英格兰萨里大学：《C语言》课程教学资源（讲义）Lecture 5 - Standard IO

rrodcscLecture 5Standard Input and Output.Manyprograms havetheform:InputDataProcess ItOutputData.Wheredatacanbereadfromthekeyboard or a file and be printed to thescreen ora fileStandard Input and OutputStandardInputandOutputLuckily some nice person has written a whole library In C, the standard input (stdin) is generally connected toofC calls which allowuse to easily perform stdiothe keyboard and the standard output (stdout) to thei.e.screen.However,inUNIX wecanredirecttheeitherorbothat.In order to use these routines we need to include therun timefromthecommand lineappropriate headerfileIfwehaveasimpleprograma.outwhichtakessometext#includefrom the keyboard and prints the results to the screen,then:Wewillstudysomeofthesemorecommonroutines,a.out >outfile.txtsends output to filenamelya.outoutrile.txt- does boh- OUTPUT:puts, printsf and putcharsends output ofprog1to input df prog2prog1 I prog2- INPUT:scanf, getcharhello.cstdout/* Hellod program */The first program we will look at uses puts to put a1N0.stringto standardouttinclude a newline is automafically added to the end of the string see puts.c (available on web)void main()printf ("Hello world");1

1 1. Introduction 2. Binary Representation 3. HardwareandSoftware 4. HighLevel Languages 5. Standard inputand output 6. Operators, expression and statements 7. MakingDecisions 8. Looping 9. Arrays 10. Basics of pointers 11. Strings 12. Basics of functions 13. Moreabout functions 14. Files 14. DataStructures 16. Casestudy:lottery number generator Lecture 5 Standard Input and Output • Many programs have the form: Input Data Process It Output Data • Where data can be read from the keyboard or a file and be printed to the screen or a file Standard Input and Output • In C, the standard input (stdin) is generally connected to the keyboard and the standard output (stdout) to the screen. • However, in UNIX we can redirect the either or both at run time from the command line • If we have a simple program a.out which takes some text from the keyboard and prints the results to the screen, then a.out > outfile.txt - sends output to file a.out outfile.txt - does both prog1 | prog2 - sends output of prog1 to input of prog2 Standard Input and Output • Luckily some nice person has written a whole library of C calls which allow use to easily perform stdio i.e. • In order to use these routines we need to include the appropriate header file #include • We will study some of these more common routines, namely – OUTPUT: puts, printsf and putchar – INPUT: scanf, getchar hello.c /* Hello world program */ #include void main() { printf("Hello world"); } stdout • The first program we will look at uses puts to put a string to standard out – a newline is automatically added to the end of the string – see puts.c (available on web) /* Example: outputting strings using puts */ #include void main() { puts("To be or not to be: that is the question:"); puts("Whether 'tis nobler in the mind to suffer"); puts("The slings and arrows of outrageous fortune,"); puts("Or to take arms against a sea of troubles,"); puts("And by opposing end them? To die: to sleep;"); puts("No more; and by a sleep to say we end"); puts("The heart-ache and the thousand natural shocks"); puts("That flesh is heir to, 'tis a consummation"); puts("Devoutly to be wished. To die, to sleep;"); puts("To sleep: perchance to dream: ay, there's the rub."); puts("For in that sleep of death what dreams may come"); puts("When we have shuffled off this mortal coil,"); puts("Must give us pause."); /* Hamlet */ }

stdoutprintf1.c:Often we need to print data that the program hascalculated and we need to control the formatting ofthis data. e.g. the value 30 could be prnted as 30 or30.00 or+3E+01this can all be done with print f which prints formattedoutputtostandardoutnote: print f does not automafically add a new line for usBasic printingprintrc%i",): for an int j:forafloatxprintr(*%f,x):The %-string is a format conversion stringprintf3.cprintf2.c电:a010:82:1301:140.1: 1--T: 10812:1-1prinf.bugpucharputchar:puta character (to stdout)w... Remember a char can be treated as a characterorasanumbersoputchar(A');andputchar (65);areequivalent2

