Lecture 2-Character Input-Output in C.ppt

1、1,Lecture 2: Character Input/Output in C,Professor Jennifer Rexford COS 217,http:/www.cs.princeton.edu/courses/archive/spring08/cos217/,2,Overview of Todays Lecture,Goals of the lecture Important C constructs Program flow (if/else, loops, and switch) Character input/output (getchar and putchar) Dete

2、rministic finite automata (i.e., state machine) Expectations for programming assignments C programming examples Echo the input directly to the output Put all lower-case letters in upper case Put the first letter of each word in upper case Glossing over some details related to “pointers” which will b

3、e covered in the next lecture,3,Echo Input Directly to Output,Including the Standard Input/Output (stdio) library Makes names of functions, variables, and macros available #include Defining procedure main() Starting point of the program, a standard boilerplate int main(void) int main(int argc, char

4、*argv) Hand-waving: argc and argv are for input arguments Read a single character Returns a single character from the text stream “standard in” (stdin) c = getchar(); Write a single character Writes a single character to “standard out” (stdout) putchar(c);,4,Putting it All Together,#include int main

5、(void) int c;c = getchar();putchar(c);return 0; ,5,Why is the Character an “int”,Meaning of a data type Determines the size of a variable and how it is interpreted and manipulated Difference between char and int char: character, a single byte (256 different values) int: integer, machine-dependent (e

6、.g., -32,768 to 32,767) One byte is just not big enough Need to be able to store any character plus, special value like End-Of-File (typically “-1”) Well see an example with EOF in a few slides,6,Read and Write Ten Characters,Loop to repeat a set of lines (e.g., for loop) Three arguments: initializa

7、tion, condition, and re-initialization E.g., start at 0, test for less than 10, and increment per iteration,#include int main(void) int c, i;for (i=0; i10; i+) c = getchar();putchar(c);return 0; ,7,Read and Write Forever,Infinite for loop Simply leave the arguments blank E.g., for ( ; ; ),#include i

8、nt main(void) int c;for ( ; ; ) c = getchar();putchar(c);return 0; ,8,Read and Write Till End-Of-File,Test for end-of-file (EOF) EOF is a special global constant, defined in stdio The break statement jumps out of the current scope,#include int main(void) int c;for ( ; ; ) c = getchar();if (c = EOF)

9、break;putchar(c);return 0; ,9,Many Ways to Say the Same Thing,for (;) c = getchar();if (c = EOF)break;putchar(c); ,for (c=getchar(); c!=EOF; c=getchar() putchar(c);while (c=getchar()!=EOF)putchar(c);,Very typical idiom in C, but messy side-effects in loop test,c = getchar(); while (c!=EOF) putchar(c

10、);c = getchar(); ,10,Review of Example #1,Character I/O Including stdio.h Functions getchar() and putchar() Representation of a character as an integer Predefined constant EOF Program control flow The for loop and while loop The break statement The return statement Assignment and comparison Assignme

11、nt: “=” Increment: “i+” Comparing for equality “=” Comparing for inequality “!=”,11,Example #2: Convert Upper Case,Problem: write a program to convert a file to all upper-case (leave nonalphabetic characters alone) Program design:repeatread a characterif its lower-case, convert to upper-casewrite th

12、e characteruntil end-of-file,12,ASCII,American Standard Code for Information Interchange0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150 NUL SOH STX ETX EOT ENQ ACK BEL BS HT LF VT FF CR SO SI16 DLE DC1 DC2 DC3 DC4 NAK SYN ETB CAN EM SUB ESC FS GS RS US32 SP ! “ # $ % ? 64 A B C D E F G H I J K L M N O 80 P Q

13、 R S T U V W X Y Z _ 96 a b c d e f g h i j k l m n o 112 p q r s t u v w x y z | DEL,Lower case: 97-122 and upper case: 65-90 E.g., a is 97 and A is 65 (i.e., 32 apart),13,#include int main(void) int c;for ( ; ; ) c = getchar();if (c = EOF) break;if (c = 97) ,Implementation in C,14,Thats a B-minus,

