C-Language string manipulation function collation

#include <stdio.h> #include <string.h> #include <stdlib.h>int main () {     char *str1= "hello world!";     char *str2= "hello world!";     int len=strlen (STR1);//The string length to be asked, excluding '     printf ' ("len=%d\n" , Len);     int cmp=strcmp (STR1,STR2);     printf ("cmp=%d\n", CMP); &NBSP;&NBSP;&NBSP;&NBSP;//STR1&GT;STR2, returns a value greater than 0, STR1&LT;STR2 returns a value less than 0, which is equal to return 0. The comparison is by character comparison     //int strcmp (Char *str1,char * str2,int n), Just can control the number, other and strcmp () like     char str3[100]= "hello world!";     char str4[100]= "hello world!";     strcat (STR3,STR4);     printf ("str3=%s\n", STR3);    The  //function connects the string str4  to the end of the STR3, which erases the str3 end of ' STR4 ', but it retains and returns the pointer str3. The &NBSP;&NBSP;&NBSP;&NBSP;//STR4 should be const. STR3 space is enough to accommodate STR3 and STR4, or it will sendOverflow error, this function is unsafe     char str5[100]= "hello world!";     char str6[100]= "hello world!";     strncat (str5,str6,5);     printf ("str3=%s\n", STR5);     //This function, like strcat (), is only relatively safe and can explicitly control the number of link characters.     //will erase the STR5 ' ", and will automatically add '     char str7[100]= ' hello  World! ";     char str8[100]= "hello world!";     strcpy (STR7,STR8);     printf ("str7=%s\n", STR7);     //Copy the contents of the Str8 to STR7, including ' \ ', return the STR7 pointer, that is, all erase STR7 content, change to str     // The premise is that the STR7 space should be large enough to accommodate the contents of str8 and ' s ', so this function is relatively unsafe     char str9[100]= "hello world!";     char str10[100]= "hello worldddd!";     strncpy (str9,str10,14);     len=strlen (STR9);     printf ("str9=%s\n", STR9);     printf ("str9 len=%d\n", Len);    /*         : Copy  source  first  num  characters to  destination . If you encounter          to ';  ' in a copy  num  character, then fill ' + ' if  source The length of   is greater than  num, and after the  num  characters are copied, it is not          will be in   Add ' destination  ' to the back. In this case, the,destination  does not end with '/', read it may          be out of bounds.          the usual practice is to set the  n according to the size of  dest  and then read  N-1  characters. Then will  dest[N-1]=         ';    */   '   //Teacher's document as described above, but his own test several times without error, and with strlen () statistics is also correct     //use this function premise is also the target string space to be enough, It's just relative safety     int a=192,b=168,c=100,d=111;    char str11[18];     sprintf (Str11, "%d.%d.%d.%d", A,B,C,D);     len=strlen (STR11);     printf ("str11=%s\n", Str11);     printf ("str11 len=%d\n", len);     //function function: Writes formatted data to a string buffer. The premise is the buffer to be stored under the     char str12[100]= "&NBSP;&NBSP;&NBSP;123ABC";     int  data=atoi (Str12);     printf ("str12 to data=%d\n", data);     /*     *  function: Converts a string to an integer number; Atoi () scans the parameters  nptr  string, skipping the preceding space-word       *  until a number or sign is encountered to start the conversion, and then the non-numeric or string ('/') ends      *  conversions and returns the result (returns the converted integer number), and if the first is a character, it returns 0     *  in Stdlib.h     */     data=123;    char str13[100];    itoa ( data,str13,10)     printf ("base=10,str13=%s\n", Str13);     itoa (data, str13,2);     printF ("base=2,str13=%s\n", Str13),     /*     * converts the integer data to a ' A string that ends in a  str  to the character array that is pointed to.     */    char  in      * stdlib.h Str15[100]= "hello world!"; &NBSP;&NBSP;&NBSP;&NBSP;CHAR&NBSP;*P=STRCHR (Str15, ' l ');     printf ("L in str15  first=%s\n ", p);     //function: Returns the position pointer for the first occurrence of the character  c  in the string  str , not returning to the return   Null.     char str16[100]= "LL";     p=strstr (str15,str16);     printf ("str16 in str15 first=%s\n", p);     //function function: strstr ()   Function search string  str2  Whether it appears in string  str1 . When the searched character     //string is found, the function returns the address of the first matching string, or if the searched string is not found, returns null    char  Str17[100]= "[email protected]@[email protected]";     char *str18=strtok ( Str17, "@");nbsp;   printf ("str18=%s\n", Str18);    int i=0;     for (i=0;i<3;i++) {        str18=strtok (NULL, "@");         printf ("str18=%s\n", Str18);    }     /*     *  function: decomposes a string into a set of strings. s  the string to be decomposed as a delimiter string,delim .          Description:,s  points to the string to decompose when first called, and then calls to set  s  to  null.         strtok  find the characters contained in  delim  in  s  and use   NULL (',  ') to replace the entire string until it is searched.          return value: A split string from the beginning of the  s . Returns  null when there is no split string.          all the characters contained in  delim  are filtered out and set to a segmented node in the filter area.     */    char str19[20]={0};    //memset ( Str19, ' y ', sizeof (STR19));    memset (str19, ' Y ', 5);     printf ("memset str19=%s\n", Str19);     //function:  function copy ch  to buffer  count  Word character,  and returns the buffer pointer from the beginning.     //memset ()   can be applied to initialize a section of memory to a value.     //For example: memset ( the_array,  ',  sizeof (the_array)  );     //This is a convenient way to set the component of an array to 0.     char str20[100]= "hello world!";     char str21[100]= "hello world!";     memcpy (str20,str21,5);     printf ("memcpy str21 to  Str20=%s\n ", str20);     //definition: void *memcpy ( void *to, const void  *from, size_t count )     //function: The function copies the count  characters from the from into to and returns the to pointer. If to  and  from  overlap, the function behavior is indeterminate.     //applied to strings acts like strncpy ()     char str22[100]= "Hello woRld! ";     char str23[100]= "hello world!";     memmove (str22,str23,5);     printf ("Memmove str23 to  str22=%s\n ", str22),     //function:  and mencpy, different when to  and  from  overlap, The function will still work properly.     return 0;}

C-Language string manipulation function collation