2 stdout • Often we need to print data that the program has calculated and we need to control the formatting of this data. – e.g. the value 30 could be printed as 30 or 30.00 or +3E+01 – this can all be done with printf which prints formatted output to standard out – note: printf does not automatically add a new line for us • Basic printing – for an int j: printf(“%i”,j); – for a float x: printf(“%f”,x); • The %-string is a format conversion string printf1.c /* Example: outputting numeric data using printf */ #include void main() { int j = 5; float x = 123.4567890123456789; double z = 123.4567890123456789; char c = 'A'; printf("The value of j is %i\n\n", j); printf("The value of x is %f\n", x); printf(".or %E\n", x); printf(".or %30.27f\n\n", x); printf("The value of z is %f\n", z); printf(".or %30.27f\n\n", z); printf("\nThe value of c is %c or %i\n", c, c); c++; printf("The new value of c is %c or %i\n\n", c, c); printf("0.05 is equivalent to %i%%\n\n", j); printf("Hello "); printf("world!\n"); } The value of j is 5 The value of x is 123.456787 .or 1.234568E+002 .or 123.456787109375000000000000000 The value of z is 123.456789 .or 123.456789012345680000000000000 The value of c is A or 65 The new value of c is B or 66 0.05 is equivalent to 5% Hello world! printf2.c /* Example: formatting integer data using printf */ #include main() { int j = 1234; int k = -5678; puts("j = 1234"); puts("k = -5678"); puts("\n|.| is used to show the field width.\n"); printf("Using %%i, j = |%i|\n", j); printf(" k = |%i|\n\n", k); printf("Using %%+i, j = |%+i|\n", j); printf(" k = |%+i|\n\n", k); printf("Using %% i, j = |% i|\n", j); printf(" k = |% i|\n\n", k); printf("Using %%8i, j = |%8i|\n", j); printf(" k = |%8i|\n\n", k); printf("Using %%-8i, j = |%-8i|\n", j); printf(" k = |%-8i|\n\n", k); printf("Using %%- 8i, j = |%- 8i|\n", j); printf(" k = |%- 8i|\n\n", k); printf("Using %%08i, j = |%08i|\n", j); printf(" k = |%08i|\n\n", k); printf("Using %%8.6i, j = |%8.6i|\n", j); printf(" k = |%8.6i|\n\n", k); } j = 1234 k = -5678 |.| is used to show the field width. Using %i, j = |1234| k = |-5678| Using %+i, j = |+1234| k = |-5678| Using % i, j = | 1234| k = |-5678| Using %8i, j = | 1234| k = | -5678| Using %-8i, j = |1234 | k = |-5678 | Using %- 8i, j = | 1234 | k = |-5678 | Using %08i, j = |00001234| k = |-0005678| Using %8.6i, j = | 001234| k = | -005678| printf3.c /* Example: formatting float/double data using printf */ #include main() { double a = 123.4; /* These could all be floats */ double b = -567.8; double c = 987654321; double d = -0.00008765; puts("a = 123.4"); puts("b = -567.8"); puts("c = 987654321"); puts("d = -0.00008765"); puts("\n|.| is used to show the field width.\n"); printf("Using %%f, a = |%f|\n", a); printf(" b = |%f|\n", b); printf(" c = |%f|\n", c); printf(" d = |%f|\n\n", d); printf("Using %%8.3f, a = |%8.3f|\n", a); printf(" b = |%8.3f|\n", b); printf(" c = |%8.3f|\n", c); printf(" d = |%8.3f|\n\n", d); printf("Using %%E, a = |%E|\n", a); printf(" b = |%E|\n", b); printf(" c = |%E|\n", c); printf(" d = |%E|\n\n", d); printf("Using %%12.3E, a = |%12.3E|\n", a); printf(" b = |%12.3E|\n", b); printf(" c = |%12.3E|\n", c); printf(" d = |%12.3E|\n\n", d); printf("Using %%G, a = |%G|\n", a); printf(" b = |%G|\n", b); printf(" c = |%G|\n", c); printf(" d = |%G|\n\n", d); } a = 123.4 b = -567.8 c = 987654321 d = -0.00008765 |.| is used to show the field width. Using %f, a = |123.400000| b = |-567.800000| c = |987654321.000000| d = |-0.000088| Using %8.3f, a = | 123.400| b = |-567.800| c = |987654321.000| d = | -0.000| Using %E, a = |1.234000E+002| b = |-5.678000E+002| c = |9.876543E+008| d = |-8.765000E-005| Using %12.3E, a = | 1.234E+002| b = | -5.678E+002| c = | 9.877E+008| d = | -8.765E-005| Using %G, a = |123.4| b = |-567.8| c = |9.87654E+008| d = |-8.765E-005| printf.bug /* BUG ZONE!!! Example: outputting numeric data using printf */ #include main() { int j = 5; float x = 123.4567890123456789; double z = 123.4567890123456789; char c = 'A'; printf("The value of j is %f\n\n", j); /* BUG */ printf("The value of x is %i\n", x); /* BUG */ printf("The value of z is %c\n", z); /* BUG */ printf("\nThe value of c is %f or %s\n", c, c); /* 2 BUGS */ printf("0.05 is equivalent to 5%"); /* BUG */ } The value of j is 0.000000 The value of x is 0 The value of z is Ö The value of c is 0.000000 or 0.05 is equivalent to puchar • putchar : put a character (to stdout) – Remember a char can be treated as a character or as a number so putchar(‘A’); and putchar(65); are equivalent /* Example: outputting characters using putchar */ #include main() { putchar('H'); putchar('e'); putchar(108); putchar(108); putchar('o'); putchar('!'); putchar('\t'); putchar('*'); putchar('\n'); } Hello! *