14、Programming well means programs that are Clean Readable Maintainable Its not enough that your program works! We take this seriously in COS 217.,15,#include int main(void) int c;for ( ; ; ) c = getchar();if (c = EOF) break;if (c = 97) ,Avoid Mysterious Numbers,Correct, but ugly to have all these hard

15、-wired constants in the program.,16,#include int main(void) int c;for ( ; ; ) c = getchar();if (c = EOF) break;if (c = a) ,Improvement: Character Literals,17,Standard C Library Functions ctype(3C)NAMEctype, isdigit, isxdigit, islower, isupper, isalpha, isalnum, isspace, iscntrl, ispunct, isprint, is

16、graph, isascii - character handlingSYNOPSIS#include int isalpha(int c);int isupper(int c);int islower(int c);int isdigit(int c);int isalnum(int c);int isspace(int c);int ispunct(int c);int isprint(int c);int isgraph(int c);int iscntrl(int c);int toupper(int c);int tolower(int c);,Improvement: Existi

17、ng Libraries,DESCRIPTIONThese macros classify character-coded integer values. Each is a predicate returning non-zero for true, 0 for false.The toupper() function has as a domain a type int, the value of which is representable as an unsigned char or the value of EOF If the argument of toupper() repre

18、sents a lower-case letter . the result is the corresponding upper-case letter. All other arguments in the domain are returned unchanged.,18,Using the ctype Library,#include #include int main(void) int c;for ( ; ; ) c = getchar();if (c = EOF) break;if (islower(c)c = toupper(c);putchar(c);return 0; ,1

19、9,% ls get-upper.c % gcc get-upper.c % ls a.out get-upper.c % a.out Well be on time today! WELL BE ON TIME TODAY! D %,Compiling and Running,20,% a.out #INCLUDE INT MAIN(VOID) INT C;FOR ( ; ; ) C = GETCHAR();IF (C = EOF) BREAK;IF (ISLOWER(C)C = TOUPPER(C);PUTCHAR(C);RETURN 0; ,Run the Code on Itself,

20、21,% a.out test.c % gcc test.c test.c:1:2: invalid preprocessing directive #INCLUDE test.c:2:2: invalid preprocessing directive #INCLUDE test.c:3: syntax error before “MAIN“ etc.,Output Redirection,22,Review of Example #2,Representing characters ASCII character set Character constants (e.g., A or a)

21、 Manipulating characters Arithmetic on characters Functions like islower() and toupper() Compiling and running C code Compile to generate a.out Invoke a.out to run program Can redirect stdin and/or stdout,23,Example #3: Capitalize First Letter,Capitalize the first letter of each word “cos 217 rocks”

22、 “Cos 217 Rocks” Sequence through the string, one letter at a time Print either the character, or the upper-case version Challenge: need to remember where you are Capitalize “c” in “cos”, but not “o” in “cos” or “c” in “rocks” Solution: keep some extra information around Whether youve encountered th

23、e first letter in the word,24,Deterministic Finite Automaton,Deterministic Finite Automaton (DFA),1,2,letter,letter,not-letter,not-letter,State #1: before the 1st letter of a word State #2: after the 1st letter of a word Capitalize on transition from state 1 to 2“cos 217 rocks” “Cos 217 Rocks”,25,#i

24、nclude #include int main (void) int c;for ( ; ; ) c = getchar();if (c = EOF) break;return 0; ,Implementation Skeleton,26, = switch (state) case 1: break;case 2:break;default:,Implementation,if (isalpha(c) putchar(toupper(c);state = 2; else putchar(c);,if (!isalpha(c)state = 1; putchar(c);,27,#includ

25、e #include int main(void) int c; int state=1;for ( ; ; ) c = getchar();if (c = EOF) break;switch (state) case 1: if (isalpha(c) putchar(toupper(c);state = 2; else putchar(c);break;case 2:if (!isalpha(c) state = 1;putchar(c);break;return 0; ,Complete Implementation,28,Running Code on Itself,% gcc upp

26、er1.c % a.out #Include Int Main(Void) Int C; Int State=1;For ( ; ; ) C = Getchar();If (C = EOF) Break;Switch (State) Case 1: If (Isalpha(C) Putchar(Toupper(C);State = 2; Else Putchar(C);Break;Case 2:If (!Isalpha(C) State = 1;Putchar(C);Break;Return 0; ,29,OK, Thats a B+,Works correctly, but Mysterio

