Upload
zoe-maxwell
View
221
Download
0
Embed Size (px)
Citation preview
ECE 103 Engineering ProgrammingChapter 13
Input Output
Herbert G. Mayer, PSU CSStatus 7/13/2015
Initial content copied verbatim fromECE 103 material developed by
Professor Phillip Wong @ PSU ECE
Syllabus Console I/O Output Functions Input Examples
3
Common Console I/O Functions
The C standard library contains functions to print output to and read input from the console
“Console” refers to: Output device (e.g., monitor or output stream) Input device (e.g., keyboard or input stream)
The <stdio.h> header file contains prototypes for the console I/O functions
#include <stdio.h>
4
Output Functions int putchar( int c );
putchar() writes a character c to the console; the “file” is named stdout
If successful, the character written is returned.If not successful, the EOF value is returned
Note: EOF is a pre-defined macro
The expected argument c is of type integer. If c is type char, it is converted to an integer first
Make sure no integer of value > 255 is passed! Best to pass a char literal or char type object
5
Example:
#include <stdio.h>
int main( void )
{ // main
int x = 'e';
char y = 's';
putchar( 'Y' ); // output: ‘Y’
putchar( x ); // ouptut: ‘e’
putchar( y ); // output: ‘s’
putchar( 33 ); // 33 is '!' in ASCII
return 0;
} //end main
Output:
!
6
int printf( const char * format, … ); printf() writes formatted output to the console; i.e.
stdout If successful, the number of characters written is
returned; else a negative value is returned, indicating error!
format is a string that contains any combination of literal text and conversion specifiers
… is an optional argument list of expressions whose values are to be printed
For each item in the list, there must be a corresponding conversion specifier in format
7
A conversion specifier inside printf() string: Begins with % Ends with a conversion character Unix has pretty incomplete printf man-page
Between the % and the conversion character are optional formatting instructions in this order: – to left-justify output (default is right-justify) + to force display of + or – sign Number that specifies a minimum field width . (period), which separates field width from precision Number that specifies the precision
(# of digits printed after a decimal point)
8
Variable Type printf() Specifier Comment
Integer Values
short, int %d, %i
long %ld, %li
int %u unsigned
int %x, %X hexadecimal
Floating-Point Values
float, double %f%e, %E%g, %G
fixed decimal formatscientific notationshorter of %f or %e
long double %Lf,%Le, %LE%Lg, %LG
Character & String Values
char %c
char * %s string
Miscellaneous
address %p memory address
To print % symbol % %
9
An escape sequence embedded in the format string performs special actions
A sequence starts with the '\' character
Sequence Description Sequence Description\a alert (bell) character \v vertical tab
\b backspace \\ backslash
\f formfeed \' single quote
\n newline \" double quote
\r carriage return
\t horizontal tab \xhh hexadecimal number
10
Example:
#include <stdio.h>
int main( void )
{ // main
printf("Wake up!\n");
printf("\n"); /* Blank */
printf("Summer ");
printf("is coming.\n");
printf(" See \"you\"\nlater.\n");
return 0;
} //end main
Wake up!
Summer is coming.
See "you"later.
11
Example:
#include <stdio.h>
int main( void )
{ // main
int c = 65; // an ’A’
char ch = 'B'; // obvious
printf( "c = %c\n", c );
printf( "c = %d\n", c );
printf( "ch = %c\n", ch );
printf( "ch = %d\n", ch );
return 0;
} //end main
c = Ac = 65ch = Bch = 66
12
Example:
#include <stdio.h>
int main( void )
{ // main
int x = 5, y = 9;
printf( "%d ", 12 );
printf( "[%d]\n", 3*x+y );
printf( "x equals %d\n", x );
printf( "x=%d y=%d\n", x, y );
printf( "x=%3d y=%3d\n", x, y );
printf( "x=%3d y=%3d\n", 12, 7 );
return 0;
} //end main
12 [24]
x equals 5x=5 y=9x= 5 y= 9x= 12 y= 7
13
Example:#include <stdio.h>
int main( void )
{ // main
float u = 5.0;
double v = 1.75;
const char * str = "PSU rocks!";
printf( "%c %d %f\n", 65, 65, (float)65 );
printf( "%f\n", 2/3.0 );
printf( "u=[%f] v=[%f]\n", u, v );
printf( "%.1f %.2f %.3f\n", v, v, v );
printf( "%12.3f\n", v );
printf( "%12.3e\n", 254*v );
printf( "%s\n", str );
printf( "[%15s]\n", str );
printf( "[%-15s]\n", str );
return 0;
} //end main
A 65 65.0000000.666667u=[5.000000] v=[1.750000]1.8 1.75 1.750 1.750 4.445e+02PSU rocks![ PSU rocks!][PSU rocks! ]
14
Example:#include <stdio.h>
#define V1 1.0
#define V2 1.5e-7
#define V3 1.5e7
