1 The substring of is less than string2 The string of ;

The return value is 0, Express string1 The substring of is equal to string2 The string of ;

Return value > 0, Express string1 The substring of is greater than string2 The string of .

int strnicmp(const char *string1, const char *string2, size_t count); 

Compare strings string1 and string2 size , Just compare the front count Characters . And strncmp The difference is , Compare their lowercase versions . Return value and strncmp identical .

char *strtok(char *strToken, const char *strDelimit); 

stay strToken Find the next tag in the string , strDelimit The character set specifies the delimiters that may be encountered in the current lookup call . Return a pointer , Point at strToken Next tag found in . If the tag is not found , Just go back to NULL value . Each call modifies strToken Content , use NULL Every delimiter encountered by character substitution .

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