27、us integer constants (“magic numbers”) No modularization No checking for states besides 1 and 2What now? States should have names, not just 1,2 Should handle each state in a separate function Good to check for unexpected variable value,30,Improvement: Names for States,Define your own named constants

28、 Enumeration of a list of items enum Statetype NORMAL,INWORD; Declare a variable of that type enum Statetype state;,31,#include #include enum Statetype NORMAL,INWORD; int main(void) int c; enum Statetype state = NORMAL;for ( ; ; ) c = getchar();if (c = EOF) break;switch (state) case NORMAL: if (isal

29、pha(c) putchar(toupper(c);state = INWORD; else putchar(c);break;case INWORD:if (!isalpha(c) state = NORMAL;putchar(c);break;return 0; ,Improvement: Names for States,32,Improvement: Modularity,#include #include enum Statetype NORMAL,INWORD; enum Statetype handleNormalState(int c) . enum Statetype han

30、dleInwordState(int c) . int main(void) int c;enum Statetype state = NORMAL; for ( ; ; ) c = getchar();if (c = EOF) break;switch (state) case NORMAL: state = handleNormalState(c);break;case INWORD:state = handleInwordState(c);break;return 0; ,33,Improvement: Modularity,enum Statetype handleNormalStat

31、e(int c) enum Statetype state;if (isalpha(c) putchar(toupper(c);state = INWORD; else putchar(c);state = NORMAL;return state; ,34,Improvement: Modularity,enum Statetype handleInwordState(int c) enum Statetype state;putchar(c);if (!isalpha(c)state = NORMAL;elsestate = INWORD;return state; ,35,Improvem

32、ent: Defensive Programming,Assertion checks for diagnostics Check that that an expected assumption holds Print message to standard error (stderr) when expression is false E.g., assert(expression); Makes program easier to read, and to debug,switch (state) case NORMAL: break;case INWORD:break;default:

33、 assert(0);,36,#include #include enum Statetype NORMAL,INWORD;enum Statetype handleNormalState(int c) enum Statetype state;if (isalpha(c) putchar(toupper(c);state = INWORD; else putchar(c);state = NORMAL;return state; ,Putting it Together: An “A” Effort,37,enum Statetype handleInwordState(int c) enu

34、m Statetype state;putchar(c);if (!isalpha(c)state = NORMAL;elsestate = INWORD;return state; ,Putting it Together: An “A” Effort,38,int main(void) int c;enum Statetype state = NORMAL; for ( ; ; ) c = getchar();if (c = EOF) break;switch (state) case NORMAL: state = handleNormalState(c);break;case INWO

35、RD:state = handleInwordState(c);break;return 0; ,Putting it Together: An “A” Effort,39,Review of Example #3,Deterministic Finite Automaton Two or more states Actions in each state, or during transition Conditions for transitioning between states Expectations for COS 217 assignments Modularity (break

36、ing into distinct functions) Readability (meaningful names for variables and values) Diagnostics (assertion checks to catch mistakes) See K&P book for style guidelines specification,40,Another DFA Example,Does the string have “nano” in it? “banano” yes “nnnnnnnanofff” yes “banananonano” yes “bananan

37、anashanana” no,S,2,3,n,n,1,a,n,o,F,a,n,41,Yet Another DFA Example,Valid numbers “-34” “78.1” “+298.3” “-34.7e-1” “34.7E-1” “7.” “.7” “999.99e99”,Invalid numbers “abc” “-e9” “1e” “+” “17.9A” “0.38+” “.” “38.38f9”,Question #4 from fall 2005 midterm Identify whether or not a string is a floating-point

38、number,http:/www.cs.princeton.edu/courses/archive/fall05/cos217/exams/fall05exam1ans.pdf,42,Conclusions,Lectures this week C fundamentals Character I/O Deterministic Finite Automata Reading this week King book: chapters 1-3 Lectures next week Variables pointers and arrays Good programming Reading for next week King book: chapters 4-7 K&P book: chapters 4 and 5,