1. Module 12 Input and Output

2. Communications Channels
stdin – standard input device (keyboard)
stdout – standard output device (monitor)
stderr – standard error device (monitor)
Defined in stdio.h
Also includes prototypes for input/output functions
@Copyright UMBC Training Centers 2012

3. @Copyright UMBC Training Centers 2012
Character I/O
getchar( ) to read a single character from stdin
putchar( char ) to output a single char to stdout
@Copyright UMBC Training Centers 2012

4. @Copyright UMBC Training Centers 2012
Formatted output
printf( “format string”, var1, var2, … );
Outputs to stdout
“format string” consists of
Literal text to be output
Format specifiers for expressions
General form of format specifiers
%[flags][width][.precision][hlL][type]
@Copyright UMBC Training Centers 2012

5. @Copyright UMBC Training Centers 2012
Common Flags
Flag
Meaning -
Left justify +
Output values with +/-
Leading 0s for values
@Copyright UMBC Training Centers 2012

6. @Copyright UMBC Training Centers 2012
Width and Precision
Width
number = minimum number of characters to output
Precision
.number =
min number of digits for integer
number of decimal places for float/double
max characters for string
@Copyright UMBC Training Centers 2012

7. Types Common Type Specifiers Specifier To display %d, %i integer %ld
long integer
%hd
short integer %u
unsigned integer %f
float/double %c
A single character %s
A NUL terminated string %p
pointer
%x, %X
Hexadecimal integer %o
Octal integer
@Copyright UMBC Training Centers 2012

8. @Copyright UMBC Training Centers 2012
Let’s Take a Look
C::B printDemo
Go over basic printf output format strings – see comments in code
Start with default output and uncomment each section as you go
Make a copy or be sure to put comments back for next time
@Copyright UMBC Training Centers 2012

9. @Copyright UMBC Training Centers 2012
Unformatted Output
puts(char *string);
Outputs string to stdout
Appends a newline character to output
@Copyright UMBC Training Centers 2012

10. @Copyright UMBC Training Centers 2012
Formatted Input
scanf( “format string”, var1, var2, … );
Inputs from stdin
“format string” consists of
Literal text to be input
Format specifiers for interpreting input
Type specifiers same as printf
Use %lf to input double
@Copyright UMBC Training Centers 2012

11. @Copyright UMBC Training Centers 2012
Scanning input
scanf skips leading whitespace
Except for %c or bracketed chars
Reads characters until
max characters specified by field width are read
An invalid character for the specified type is encountered
Whitespace terminates a string (%s)
Next scanf starts where previous scanf stopped
Lots of scanf example in C::B
@Copyright UMBC Training Centers 2012

12. @Copyright UMBC Training Centers 2012
Let’s Take a Look
C::B
Scanf1
Nothing new
Scanf2
Nothing new, multiple scans in ONE line
Scanf3
@Copyright UMBC Training Centers 2012

13. @Copyright UMBC Training Centers 2012
I/O Redirection
Output to stdout written to a file
myProgram > outputFile
Input from stdin read from a file
myProgram < inputFile
Redirect both input and output
myProgram < inputFile > outputFile
@Copyright UMBC Training Centers 2012

14. @Copyright UMBC Training Centers 2012
Streams
In C, all I/O is performed using streams
A sequence of bytes that “flows” between your program and an I/O device
A high level abstraction representing a channel to a file or I/O device
Some I/O functions such as printf and scanf use the standard streams (stdout/stdin) by default
opened when your program begins execution
@Copyright UMBC Training Centers 2012

15. @Copyright UMBC Training Centers 2012
Lots of Streams
C Application
Keyboard
data.txt
Monitor
output.txt
stdin
FILE*
stdout
I/O
Device
I/O Device
stderr
@Copyright UMBC Training Centers 2012

16. @Copyright UMBC Training Centers 2012
File I/O
Files must be opened to create a stream before I/O may be performed
Functions that perform I/O with files are similar to those for stdout/stdin, but must specify the stream to which the I/O is performed
Streams are designated as type FILE*
@Copyright UMBC Training Centers 2012

17. @Copyright UMBC Training Centers 2012
Opening a Stream
FILE* fopen(char fileName[], char mode[]);
fileName is the path to the file
Windows: C:\folder\filename
Unix: ../directory/filename
mode is a 1- or 2-character string
Return value FILE* is the stream used as the argument to other functions
Returns NULL if an error occurs
@Copyright UMBC Training Centers 2012

