1. I/O in C + Misc
Lecture 13
Sudeshna Sarkar, IIT Kharagpur

2. Input : movement of data into memory from the outside world.
read operation
Changes the value of the variable
Output: movement of data from memory to outside world.
write operation
does not change value of memory
Sudeshna Sarkar, IIT Kharagpur

3. Conceptually, C performs all I/O on text streams.
In C, input and output is handled by a set of standard library functions.
Conceptually, C performs all I/O on text streams.
The standard input stream from the keyboard is referred to as stdin.
The standard output stream is referred to as stdout,
Sudeshna Sarkar, IIT Kharagpur

4. Single character input : getchar
getchar reads a single character from stdin
char ch;
ch = getchar();
The getchar function accepts any character types in including newline, blank, and tab.
Sudeshna Sarkar, IIT Kharagpur

5. Single character output : putchar
putchar prints a single character to stdout
char ch;
. . .
putchar (ch) ;
Sudeshna Sarkar, IIT Kharagpur

6. Example
/* Read a character from the keyboard and output it in reverse case */
#include <stdio.h>
#include <ctype.h>
main() {
char alpha;
printf (“\n Enter a alphabet (lowercase or uppercase): “);
alpha = getchar();
if (islower(alpha))
putchar (toupper(alpha));
else
putchar (tolower(alpha)); }
Sudeshna Sarkar, IIT Kharagpur

7. Display formatted output (printf)
printf (“Enter %d positive numbers:”, num1*2);
printf (“control string”, list of expressions) ;
Control string gives the format of output.
Expressions are what to output
%d is a placeholder for an int value.
printf (“The %d-th character is %c\n”, i, ch);
% placeholders in format string match expressions in output list in number, order, type.
Sudeshna Sarkar, IIT Kharagpur

8. Read input : scanf scanf (“control string”, &input list) ;
scanf(“%d”, &number);
scanf(“%d%lf”,&studentId, &grade);
Input list variables must be preceded by an &
% placeholders in the format must match variables in the input list.
one-for-one in number, order, and type.
Sudeshna Sarkar, IIT Kharagpur

9. Whitespace “Whitespace”: space ‘ ‘, tab ‘\t’, newline ‘\n’
Skipped by scanf for int (%d) and double (%lf)
user can type spaces before a number and they are ignored.
Not skipped for char input (%c)
each character typed, including spaces, is used
Sudeshna Sarkar, IIT Kharagpur

10. Review: Mixed Mode Arithmetic
What is 2 * 3.14 ?
The compiler implicitly (automatically) converts int to double when they occur together.
Generally in mixed expressions, the variable whose type has lower precision is promoted to the type of the variable with higher precision.
int
unsigned int
unsigned long int
long int
float
double
long double
higher precision
Sudeshna Sarkar, IIT Kharagpur

11. double d; int x=5, y=2; double dx=5.0, dy=2.0; d = x/y;
printf(“1. d =%f\n”, d);
d = dx/y;
printf(“2. d =%f\n”, d);
d = x/dy;
printf(“3. d =%f\n”, d);
Sudeshna Sarkar, IIT Kharagpur

12. Assignment : mixed mode
float i, j;
i = 1.99; j = -1.99;
printf (“i = %d, j = %d\n”, i, j);
/* floating point numbers are truncated while converting to int */
Assignment rules:
If the two operands in an assignment expression are of different data types, then the value of the righthand operand will be converted to the type of the operand on the left.
A float value is truncated if assigned to an int.
A double identifier is rounded if assigned to a float.
An int may be altered if assigned to a short int or to a char.
Sudeshna Sarkar, IIT Kharagpur

13. Explicit conversions: case
(type) expression;
double avg = total/count;
double avg = (double)total / (double)count;
((int)(i+f)) % 4 /* i is int, f is float */
Sudeshna Sarkar, IIT Kharagpur

14. Errors Syntax : the required form of the program
The C compiler catched these “syntax errors”
Semantics and logic: what the program means
what you want it to do
The C compiler cannot catch these kinds of errors.
Sudeshna Sarkar, IIT Kharagpur

15. Library functions: stdio.h
int getchar (void)
int printf(...)
int putchar(c)
int rand(void)
int scanf(...)
Sudeshna Sarkar, IIT Kharagpur

16. Library functions: math.h
double acos (double)
double asin (double)
double atan (double)
double ceil (double)
double floor (double)
double cos(double)
double exp(double)
double fabs (double)
double log (double)
double log10 (double)
double pow (double, double)
double sin (double)
double sinh (double)
double cosh (double)
double tan (double)
double tanh (double)
double sqrt(double)
Sudeshna Sarkar, IIT Kharagpur

17. Library functions: ctype.h
int isalnum (char)
int isalpha (char)
int isascii (char)
int isdigit (char)
int islower (char)
int ispunct (char)
int isspace (char)
int isupper (char)
int toascii (char)
int tolower (char)
int toupper (char)
Sudeshna Sarkar, IIT Kharagpur