1、A First Book of ANSI C Fourth Edition,Chapter 9 Character Strings,A First Book of ANSI C, Fourth Edition,2,Objectives,String Fundamentals Library Functions Input Data Validation Formatting Strings (Optional) Case Study: Character and Word Counting Common Programming and Compiler Errors,A First Book
2、of ANSI C, Fourth Edition,3,String Fundamentals,A string literal is any sequence of characters enclosed in double quotes “Good Morning!“ Also called: string constant, string value, string A string is stored as an array of characters terminated by an end-of-string symbolic constant named NULL (0),A F
3、irst Book of ANSI C, Fourth Edition,4,String Input and Output,gets() accepts and stores the characters typed at the terminal into the character array Pressing the Enter key generates a newline character, n, which is interpreted by gets() as the end-of-character entry All the characters encountered b
4、y gets(), except the newline character, are stored in the message array,A First Book of ANSI C, Fourth Edition,5,String Input and Output (continued),A First Book of ANSI C, Fourth Edition,6,String Input and Output (continued),Sample run: Enter a string: This is a test input of a string of characters
5、. The string just entered is: This is a test input of a string of characters.,A First Book of ANSI C, Fourth Edition,7,String Input and Output (continued),A printf() function call can be used in place of a puts() function call printf(“%sn“,message); puts(message); This correspondence between the out
6、put functions is not duplicated by the input functions scanf() and gets() scanf() reads a set of characters up to either a blank space or a newline character scanf(“%s“,message); /No & is required gets() stops accepting characters only when a newline is detected,A First Book of ANSI C, Fourth Editio
7、n,8,String Input and Output (continued),A First Book of ANSI C, Fourth Edition,9,String Processing,A First Book of ANSI C, Fourth Edition,10,String Processing (continued),A First Book of ANSI C, Fourth Edition,11,NOTE: Because the expression string2i is only 0 at the end of a string and non-0 for ev
8、ery other character, the expression while (string2i != 0) can be replaced by the simpler expression while (string2i).,String Processing (continued),A First Book of ANSI C, Fourth Edition,12,String Processing (continued),A First Book of ANSI C, Fourth Edition,13,Be careful: omitting the parentheses c
9、auses the entire expression to be equivalent to c = (getchar() != n),String Processing (continued),A First Book of ANSI C, Fourth Edition,14,String Processing (continued),A First Book of ANSI C, Fourth Edition,15,Library Functions,Note: Attempting to copy a larger string into a smaller string causes
10、 the copy to overflow the destination array beginning with the memory area immediately following the last array element.,A First Book of ANSI C, Fourth Edition,16,Library Functions (continued),A First Book of ANSI C, Fourth Edition,17,Library Functions (continued),When comparing strings, their indiv
11、idual characters are evaluated in pairs; if a difference is found, the string with the first lower character is the smaller one “Good Bye“ is less than “Hello“ because the first G in Good Bye is less than the first H in Hello “Hello“ is less than “Hello “ because the 0 terminating the first string i
12、s less than the in the second string “123“ is greater than “122“ because 3 in 123 is greater than 2 in 122 “1237“ is greater than “123“ because 7 in 1237 is greater than 0 in 123,A First Book of ANSI C, Fourth Edition,18,Library Functions (continued),A First Book of ANSI C, Fourth Edition,19,Library
13、 Functions (continued),A First Book of ANSI C, Fourth Edition,20,Library Functions (continued),Sample output: Hello is less than Hello thereThe length of string1 is 5 characters The length of string2 is 11 charactersAfter concatenation, string1 contains the string value Hello there World! The length
14、 of this string is 18 charactersType in a sequence of characters for string2: Its a wonderful day After copying string2 to string1, the string value in string1 is: Its a wonderful day The length of this string is 20 charactersThe starting address of the string1 string is: 1244836,A First Book of ANS
15、I C, Fourth Edition,21,Character Routines,A First Book of ANSI C, Fourth Edition,22,Character Routines (continued),A First Book of ANSI C, Fourth Edition,23,Conversion Routines,A First Book of ANSI C, Fourth Edition,24,Conversion Routines (continued),A First Book of ANSI C, Fourth Edition,25,Input D
16、ata Validation,Successful programs always try to anticipate invalid data and isolate such data from being accepted and processed First validate that the data is of the correct type; if not, request the user to re-enter the data Explain why the entered data was invalid One of the most common methods
17、of validating input data is to accept all numbers as strings Each character can then be checked to ensure that it complies with the data type being requested,A First Book of ANSI C, Fourth Edition,26,Input Data Validation (continued),A First Book of ANSI C, Fourth Edition,27,Input Data Validation (c
18、ontinued),A First Book of ANSI C, Fourth Edition,28,Input Data Validation (continued),We can use isvalidInt() in a loop that continually requests an integer until a valid integer value is entered Set an integer variable named isanInt to 0 doAccept a string valueIf the string value does not correspon
19、d to an integerDisplay the error message “Invalid integer - Please re-enter: “Send control back to expression being tested by the do-while statementSet isanInt to 1 (this causes the loop to terminate) while(isanInt is 0) Return the integer corresponding to the entered string,A First Book of ANSI C,
