Take notes for the tenth day of IOS Learning (string operation), and take notes for the tenth day of ios

Take notes for the tenth day of IOS Learning (string operation), and take notes for the tenth day of ios

Knowledge Point sorting for IOS Learning (C language)

I. String operations

1) string storage, character array, applying for memory space in the heap.

Instance code:

# Include <stdlib. h> # include <string. h> int main () {char str [100] = "hello world"; char * p = "hello world"; // * p = 'H '; // The constant string cannot be modified. p points to the read-only address p = str; * p = 'H'; // printf ("% s", str ); p = (char *) malloc (100); // empty if (p = NULL) return 0; // reset memset (p, 0,100 ); printf ("p = % p \ n", p); // string assignment // memcpy (p, "helloworld", 10); // Method 1: sprintf (p, "% s", "helloworld"); // Method 2: sprintf () Output Value assignment // p = "helloworld"; // error, p points to the constant area, and the applied memory is lost printf ("p = % p \ n", p); printf ("% s", p ); // release free (p); return 0 ;}

2) character operation function. The parameter is the ASCII code value of the corresponding parameter. If the condition is met, 1 is returned. If the condition is not met, 0 is returned. The header file must be referenced. # include <ctype. h>

1. int isalnum (int); // determines whether it is a number or letter.

2. int isalpha (int); // determines whether it is an English character.

3. int isdigit (int); // determines whether the number is 0 ~ 9

4. int islower (int); // determines whether it is a lowercase letter.

5. int isupper (int); // determines whether it is a capital letter.

6. int isxdigit (int); // whether it is a hexadecimal number

7. int tolower (int); // convert it to lowercase.

8. int toupper (int); // converts it to uppercase.

9. int digittoint (int); // converts a hexadecimal number to an integer.

10. int ishexnumber (int); // equivalent to isxdigit

11. int isnumber (int); // equivalent isdigit

Instance code:

 1 #include <ctype.h> 2 int main() 3 { 4     printf("%d\n",isalnum('1'));//1 5     printf("%d\n",isalpha('1'));//0 6     printf("%d\n",isdigit('a'));//0 7     printf("%d\n",islower('A'));//0 8     printf("%d\n",isupper('a'));//0 9     printf("%d\n",isxdigit('F')); //110     printf("%c\n",tolower('A'));//a11     printf("%c\n",toupper('a'));//A12     printf("%d\n",digittoint('1'));//113     printf("%d\n",ishexnumber('a'));//114     printf("%d\n",isnumber('a'));//015     return 0;16 }

3) Use strcpy or strncpy to copy strings. You can specify the copy length.

Instance code:

1 int main () 2 {3 char str [20] = "XXXXXXXXXXXXXX"; 4 char * p = "hello"; 5 strcpy (str, p ); // copy the string from the position of p and copy the '\ 0' following p. if the received string is not large enough, it may cross the border. 6 printf ("% s \ n", str); // print from str to \ 0 end 7 for (int I = 0; I <10; I ++) {8 printf ("str [% d] = % c = % d \ n", I, str [I], str [I]); 9} 10 unsigned long len = strlen (str); 11 strncpy (str, p, len-1); // The final parameter: Max number of copy characters. typically set as the destination string bufsize-1, reserved Terminator 12 str [len-1] = '\ 0 '; // set the last element of the array to the ending character 13 printf ("% s \ n", str); // print from str to \ 0 end 14 15}

4) string comparison function strcmp. The latter of strncmp can limit the length of the comparison string. If the comparison result is equal to 0, the result of s1> s2 is greater than 0;

If the s1 <s2 result is less than 0, the comparison result is equal to the difference of the first ascii code that is not equal to the character string.

Instance code:

1 int main () 2 {3 char * p1 = "helloAworld"; 4 char * p2 = "helloB"; 5 int rst; 6 rst = strcmp (p1, p2 ); 7 printf ("rst = % d \ n", rst); // result:-1 8 rst = strncmp (p1, p2, 2 ); // 2 indicates the maximum length of the comparison 9 printf ("rst = % d \ n", rst); // result: 010 return 0; 11}

5) Search for string functions

1. strchr searches from left to right.

2. Reverse query of strrchr from right to left.

3. strstr returns the position where the s2 string appears for the first time in the s1 string. If the corresponding string is not found, NULL is returned.