stdinCan you spot the bugs in this program?BnGZOE!scanf : scan formatted (from stdin)Exanple: standaid output*/.Basic use/-2BOGS1-/Heeledaforan intjscanf("gi",&j);:Main)/-BUGT-/-forafloatx.scanf("f",&x);/* BUG1.-/?the & is very important and must not be+71:omitted(aneasymistaketomake)!char ("(s")/-BOG*/&x means"the address ofx,well see why later/-BUG-/tinea-si,9-8::Againthe%-stringisaformatconversion80-1stringPrintt"/n':scanf.bugscanf.cgetchargetchar.c getchar :get a character (from stdin).Theinput isbuffered-thismeansyouhavetopress ENTER after the character..ItcanalsocauseproblemsbecausethisENTER(='n)will bereadnexttimegetcharis called..Thesolutionisto"flushthebuffer'beforereading itusingfflush(stdin);3

3 Can you spot the bugs in this program ? /* BUG ZONE!!! Example: standard output */ #include (studio.h) /* 2 BUGS! */ Main() /* BUG! */ { puts(Multiplication); /* BUG! */ printf("9 times 7 = %c", 9 * 7); /* BUG! */ putchar("\n"); /* BUG */ print("9 times 8 = %i", 9 * 8); /* BUG */ printf("/n"); /* BUG */ } stdin • scanf : scan formatted (from stdin) • Basic use – for an int j : scanf(“%i”,&j); – for a float x : scanf(“%f”,&x); • the & is very important and must not be omitted (an easy mistake to make)! – &x means “the address of x”, well see why later • Again the %-string is a format conversion string. scanf.c /* Example: inputting numeric data using scanf */ #include #include /* defines INT_MIN, INT_MAX, LONG_MIN, LONG_MAX */ main() { int j; long int k; float x; double z; printf("Enter an integer (between %i and %i): ", INT_MIN, INT_MAX); scanf("%i", &j); printf("You entered %i\n\n", j); printf("Enter a long integer (between %li and %li): ", LONG_MIN, LONG_MAX); scanf("%li", &k); printf("You entered %li\n\n", k); printf("Enter a floating point number: "); scanf("%f", &x); printf("You entered %20.10E\n\n", x); printf("Enter a double precision floating point number: "); scanf("%lf", &z); printf("You entered %20.10E\n\n", z); puts("\n\nTry again: enter invalid data and see what happens!"); } Enter an integer (between -2147483648 and 2147483647): 10 You entered 10 Enter a long integer (between -2147483648 and 2147483647): 10 You entered 10 Enter a floating point number: 1.1 You entered 1.1000000238E+000 Enter a double precision floating point number: 1.1 You entered 1.1000000000E+000 Try again: enter invalid data and see what happens! scanf.bug /* BUG ZONE!!! Example: inputting numeric data using scanf */ #include #include /* defines INT_MIN, INT_MAX */ main() { int j; double z; printf("Enter an integer (between %i and %i): ", INT_MIN, INT_MAX); scanf("%i", j); /* BUG */ printf("You entered %i\n\n", j); printf("Enter a double precision floating point number: "); scanf("%i", &z); /* BUG */ printf("You entered %20.10E\n\n", z); } getchar • getchar : get a character (from stdin) • The input is buffered - this means you have to press ENTER after the character. • It can also cause problems because this ENTER (=‘\n’) will be read next time getchar is called. • The solution is to “flush the buffer” before reading it using fflush(stdin); getchar.c /* Example: getting a single character from the keyboard, using getchar */ #include main() { char key; printf("Press a key (then ENTER): "); key = getchar(); printf("You pressed %c\n", key); puts("-"); printf("Press another key: "); key = getchar(); printf("You pressed %c\n", key); puts("-"); puts("Oops! What went wrong?"); puts("Let's try again.\n\n"); printf("Press a key (then ENTER): "); fflush(stdin); /* flush the keyboard buffer */ key = getchar(); printf("You pressed %c\n", key); puts("-"); printf("Press another key: "); fflush(stdin); /* flush the keyboard buffer */ key = getchar(); printf("You pressed %c\n", key); puts("-"); puts("Ah, that's more like it!"); /* getchar is both echoed and buffered */ } Press a key (then ENTER): p You pressed p - Press another key: You pressed - Oops! What went wrong? Let's try again. Press a key (then ENTER): p You pressed p - Press another key: r You pressed r - Ah, that's more like it!