Strings. Chuan-Ming Liu. Computer Science & Information Engineering National Taipei University of Technology Taiwan

Transcription

1 Strings Chuan-Ming Liu Computer Science & Information Engineering National Taipei University of Technology Taiwan 1

2 Outline String Basic String Library Functions Longer Strings: Concatenation and Whole-Line Input String Comparison Array of Pointers Character Operations String-to-Number and Number-to-String Conversion String Process Illustration 2

3 Strings A string is a data structure which is a group of characters In C, the string data structure uses arrays of type char 3

4 String Basics The blanks in the string are characters just as valid as those requiring ink. String constants #define ERR_PREFIX "*****Error - " #define INSUFF_DATA "Insufficient Data" 4

5 Declaring and Initializing In C, a string is implemented as an array char string_var[30]: will hold strings from 0 to 29 characters Differentiating from other arrays with the ability to handle various lengths of string Initialization of string variables using either a braceenclosed character list, or a string constant char str[20] = "Initial value"; null character: character '\0' that marks the end of a string in C 5

6 Arrays of Strings An array of strings is a two-dimensional array of characters in which each row is one string. #define NUM_PEOPLE 30 #define NAME_LEN char names[num_people][name_len]; Initialization char month[12][10] = {"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"}; 6

7 I/O with printf and scanf (1) printf will display the characters until \0 is met Be sure that a null character is inserted at the end right-justified printf ("***%8s***%3s***\n", "Short", "Strings"); left justified printf("%-20s\n", president); 7

8 I/O with printf and scanf (2) Do not apply the address-of operator to a string argument passed to scanf, why? The approach scanf takes to string input is very similar to its processing of numeric input. scanf skips leading whitespace characters such as blanks, newlines, and tabs. 8

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10 I/O with printf and scanf (3) When it comes across a whitespace character, scanning stops, and scanf places the null character at the end of the string in its array argument. 10

11 I/O with printf and scanf (4) scanf would have difficulty if some essential whitespace between values were omitted or if a nonwhitespace separator were substituted. For Fig. 9.2: MATH 1270 TR 1800 MATH1270 TR 1800 Even more out of space problem! MATH,1270,TR,

12 Example int ages[num_people]; 12

13 Outline String Basic String Library Functions Longer Strings: Concatenation and Whole-Line Input String Comparison Array of Pointers Character Operations String-to-Number and Number-to-String Conversion String Process Illustration 13

14 String Library Functions String library: string.h A declaration of a string variable with initialization is the only place in which the operator means to copy the string. An array name with no subscript represents the address of the initial array element, which is constant and cannot be changed through assignment. Invalid target of assignment: char one_str[20]; one_str = "Test string"; /* Does not work */ 14

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16 String Assignment Suppose char one_str[20]; strcpy copies the string that is its second argument into its first argument. strcpy(one_str, "Test String"); Overflow will occur if the copied one has longer length strcpy(one_str, "A very long test string"); 16

17 Copying with Limited Size (1) strncpy takes an argument specifying the number of characters to copy strncpy(one_str, "Test string", 20); If the string to be copied (the source string) is shorter than n characters, the remaining characters stored are null. 17

18 Copying with Limited Size (2) When the source string is longer than n characters, only the first n characters are copied. strncpy(one_str, "A very long test string", 20); no terminating '\0 Solution strncpy(dest, source, dest_len - 1); dest[dest_len - 1] = '\0'; 18

19 Substrings (1) Substrings a fragment of a longer string Example char result[10], s1[15] = "Jan. 30, 1996"; strncpy(result, s1, 9); result[9] = '\0'; strncpy(result, &s1[5], 2); result[2] = '\0'; 19

20 Substrings (2) 20

21 Substrings (3) 21

22 Example: strcpy and strncpy 22

23 Substrings (4) string length the number of characters before the first null character strlen empty string a string of length zero the first character of the string is the null character 23

24 Outline String Basic String Library Functions Longer Strings: Concatenation and Whole-Line Input String Comparison Array of Pointers Character Operations String-to-Number and Number-to-String Conversion String Process Illustration 24

25 Concatenation (1) Concatenation: joining of two strings Related functions: strcat, strncat Example #define STRSIZ 15 char f1[strsiz] = "John ", f2[strsiz] = "Jacqueline ", last[strsiz] = "Kennedy"; strcat(f1, last); strcat(f2, last); /* invalid overflow of f2 */ strncat(f2, last, 3); 25

26 Concatenation (2) Example of overflow prevention if (strlen(s1) + strlen(s2) < STRSIZ) { strcat(s1, s2); } else { strncat(s1, s2, STRSIZ - strlen(s1) - 1); s1[strsiz - 1] = '\0'; } Always be in the mind Is there enough space in the output parameter? Does the created string end in '\0'? 26

