Standard C language 11

thirty 、 character string

    character ： Patterns that people can understand

        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

    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

        The processing of this data structure is batch , From the beginning to the end

    character string ：

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

    The existence form of string

        A character array ：

            char str[5] = {'1','3','4','2'};

            from char The formed string should be given '\0' Leave a place , Stack memory is used , Data can be modified halfway

        string literal ：

            " Several characters contained in double quotation marks "   Auto complement at the end '\0'

            String literals exist as addresses , Character data exists in code segment , Do not modify the . Correct some errors

            Be careful ： Exactly the same string literal , There is only one copy in the code segment

            const char* p = " string literal ";

            sizeof(p)       Operating system bits ？4:8

            sizeof("1234")  5   Calculate the number of memory bytes occupied by the string literal in the code segment , Include '\0'

    Commonly used way ：

        A character array [] = " string literal ";

        Will automatically give '\0' Leave a place , You can modify the content , Initialization is simple

        There are two copies of the string after the assignment , One in the code segment , One in stack memory （ Modifiable ）

    Input and output of string ：

    Input ：

        char str[200]={};

        scanf("%s",str);

        You cannot enter a string with spaces ,

        char *gets(char *s);

        Return value ： return s, For chain calls , The return value of one function is directly used as the parameter of another function .

            Can input a string with spaces , Do not limit the length of input , There are security risks , The compiler will have a warning .

        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 received +1

        stream：stdin Equivalent to keyboard file Fixed write

        Return value ： For chain calls , The return value of one function is directly used as the parameter of another function .

        If the input exceeds size-1 Characters , The extra ones will not be accepted

        If not enough size-1,'\n' Will also be received

    Output :

        printf("%s", A character array | string literal );

        int puts(const char *s);

        function ： Output string

        Return value ： Number of characters successfully output

        Be careful ： Will automatically print line breaks

Thirty-one 、 Output buffer

    The content to be output 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, it will be displayed on the screen from the buffer .

    1. encounter '\n' after , All output

    2. When encountering an input statement , Output

    3. When the output buffer （4K） Full of

    4. Program end

    5. Linux Provides fflush(stdout); Manually refresh only Linux

Thirty-two 、 Input buffer

    The program does not immediately get the input from the keyboard , But when you press enter , The content entered by the terminal will

    Store it in the input buffer first , Then execute the input function and read 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 , Read failed ;

    Data will remain in the buffer , Affect the subsequent data reading

        solve ：

        First judge whether all inputs are correct , If not , First clean up the cache and re-enter , Until the input is correct .

        int a=-1,b=-1,c=-1;

        while (3>scanf("%d%d%d",a,b,c))

        {

            printf(" There is a problem with the input , Please re-enter ");

            stdin->_IO_read_ptr = stdin->_IO_read_end;      // only Linux

        }

        printf("%d %d %d",a,b,c);

       

    2. fgets() Can receive specified size-1 Characters , If there are extra characters , Will remain in the buffer , May affect the following data

        solve ：

        1.

            fgets(str,size,stdin);

            int len = 0;

            while (str[len])

            len++;

            if('\n' != str[len-1])

            {

                scanf("%*[^\n]");( Regular expressions )

                Read any data from the buffer , If not '\n', Just keep reading , Until I met '\n', Stop reading

                scanf("%*c");

                Read a character from the buffer , And discard  

            }

        2.

            stdin->_IO_read_ptr = stdin->_IO_read_end;（ Only in Linux/UNIX System use ）

            Pointer the current position in the input buffer , Move to the end of the buffer , It is equivalent to clearing the input buffer .

    3. When entering integer or floating-point type first , Then type the characters . Enter characters in 、 When the string , The previous input will remain '\n'

    Affect the following characters 、 Input of string

        solve ：

        character ：

            Add a space before the next input

        character string :

            scanf("%d",&num);

            scanf("%*c");// This is the solution

            gets(str);

        Thirty-three 、 String related operation functions

        （ Be able to realize it by yourself ）

#include <string.h>

            size_t strlen(const char *s);

        function ： Calculate string length ( barring '\0')

        strlen And sizeof difference

            strlen Is the function

                sizeof It's the operator

#include <string.h>

            char *strcpy(char *dest, const char *src);

        function ： hold src Copy to dest, It is equivalent to giving dest assignment .

            Return value ： call chaining

#include <string.h>

            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

#include <string.h>

            int strcmp(const char *s1, const char *s2);

        function ： Compare the size of two strings .

        Start from scratch , Each character is compared one-on-one , In dictionary order , Who is in front and who is young .

        Once the results are compared , Return results immediately , There will be no comparison later .

    Return value ：

    0       s1 == s2    （ The main ）

    Positive numbers    s1 > s2

    negative    s1 < s2

    #include <stdlib.h>

    int atoi(const char *nptr);

    function ： String rotation int

    long atol(const char *nptr);

    function ： String rotation long

    long long atoll(const char *nptr);

    function ： String rotation long long

    double atof(const char *nptr);

    function ： String rotation double

    #include <string.h>

    char *strstr(const char *haystack, const char *needle);

    function ： stay haystack To find out whether there is a substring needle

    Return value ：needle stay haystrck The first place in , No return NULL

    #include <stdio.h>

    int printf(const char *format, ...);

   

    int sprintf(char *str, const char *format, ...);

    function ： Output various types of data to string str

    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 .

    #include <string.h>

    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

    #include <string.h>

    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 come out, there will be no comparison

    Return value ：

    0       s1 == s2    （ The main ）

    Positive numbers    s1 > s2

    negative    s1 < s2