18. @Copyright UMBC Training Centers 2012
Closing a Stream
fclose(FILE* stream);
stream is FILE* returned from fopen( )
@Copyright UMBC Training Centers 2012

19. Formatted Stream Output
fprintf(FILE* stream, “formatString”, var1, var2,…);
stream is FILE* returned from fopen( )
“formatString” is the same as in printf()
@Copyright UMBC Training Centers 2012

20. @Copyright UMBC Training Centers 2012
Let’s Take a Look
C::B fprintf
could get filename from user
ERROR MSGS – user fprintf(stderr,….) rather than printf —line 24
@Copyright UMBC Training Centers 2012

21. Formatted Stream Input
fscanf(FILE* stream, “formatString”, &var1, &var2,…);
stream is FILE* returned from fopen( )
“formatString” is the same as in scanf()
Returns EOF if error or end-of-file
@Copyright UMBC Training Centers 2012

22. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B – fscanf
Nothing new
Checks for EOF
@Copyright UMBC Training Centers 2012

23. @Copyright UMBC Training Centers 2012
Unformatted Input
int getc(FILE* stream) int fgetc(FILE* stream)
Return a character from stream as int
Return EOF if error or end-of-file
fgets(char line[], int maxChars, FILE* stream);
inputs a line from a stream
Reads up to maxChars - 1 characters
fgets( )with STDIN as the stream is a good technique
Returns NULL if error or end-of-file
@Copyright UMBC Training Centers 2012

24. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B fgets
Change LINE_SIZE to <= 17 and run again
Also shows puts( )
Note double spacing – one \n from file, one from puts
Change while-loop to use commented code (fgetc), then to getc
@Copyright UMBC Training Centers 2012

25. @Copyright UMBC Training Centers 2012
Exercises
Ex2
Ex4
@Copyright UMBC Training Centers 2012

26. @Copyright UMBC Training Centers 2012
Unformatted Output
putc(int c, FILE* stream) fputc(int c, FILE* stream)
Output an unformatted character to stream
fputs(char string[], FILE* stream);
Output an unformatted string to stream
@Copyright UMBC Training Centers 2012

27. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B fputs
Fputs (outputs a String)
Fputc (outputs a Character)
@Copyright UMBC Training Centers 2012

28. Other stream Functions
int ungetc( int c, FILE* stream)
Pushes c back onto the stream
Cannot push EOF
Returns char pushed or EOF on error
int fflush( FILE *stream )
For output stream, causes buffered output to be written
For input stream, undefined
@Copyright UMBC Training Centers 2012

29. @Copyright UMBC Training Centers 2012
sscanf( )
sscanf(char string[], “formatString”, &var1, &var2, …);
“reads” input from string
Returns the number of items successfully read
Combine with fgets for input error checking
@Copyright UMBC Training Centers 2012

30. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B sscanf
Notice that input MUST be all in ONE line with spaces
Legit
Lupoli
Not Legit
Lupoli 23 Lupoli
Notice the error message that appears
@Copyright UMBC Training Centers 2012

31. @Copyright UMBC Training Centers 2012
sprintf()
sprintf(char string[], “formatString”, var1, var2, …);
“writes” to string
Much like strcat
Otherwise just like printf
@Copyright UMBC Training Centers 2012

32. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B sprintf
Notice that “line will have two \n s!!!
Also notice that ADDING “age” requires &
@Copyright UMBC Training Centers 2012

33. @Copyright UMBC Training Centers 2012
I/O Function Summary
Stream Input
Stream Output
stdin
stdout
char
getc, fgetc
fscanf with %c
putc, fputc
fprintf with %c
getchar
scanf with %c
putchar
printf with %c
string
fscanf with %s
fputs
fprintf with %s
scanf with %s
puts
printf with %s
line
fgets
Same as string
Stream functions may also be used with stdin, stdout and stderr.
@Copyright UMBC Training Centers 2012

34. @Copyright UMBC Training Centers 2012
DOs and DON’Ts
DON’T use gets( )
DON’T use scanf, fscanf
DO use fgets with sscanf
@Copyright UMBC Training Centers 2012

35. @Copyright UMBC Training Centers 2012
Exercises
Ex1-NameAgeLetter – input int, char, string
Ex3-MileConversion- formatted table output
Textbook – pg 371
#6 – display file 20 lines at a time
Ex5-FileIO – reading and comparing strings (from PE5-2)
@Copyright UMBC Training Centers 2012

36. @Copyright UMBC Training Centers 2012
If you have any comments about this video, please use the contact information in your registration materials to let us know.
@Copyright UMBC Training Centers 2012