27 Characters and Strings character 'Q' and the string "Q" 27

28 Scanning a Full Line (1) gets input of one complete line of data Example char line[80]; printf("type in a line of data.\n"> "); gets(line); Input: > Here is a short sentence. \nis not stored 28

29 Scanning a Full Line (2) fgets takes three arguments the output parameter string, a maximum number of characters to store (n), and the file pointer to the data source. fgets will never store more than n -1 characters from the data file. The final character stored will always be '\0'. If fgets has room to store the entire line of data, it will include '\n' before '\0'. If the line is truncated, no '\n' is stored. When a call to fgets encounters the end of file, the value returned is the address 0, which is considered the null pointer. 29

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32 Outline String Basic String Library Functions Longer Strings: Concatenation and Whole-Line Input String Comparison Array of Pointers Character Operations String-to-Number and Number-to-String Conversion String Process Illustration 32

33 String Comparison (1) If str1 and str2 are string variables, the condition str1 < str2 is not checking whether str1 precedes str2 alphabetically. function strcmp If the first n characters of str1 and str2 match and str1[n], str2[n] are the first nonmatching corresponding characters, str1 is less than str2 if str1[n] < str2[n]. 33

34 String Comparison (2) If str1 is shorter than str2 and all the characters of str1 match the corresponding characters of str2, str1 is less than str2. str1: j o y str2: j o y o u s 34

35 String Comparison (3) 35

36 Sentinel-Controlled Loop for String Input 36

37 Outline String Basic String Library Functions Longer Strings: Concatenation and Whole-Line Input String Comparison Array of Pointers Character Operations String-to-Number and Number-to-String Conversion String Process Illustration 37

38 Arrays of Pointers (1) 38

39 Arrays of Pointers (2) char *alphap[5]; Let alphap[0] address of "daisy" alphap[1] address of "marigold" alphap[2] address of "petunia" alphap[3] address of "rose" alphap[4] address of "tulip" for (i = 0; i < 5; ++i) printf("%s\n", alphap[i]); 39

40 Arrays of Pointers (3) char * alphap[5] 40

41 Two Orderings of One List (1) 41

42 Two Orderings of One List (2) 42

43 Two Orderings of One List (3) 43

44 Arrays of Pointers (4) benefits a pointer requires less storage space than a full copy of a character string. The sorting function executes faster when it copies only pointers and not complete arrays of characters. 44

45 Arrays of String Constants Compare: char month[12][10] = {"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"}; char *month[12] = {"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"}; 45

46 Outline String Basic String Library Functions Longer Strings: Concatenation and Whole-Line Input String Comparison Array of Pointers Character Operations String-to-Number and Number-to-String Conversion String Process Illustration 46

47 Character Operations (1) Using #include <ctype.h> for character manipulation input scanf("%c", &ch) ch = getchar() ch = getc(infilep) // get a single character from a file What if the getchar came across the end of data (i.e., EOF which is a negative value)? Note that the characters are represented as integer codes in C The range of character codes is within the range [0-255] which does not include the EOF 47

48 Implementation of scanline Function Using getchar 48

49 Character Operations (2) output putchar('a'); putc('a', outp); The first argument of putchar and putc is type int character code Character Analysis and Conversion Need to know if a character belongs to a particular subset of the overall character set Is the character a letter? a digit? a punctuation mark? Character case conversion 49

50 Character Analysis and Conversion 50

51 String Function for a Greater- Than Operator (1) 51

52 String Function for a Greater- Than Operator (2) 52

53 Outline String Basic String Library Functions Longer Strings: Concatenation and Whole-Line Input String Comparison Array of Pointers Character Operations String-to-Number and Number-to-String Conversion String Process Illustration 53

54 String-to-Number and Numberto-String Conversions (1) 54

55 String-to-Number and Numberto-String Conversions (2) 55

56 sprintf() sprintf Similar to printf But instead of printing out the result, sprintf stores it in the character array accessed by its first argument 56

57 sscaf() sscanf Similar to scanf But instead of taking the data from standard input device, sscanf takes data from the string that is its first argument 57

58 Convert Representations of Dates 58

59 Convert Representations of Dates 59

60 Convert Representations of Dates 60

61 Convert Representations of Dates 61

62 Outline String Basic String Library Functions Longer Strings: Concatenation and Whole-Line Input String Comparison Array of Pointers Character Operations String-to-Number and Number-to-String Conversion String Process Illustration 62

63 Structure Chart for Text Editor Program 63