ECE 103 Engineering Programming Chapter 13 Input Output

1. ECE 103 Engineering ProgrammingChapter 13Input Output Herbert G. Mayer, PSU CS Status 7/12/2014 Initial content copied verbatim from ECE 103 material developed by Professor Phillip Wong @ PSU ECE

2. 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> 2

4. Output Functions int putchar( int c ); putchar() writes a character c to the console If successful, the character written is returned.If not successful, the EOF value is returnedNote: 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: ! 4

6. int printf( const char * format, … ); printf() writes formatted output to the console. 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 5

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 values (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) 6

8. 7

9. An escape sequence embedded in the format string performs special actions A sequence starts with the '\' character

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. 9

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 = A c = 65 ch = B ch = 66 10

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 5 x=5 y=9 x= 5 y= 9 x= 12 y= 7 11

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 65.000000 0.666667 u=[5.000000] v=[1.750000] 1.8 1.75 1.750 1.750 4.445e+02 PSU rocks! [ PSU rocks!] [PSU rocks! ] 12

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 ); // %gisshorterof: %fand %e printf( "V1 (%%.8e) = %.8e\n\n", V1 ); printf( "V2 (%%.8f) = %.8f\n", V2 ); printf( "V2 (%%.8g) = %.8g\n", V2 ); // %gisshorterof: %fand %e printf( "V2 (%%.8e) = %.8e\n\n", V2 ); printf( "V3 (%%.8f) = %.8f\n", V3 ); printf( "V3 (%%.8g) = %.8g\n", V3 ); // %gisshorterof: %fand %e printf( "V3 (%%.8e) = %.8e\n", V3 ); return 0; } //end main V1 (%.8f) = 1.00000000 V1 (%.8g) = 1 V1 (%.8e) = 1.00000000e+00 V2 (%.8f) = 0.00000015 V2 (%.8g) = 1.5e-07 V2 (%.8e) = 1.50000000e-07 V3 (%.8f) = 15000000.00000000 V3 (%.8g) = 15000000 V3 (%.8e) = 1.50000000e+07 13

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 14

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: A ch = A 15

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: A Enter ch2: ch1 = A ch2 = Why does this look wrong? 16

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' 'A' '\n' '\n' Arrow indicates "next character" to read. 17

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: A Enter ch2: B ch1 = A ch2 = B 18

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 19

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 %d 2. Convert the string value to an integer 3. Store the value at the address of int variable x

22. 21

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 22

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.c test.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.out Enter x: 45 Segmentation fault (core dumped) 23

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. 24