20、Fourth Edition,29,Input Data Validation (continued),A First Book of ANSI C, Fourth Edition,30,Creating a Personal Library,Programmers create their own libraries of functions This permits the functions to be incorporated in any program without further expenditure of coding time Each file in a library
21、 contains related functions #include #include “C:mylibrarydataChecks.h“ The #include statement for dataChecks.h must be placed after the #include statements for the stdio.h and stdlib.h header files (the functions in dataChecks.h require stdio.h and stdlib.h functions to correctly compile),A First B
22、ook of ANSI C, Fourth Edition,31,Formatting Strings,Examples: printf(“|%25s|“,“Have a Happy Day“); | Have a Happy Day| printf(“|%-25s|“,“Have a Happy Day“); |Have a Happy Day | printf(“|%25.12s|“,“Have a Happy Day“); | Have a Happy| printf(“|%.12s|“,“Have a Happy Day“); |Have a Happy|,A First Book o
23、f ANSI C, Fourth Edition,32,In-Memory String Conversions,The sprintf() and sscanf() functions provide capabilities for writing and scanning strings to and from memory variables sprintf(disStrn,“%d %d“, num1, num2); sscanf(data,“%c%lf %d“, “07/01/94“,A First Book of ANSI C, Fourth Edition,33,Format S
24、trings,The control string containing the conversion control sequences need not be explicitly contained within the function printf(“$%5.2f %d“,num1,num2); Or, char fmat = “$%5.2f %d“; printf(fmat,num1,num2); Useful for listing format strings with other variable declarations at the beginning of a func
25、tion If you need to change a format, it is easy to find the desired control string without searching to locate the appropriate printf() or scanf() function calls,A First Book of ANSI C, Fourth Edition,34,Case Study: Character and Word Counting,We construct two string-processing functions Count the n
26、umber of characters in a string Count words in a sting What constitutes a word?,A First Book of ANSI C, Fourth Edition,35,Program Requirement: Character Counting,Pass a string to a function and have the function return the number of characters in the string Any character in the string (blank, printa
27、ble, or nonprintable character) is to be counted The end-of-string NULL character is not to be included in the final count,A First Book of ANSI C, Fourth Edition,36,Analyze the Problem,Determine the input data Determine the required outputs List the algorithm(s) relating the inputs to the outputs,A
28、First Book of ANSI C, Fourth Edition,37,Analyze the Problem (continued),A First Book of ANSI C, Fourth Edition,38,Code the Function,int countchar(char list) int i, count = 0;for(i = 0; listi != 0; i+)count+;return(count); ,A First Book of ANSI C, Fourth Edition,39,Test and Debug the Function,A First
29、 Book of ANSI C, Fourth Edition,40,Requirement Specification: Word Counting,The last word does not have a trailing blank More than one blank may be used between words Leading blanks may be used before the first word,A First Book of ANSI C, Fourth Edition,41,Analyze the Problem,Determine the input da
30、ta Determine the required outputs Algorithm: Set an integer variable named inaword to the symbolic constant NO Set the word count to 0 For all the characters in the arrayIf the current character is a blankset inaword to NOElse if (inaword equals NO)set inaword to the symbolic constant YESincrement t
31、he word countEndIf EndFor Return the count,A First Book of ANSI C, Fourth Edition,42,Code the Function,int countword(char list) #define YES 1 #define NO 0 int i, inaword, count = 0;inaword = NO;for(i = 0; listi != 0; i+)if (listi = )inaword = NO;else if (inaword = NO)inaword = YES;count+;return(coun
32、t); ,A First Book of ANSI C, Fourth Edition,43,Test and Debug the Function,A First Book of ANSI C, Fourth Edition,44,Test and Debug the Function (continued),A sample run using Program 9.11 follows: Type in any number of words: This is a test line with a bunch of words The number of words just entere
33、d is 10 Further tests that should be performed are Enter words with multiple spaces between them Enter words with leading spaces before the first word Enter words with trailing spaces after the last word Enter a sentence that ends in a period or question mark,A First Book of ANSI C, Fourth Edition,4
34、5,Common Programming Errors,Forgetting the terminating NULL character, 0, when processing existing strings in a character-by-character manner Forgetting to terminate a newly created character string with the NULL character Forgetting that the newline character, n, is a valid data input character For
35、getting to include the string.h, ctype.h, and stdlib.h header files when using the string library, character library, and conversion library functions, respectively,A First Book of ANSI C, Fourth Edition,46,Common Compiler Errors,A First Book of ANSI C, Fourth Edition,47,Common Compiler Errors (cont
36、inued),A First Book of ANSI C, Fourth Edition,48,Summary,A string is an array of characters terminated by the NULL (0) character Character arrays can be initialized using a string assignment of the form char arrayName = “text“; Strings can always be processed using standard array-processing techniqu
37、es The gets(), scanf(), and getchar() library functions can be used to input a string The puts(), printf(), and putchar() functions can be used to display strings,A First Book of ANSI C, Fourth Edition,49,Summary (continued),Many standard library functions exist for processing strings as a complete
38、unit The standard C library also includes individual character-handling functions (ctype.h) One of the major uses of strings is validating user input, which is an essential part of any program The conversion routines atoi() and atof() are provided in the stdlib.h header file for converting strings to integer and double-precision numeric values,
