Character字元字符单字

1、Characters and Strings in CCharacter字元/字符/單字A character constant is represented as a character in single quotes單引號. Example: 'a', '5'The data type for character in C is charTo declare宣告 a character variable變量/變數char abc;To assign value to a character variableabc = 'A'String字符

2、串/文字A string constant常量 is represented as a string of characters enclosed by two double quotation marks雙引號.Example: "I am happy".You may view a string as a pointer指標 or an array陣列.· View a string as an array陣列A string in C is actually an array of characters單字陣列.Every string in C is nu

3、ll-terminated with the null character '0' 結束字符.char st="I am happy"Iamhappy0char st='I',' ','a','m',' ','h','a','p','p','y','0'The size of the st variable will be fixed to 11.To make a reference

4、 to a character in a string is similar to make a reference to an element in an array.st0 = 'i'st2 = 'a'st7 = 'p'· View a string as a pointer指標The value of a string is the address地址 of its first character首字元.char *colorPtr = "blue"But you cannot do the following

5、 thing colorPtr1 = 'r'/ 不合法*(colorPtr+1) = 'r'/ 合法as colorPtr points to a string constant and you cannot change part of the constant.Character operations字元運算For character operations, there is a library of character functions to find out if a character is a digit, a lowercase letter,

6、etc.In order to use the functions in the library, you have to add the following line to your C source file.#include <ctype.h>Some of the functions in this library are shown below:Function prototypeFunction descriptionint isdigit(int c)是否數字Returns a true value if c is a digit, and 0 otherwise.i

7、nt isalpha(int c)是否字母Returns a true value if c is a letter, and 0 isalnum(int c)是否數字或字母Returns a true value if c is a digit or a letter, and 0 islower(int c)是否小楷字母Returns a true value if c is a lowercase letter, and 0 isupper(int c)是否大楷字母Returns a true value

8、 if c is an uppercase letter, and 0 tolower(int c)轉小楷字母If c is an uppercase letter, tolower returns c as a lowercase letter. Otherwise, tolower returns the argument toupper(int c)轉大楷字母If c is a lowercase letter, toupper returns c as an uppercase letter. Otherwise, toupper

9、 returns the argument unchanged.String operations文字運算There are several functions to convert strings of digits to integer or floating-point values.Function prototypeFunction descriptiondouble atof(const char *nPtr)文字轉小數Converts the string nPtr to atoi(const char *nPtr)文字轉整數Converts the str

10、ing nPtr to int.long atol(const char *nPtr)文字轉長整數Converts the string nPtr to long int.For example,atof("1.234") will return the floating-point number 1.234. atoi("532") will return the integer number 532.For more string operations, there is a library of string functions for compa

11、ring strings, searching strings, etc.In order to use the functions in that library, you have to add the following line to your C source file.#include <string.h>Some of the functions in this library are shown below:· Find out the length of a string字長/字數The length of a string does not inclu

12、de the '0' character.Function prototypeFunction descriptionsize_t strlen(const char *s)字長/字數Determines the length of string s. The number of characters preceding the terminating '0' character is returned.For example,strlen("ABCDEFG") will return 7.Another example,char temp1

13、9 = "CSC2100b"strlen(temp) will return 8.· Copying strings複製文字In C, you have to use the following two functions for copying strings.Function prototypeFunction descriptionchar *strcpy(char *s1, const char *s2)複製全部Copies the string s2 into the array s1. The value of s1 is returned (s1&#

14、172;s2)char *strncpy(char *s1, const char *s2, size_t n)複製部份Copies at most n characters of the string s2 into the array s1. The value of s1 is returnedFor example, to copy the whole string複製全部from s2 to s1 (s1¬s2)strcpy(s1, s2);For example, to copy only the first 3 characters複製部份(首3字元)in s2 to

15、s1strncpy(s1, s2, 3);As mentioned before, strings in C are simply array of characters. You need to be careful on the size of the string小心字數/長度.For example,char temp14;char temperror3;char temp2 = "ABC"char temp3100;strcpy(temp1, temp2);/* temp1 = temp2 = "ABC" */strcpy(temperror,

16、 temp2);/* 沒有 '0' */strcpy(temp3, temp2);/* temp3 = temp2 = "ABC" */· Concatenating strings文字合併In C, you have to use the following two functions for concatenating strings.Function prototypeFunction descriptionchar *strcat (char *s1, const char *s2)(s1¬s1+s2) 全部合併Appends t

17、he string s2 to the array s1. The first character of s2 overwrites the terminating '0' character of s1. The value of s1 is returned.char *strncat (char *s1, const char *s2, size_t n)(s1¬s1+s2) 部份合併Appends at most n characters of string s2 to array s1. The first character of s2 overwrite

18、s the terminating '0' character of s1. The value of s1 is returned.For example, char s1100 = "Happy "char s2100 = "Happy "char s3 = "Chinese New Year!"strcat(s1, s3); /* s1 become "Happy Chinese New Year!"*/strncat(s2, s3, 7);s213 = '0' /* s2 b

19、ecome "Happy Chinese" */· Comparing strings比較文字In C, you have to use the following two functions for comparing two strings.Function prototypeFunction descriptionint strcmp(const char *s1, const char *s2)比較全部 (s1-s2)Compares the string s1 to the string s2. The function returns 0, less

20、than 0, or greater than 0 if s1 is equal to, less than, or greater than s2, strncmp(const char *s1, const char *s2, size_t n)比較部份Compares up to n characters of the string s1 to the string s2. The function returns 0, less than 0, or greater than 0 if s1 is equal to, less than, or gre

21、ater than s2, respectively.For example,char *s1 = "Happy New Year"/ N=78char *s2 = "Happy New Year"char *s3 = "Happy Birthday"/ B=66n=strcmp(s1, s2);/* 0 (s1=s2) */n=strcmp(s1, s3);/* +1 (s1>s3) */n=strcmp(s3, s1);/* 1 (s3<s1) */n=strncmp(s1, s3, 6);/* 0 "Hap

22、py " */n=strncmp(s1, s3, 7);/* +1 "Happy N" > "Happy B" */n=strncmp(s3, s1, 7);/* 1 "Happy B" < "Happy N" */· Searching找尋/檢索In C, you may use the following functions for searching strings for characters and other strings.Function prototypeFunct

23、ion descriptionchar *strchr(const char *s, int c)文字s中找字元cLocates the first occurrence of character c in string s. If c is found, a pointer to c in s is returned. Otherwise a NULL pointer is returned.char *strstr(const char *s1, const char *s2)文字s1中找文字s2Locates the first occurrence in string s1 of st

24、ring s2. If the string is found, a pointer to the string in s1 is returned. Otherwise a NULL pointer is returned.char *strtok(char *s1, const char *s2)打散文字s1A sequence of calls to strtok breaks string s1 into "tokens" logical pieces such as words in a line of text separated by characters c

25、ontained in string s2. The first call contains s1 as the first argument, and subsequent calls to continue tokenizing the same string contain NULL as the first argument. A pointer to the current token is returned by each call. If there are no more tokens when the function is called, NULL is returned.For example