int main( void )
{ // main
printf( "V1 (%%.8f) = %.8f\n", V1 );
printf( "V1 (%%.8g) = %.8g\n", V1 ); // %g is shorter of: %f and %e
printf( "V1 (%%.8e) = %.8e\n\n", V1 );
printf( "V2 (%%.8f) = %.8f\n", V2 );
printf( "V2 (%%.8g) = %.8g\n", V2 ); // %g is shorter of: %f and %e
printf( "V2 (%%.8e) = %.8e\n\n", V2 );
printf( "V3 (%%.8f) = %.8f\n", V3 );
printf( "V3 (%%.8g) = %.8g\n", V3 ); // %g is shorter of: %f and %e
printf( "V3 (%%.8e) = %.8e\n", V3 );
return 0;
} //end main
V1 (%.8f) = 1.00000000V1 (%.8g) = 1V1 (%.8e) = 1.00000000e+00
V2 (%.8f) = 0.00000015V2 (%.8g) = 1.5e-07V2 (%.8e) = 1.50000000e-07
V3 (%.8f) = 15000000.00000000V3 (%.8g) = 15000000V3 (%.8e) = 1.50000000e+07
15
Input Functions
int getchar( void ); getchar() reads a single character from the console If successful, the next character from the console is
read and returned, converted to int If not successful, the EOF value is returned
16
Example:
#include <stdio.h>
int main( void )
{ // main
int ch;
printf( "Enter character: ” );
ch = getchar();
printf( "ch = %c\n", ch );
return 0;
} //end main
Enter character: Ach = A
17
Example:
#include <stdio.h>
int main( void )
{ // main
int ch1, ch2;
printf( "Enter ch1: ” );
ch1 = getchar();
printf( "Enter ch2: ” );
ch2 = getchar();
printf( "ch1 = %c ch2 = %c\n", ch1, ch2 );
return 0;
} //end main
Enter ch1: AEnter ch2: ch1 = A ch2 =
Why does this look wrong?
18
getchar() works with buffered line input:
Buffer contents after pressing 'A' and 'Enter':
ch1=getchar() gets the 'A' for ch1
After reading the first character in the buffer, the "next character" arrow is updated:
ch2=getchar() gets the '\n' for ch2
'A' '\n'
'A' '\n'
Arrow indicates "next character" to read.
19
A workaround for the buffer problem:
#include <stdio.h>
int main( void )
{ // main
int ch1, ch2;
printf( "Enter ch1: " );
ch1 = getchar();
getchar(); // Remove pending '\n’
printf( "Enter ch2: " );
ch2 = getchar();
printf( "ch1 = %c ch2 = %c\n", ch1, ch2 );
return 0;
} //end main
Enter ch1: AEnter ch2: Bch1 = A ch2 = B
20
int scanf( const char * format, … ); scanf() reads formatted input from the console If successful, the number of items read is returned If not successful, EOF is returned, -1 on Unix
format is a string that contains any combination of conversion specifiers
… is an optional argument list of variable addresses where the input values are to be stored
For each item in the list, there must be a corresponding conversion specifier in format
21
The “address” of a variable is the memory location that holds the variable's value
The & character is C’s “address-of” operator
When storing the value read by scanf(), you must specify the address of the variable that will receive the value
Example: int x; ... scanf( "%d", &x );
1. Read character string of digits ‘0’..‘9’ into the input buffer; only ‘0’..’9’ due to %d2. Convert the string value to an integer3. Store the value at the address of int variable x
22
Variable Type scanf() Specifier
Integer Values
int %d, %i
short %hd, %hi
long int %ld, %li
unsigned int %u
Floating-Point Values
float %f, %e, %E, %g, %G
double %lf, %le, %lE, %lg, %lG
long double %Lf, %Le, %LE, %Lg, %LG
Character Values
char %c
Character String (whitespace delimited – not full text)
char * %s
23
Example:
#include <stdio.h>
int main( void )
{ // main
char cvar;
int ivar;
float fvar, fv2;
double dvar;
scanf( "%c", &cvar );
scanf( "%f", &fvar ); /* Use %f for floats */
scanf( "%lf", &dvar ); /* Use %lf for doubles */
scanf( "%d %f %f", &ivar, &fvar, &fv2 );
return 0;
} //end main
24
Example: Common error – missing & in scanf()// source file: test.c
#include <stdio.h>
int main( void )
{ // main
int x; // OK to leave uninitialized
printf( "Enter x: ” );
scanf( "%d", x );
printf( "%d\n", x );
return 0;
} //end main
Notice x instead of &x
$ gcc -ansi -Wall -pedantic test.ctest.c: In function 'main':test.c:7: warning: format argument is not a pointer (arg 2)test.c:7: warning: format argument is not a pointer (arg 2)
$ ./a.outEnter x: 45Segmentation fault (core dumped)
25
Reminder for printf and scanf Specifiers
Output → If value to display with printf is of type: int : %d long int : %ld float, double : %f %e %g long double : %Lf %Le %Lg
Input → If value to input with scanf is of type: int : %d long int : %ld float : %f %e %g double : %lf %le %lg long double : %Lf %Le %Lg
printf uses the same specifiers for both float and double types.
scanf uses separate specifiers for the float and double types.