C language string introduction and related operation functions

1. character string

1. character

     In a computer, characters are stored as integers , When it needs to be displayed as characters , Will be based on ASCII The corresponding relationship in the code table shows the corresponding symbols or patterns 
    '\0'  0         Null character 
    '0'   48
    'A'   65
    'a'   97

2. strand

It's a data structure , It consists of a continuous set of data of the same type , There must be an end sign at the end
For this kind of data structure, it is batch , From the beginning to the end of the processing flag stop

3. character string

A serial structure consisting of characters , The end sign is ’\0’

2. The existence form of string

1. A character array

    char  str[5]={'a','b','c','d','e'};
     from char An array of types , for '\0' Reserved location 
     Use stack memory , Data can be modified halfway 

2. string literal

“ Several characters contained in double quotation marks ”( There is text Cannot modify in segment , String constant )
The end will automatically fill ’\0’
String literals exist as addresses , There is a code segment for character data , If you modify it, it will definitely produce segment errors
Be careful ： Exactly the same string literal , There is only one copy in the code segment
const char* p=“ string literal ”;
sizeof§; 4/8
sizeof(“abcd”); 5 Calculate the number of memory bytes occupied by the string literal in the code segment , Include ’\0’

3. Commonly used way

A character array []=“ string literal ”;
Will automatically give ’\0’ Reserved location 、 You can modify the content 、 Initialization is simple
There are two copies of the string after the assignment , One in the code segment 、 A section of memory on the stack ( Can be modified )

3. Input of string 、 Output

1. Input ：

    char str[256]={};
    scanf("%s",str);
     shortcoming ： You cannot enter a string with spaces 

    char *gets(char *s);
     function ： A string with spaces can be entered 
     shortcoming ： Do not limit the input length , There are security risks , The compiler will have a warning 
     Return value ： return s, For chain calls , The return value of one function is directly used as the parameter of another function 

    char *fgets(char *s,int size,FILE *stream);
     function ： from stream Maximum input in size-1 Characters to s in 
    s: A character array 
    size： The maximum number of characters accepted +1
    stream:stdin  Equivalent to keyboard file   Fixed write 
     Return value ： call chaining 
     Be careful ： If the input exceeds size-1 Characters , Don't accept the extra ; If the input is insufficient size-1 individual ,'\n' Will also receive 

2. Output ：

    printf("%s", A character array / string literal );
    int puts(const char *s);
     function ： Output string 
     Return value ： The number of characters successfully output 
     Be careful ： Will automatically print line breaks 

4. String related operation functions

#include <string.h>
size_t strlen(const char *s);
function ： Calculate the string character length , barring ’\0’

char *strcpy(char *dest,const char *src);
function ： hold src Copy to dest, It is equivalent to giving dest assignment =
Return value ： call chaining

char *strcat(char *dest,const char *src);
function ： hold src Append to dest At the end of , It is equivalent to merging two strings ,+=
Return value ： call chaining

int strcmp(const char *s1,const char *s2);
function ： Compare the size of two strings
Start from scratch , Compare each character one-on-one , In dictionary order , Who appears in front, who is small , Once the results are compared , Return results immediately , The later ones are no longer compared
Return value ：

    0                           s1==s2
     Positive numbers                          s1>s2
     negative                          s1<s2

int atoi(const char *nptr);
function ： String rotation int type
long atol(const char *nptr);
function ： String rotation long type
long long atoll(const char *nptr);
function ： String rotation long long type
double atof(const char *nptr);
function ： String rotation double type

char *strstr(const char *haystack,const char *needle);
function ： stay haystack To find out whether there is a substring needle
Return value ：needle stay haystack The first place in , Return if not found NULL

int sprintf(char *str,const char *format,...);
function ： Output various types of data to string str
Return value ： Number of converted characters

int sscanf(const char *str,const char *format);
function : from str Parse and read data into variables
Return value ： The number of successfully resolved variables

```void *memcpy(void *dest,const void *src,size_t n);``
function ： from src Location copy n Byte to dest The location of
function ： call chaining

int memcmp(const void *s1,const void *s2,size_t n);
function ： Compare the values of two pieces of memory , Compare by byte , Once the results are compared, there will be no comparison later
Return value ：

         s1 == s2     0
         s1 < s2       negative 
         s1 > s2       Positive numbers 

5. Input buffer

The program does not immediately get the input from the keyboard , But when you press enter , The contents entered by the terminal will be stored in the input buffer first , Then the input function reads data from the input buffer into the content .

1、 When you want to read integer or floating-point data , But when the data in the buffer is symbols or letters , Reading will fail , Data will remain in the buffer , Affect the reading of all data next 
     solve ： Judge scanf Whether all the returned values of are entered correctly , If not , First clear the input buffer and then re-enter , Until all are correct 
2、fgets It is acceptable to specify size-1 Characters , If there are extra characters, they will remain in the buffer , It may affect the next input 
     resolvent 1：
    1、 First judge '\n' In or out of the string , If not , It means that it is in the buffer , The buffer needs to be cleaned up 
    2、 If in the buffer , be 
    scanf("%*[^\n]");
     Read arbitrary data from the buffer and discard , If not '\n', Continue reading , Until I met '\n', Stop reading ( Regular expressions )
    scanf("%*c")
     Read a character from the buffer , And discard 
     Be careful ： Consider packaging new fgets Function to solve the problem of too many inputs 
     Solution 2：
         Pointer the current position in the input buffer , Move to the end of the buffer , It is equivalent to clearing the input buffer , But only in Linux/Unix These two position pointers are used in the system 
        stdin->_IO_read_ptr = stdin->_IO_read_end;
3、 Enter an integer first 、 Floating point data , Then type the characters 、 When the string , The previous input will remain '\n', Affect the following characters 、 Input of string 
     solve ：scanf(" %c",&ch);

6. Output buffer

The content to be output and displayed in the program will not be immediately displayed on the screen , Instead, it is stored in the output buffer first , When certain conditions are met, the content will be displayed from the buffer to the screen
1、 encounter ’\n’ after
2、 When encountering an input statement
3、 When the output buffer is full 4k
4、 At the end of the program
5、 Manually refresh fflush(stdout);