Instance code:

1 int main () 2 {3 char * p = "hello china world"; 4 char * pRst; 5 pRst = strchr (p, 'x '); // return NULL 6 printf ("% s \ n", pRst); 7 printf ("% s \ n", strchr (p, 'L') if not found ')); // forward 8 printf ("% s \ n", strrchr (p, 'L'); // reverse 9 pRst = strstr (p, "china "); 10 printf ("% s \ n", pRst); 11 return 0; 12}

6) String concatenation Function

1. strcat: The structure char * strcat (char * dst, const char * src) the remaining memory space pointed to by dst should be sufficient to accommodate the src character string, the memory to which dst src points cannot overlap.

2. strncat, structure char * strncat (char *, const char *, size_t n) size_t n: maximum number of Concatenated characters // array length-current valid number-1 (Reserved Terminator)

Instance code:

1 int main () 2 {3 char str [20] = "hello"; 4 strcat (str, "world"); // At the end of the original string, add the new string 5 printf ("% s \ n", str); 6 int len = (int) strlen (str); 7 strncat (str, "hello world ", 20-len-1); 8 len = (int) strlen (str); 9 printf ("strlen = % d, % s \ n", len, str); 10 return 0; 11}

7) string split function strtok structure char * strtok (char * src, const char * demi ).

1. strtok converts all separators in the src string to '\ 0'

2. directly modify the original string

3. Only the first address of the string needs to be passed during the first call, and NULL needs to be passed from the second call.

4. Write a function to intercept a string and extract any part of the truncated characters.

Implementation Code:

1 void * splitstr (char * str, char * sp, int index) {2 char * result = NULL; 3 result = strtok (str, sp); 4 int num = 0; 5 while (result! = NULL) {6 if (num = index) 7 break; 8 result = strtok (NULL, sp); 9 num ++; 10} 11 return result; 12} 13 14 int main () {15 char str [] = "I love # China # People # Republic"; 16 char sp [] = "#"; 17 char * spstr = str; 18 char * spf = sp; 19 char * result = (char *) splitstr (spstr, spf, 2); // result: people 20 printf ("% s", result); 21 return 0; 22}

8) search strings

1. strpbrk function. The structure is char * strpbrk (const char * s1, const char * s2). The characters in s1 are retrieved in sequence,

When s2 is also included, the search is stopped and the location of the character is returned.

2. strspn function. The structure is size_t strspn (const char * s1, const char * s2). // the number of consecutive characters starting with s1 in S2.

Instance code:

1 int main () 2 {3 char * p1 = "hello world"; 4 char * p2 = "girl"; 5 printf ("% s \ n", strpbrk (p1, p2); // 'L' is also available in p2. 'L' is the first common character of two strings, returns the address 6 char * p3 = "lehRwsrt"; 7 printf ("% lu \ n", strspn (p1, p3) in p1; // p1, the first four characters, which also have 8 return 0; 9} In p3}

9) extract integers starting with a number and ending with a letter.

1. atoi is used to extract int type data

2. atof is used to extract floating point data such as double and float.

3. atol is used to extract long and other long integer data.

Instance code:

 1 int main(){ 2     int a; 3     a=atoi("2adse"); 4     printf("a=%d\n",a); 5  6     double d; 7     d=atof("122dsf3.34f4"); 8     printf("d=%.3f\n",d); 9 10     long l;11     l=atol("23454df3556s");12     printf("l=%ld\n",l);13     return 0;14 }

10) reverse string

Instance code:

1 char * reverse (char * s) {2 char ch; 3 unsigned long len = strlen (s); 4 for (int I = 0; I <= len/2; I ++) {5 ch = * (s + I); 6 * (s + I) = * (s + len-1-i); 7 * (s + len-1-i) = ch; 8} 9 return s; 10} 11 12 char * strcat1 (char * s, const char * ct) {13 char * p = s; 14 while (* p! = '\ 0') {15 p ++; 16} 17 18 while (* ct! = '\ 0') {19 * p = * ct; 20 p ++; 21 ct ++; 22} 23 return s; 24} 25 26 int main () {27 // reverse string 28 char str [100] = "asdfer"; 29 printf ("% s", reverse (str); 30 char * qstr = "qianfeng "; 31 strcat1 (str, qstr); 32 printf ("% s", str); 33 return 0; 34}