1. Get the string length function

#include <string.h>
size_t strlen(const char *s);

Function: Calculate the length of a string
Parameters:
s: the specified string
Return value:
the length of the current string
Note: The string length acquired by strlen ends with the first \0 and \0 is not counted as the string length

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

int main(int argc, char *argv[])
{
    
    
    // 使用strlen 函数获取字符串长度
    // strlenn获取的字符串的长度遇到第一个\0结束
    char s1[100] = "hel\0lo";

    printf("s1_len = %d\n", strlen(s1));
    printf("s1_size = %d\n", sizeof(s1));

    char *s2 = "hello";

    printf("s2_len = %d\n", strlen(s2));
    printf("s2_size = %d\n", sizeof(s2));

    return 0;
}

Results of the
Execution result 1

Second, the string copy function

#include <string.h>
char *strcpy(char *dest, const char *src);

Function: copy src to dest
parameter:
- dest: destination string
- src: source string
return value:
- Save the first address of the dest string
Notice:
- When using the strcpy function to copy strings, dest must be large enough, otherwise memory overflow will occur
- strcpy is to copy the first \0 in the src string including \0 to dest

char *strncpy(char *dest, const char *src, size_t n);

Description of the function:
- Copy the first n bytes of the string pointed to by src to the memory pointed to by dest
return value:
- The first address of the destination memory
Notice:
- strncpydo not copy ‘\0’;
- If it nis greater than srcthe number of characters in the pointed-to string, it will destbe padded with n‐strlen(src)characters ’\0’.
  Results of the:

Third, the string append function

#include <string.h>
char *strcat(char *dest, const char *src);

Function: Append src to the end of dest
parameter:
- dest: destination string
- src: source string
return value:
- Save the first address of the dest string

char *strncat(char *dest, const char *src, size_t n);

Function: Append the srcfirst character of the pointed string nto the end of the string pointed to by dest.
Note: If the number of characters is ngreater than , the string will only be appended to the end of the pointed string when appending .srcsrcdest’\0’

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

int main(int argc, char *argv[])
{
    
    
    // 使用strcat函数追加字符串
    char s1[32] = "hello world";
    char s2[32] = "abcdefg";

    //strcat是从s1的\0的位置开始追加，直到s2的第一个\0复制完毕后结束
    strcat(s1,s2);

    printf("s1 = %s\n", s1);

    return 0;
}

Results of the:
Execution result strcat

Fourth, the string comparison function

#include <string.h>
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);

Function: strcmpto compare the contents of two strings, strncmpto compare whether the first n bytes of the two strings are the same
parameter:
- s1, s2: two strings to compare
- n: strncmpThe parameter n in represents the number of bytes to be compared
return value:
- 0: s1 = s2
- >0: s1 > s2
- <0: s1 < s2

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

int main(int argc, char *argv[])
{
    
    
    //使用strcmp比较两个字符串的内容是否一致
    //strcmp函数一个字符一个字符比较，只要出现不一样的，就会立即返回
    char s1[] = "hello";
    char s2[] = "a";

    int ret = strcmp(s1,s2);

    if(ret == 0)
    {
    
    
        printf("s1 = s2\n");
    }
    else if(ret > 0)
    {
    
    
        printf("s1 > s2\n");
    }
    else
    {
    
    
        printf("s1 < s2\n");
    }


    return 0;
}

Results of the:
Execution result strcmp

Five, character lookup function

#include <string.h>
char *strchr(const char *s, int c);

Function: Find the character whose ascii code is c in the string pointed to by the character pointer s
parameter:
- s: the specified string
- c: the character to look for
return value:
- success: address of character found
- Failed: NULL
Note: there are multiple characters whose ASCII is c in the string pointed to by s, then the first character is found

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

Function: In the string pointed to by s, find the last occurrence of the character whose ASCII is c.

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

int main(int argc, char *argv[])
{
    
    
    //使用strchr函数在一个字符串中查找字符
    char s[] = "hel6lo wor6ld";

    //找第一个匹配的字符
    char *ret = strchr(s, '6');

    //找最后一个匹配的字符
    //char *ret = strrchr(s, '6');

    if(ret == NULL)
    {
    
    
        printf("not found\n");
    }
    else
    {
    
    
        printf("found, at the %dth position of string s\n", ret-s);
    }

    return 0;
}

Results of the:
Execution result strchr

6. String matching function

#include <string.h>
char *strtsr(const char *haystack, const char *needle);

Function description: Find the string pointed to by needle in the string pointed to by haystack, which is also the first match
return value:
- Found: the first address of the found string
- Not found: return NULL

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

int main(int argc, char *argv[])
{
    
    
    //使用strstr函数在一个字符串中查找另一个字符串
    char s[] =  "1234:4567:666:789:666:7777";

    //strstr查找的时候，查找的是第二个参数的第一个\0之前的内容
    char *ret =  strstr(s, "666");

    if(ret == NULL)
    {
    
    
        printf("not found\n");
    }
    else
    {
    
    
        printf("found, at the %dth position of string s\n", ret - s);
    }

    return 0;
}

Results of the: Execution result strstr

Seven, string conversion value

#include <stdlib.h>
int atoi(const char *nptr);

Function: Convert a numeric string to integer data
parameter:
- nptr: the specified string
return value:
- The obtained shaping data

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    
    
    //使用atoi将数字型字符串转化为整形数据
    char s1[] = "7856";
    int ret1 = atoi(s1);

    printf("ret1 = %d\n",ret1);

    //使用atof将浮点型的字符串转化为浮点型数据
    char s2[] =  "3.1415926";
    double ret2 = atof(s2);

    printf("ret2 = %lf\n", ret2);

    return 0;
}

Results of the:
Execution result atoi

[String processing function-part1]