1. DCP2073 Asas Pengaturcaraan C Lecture 6: Components of a C Program (Part 3)
Lecture 6: C Primitives 6

2. Topics Type Variables Keywords and Identifiers Assignments
Constant Variables
Lecture 6: C Primitives 6

3. Type The kind of a value is its “type”
Not all values are of the same kind
For example: 7 + “cat” makes no sense
Value
Variable

4. Type Built-in types: char, int, float
Type modifiers: long, short, const
User-defined types (arrays and records)
What about “strings”?
Strings are arrays of char (discussed later)

5. Character Representation
Characters are stored as a small integer
Each character has a unique integer equivalent specified by its position in the ASCII table (pronounced “as-key”)
American Standard Code for Information Interchange
see Appendix D of D&D

7. Character Representation
The ASCII values range from 0 to 127
value 0: special character ’\0’ (a.k.a. NUL character)
value 127: special character <DEL>
other special characters: ’\n’ ’\t’ ’\’’ ’\\’ etc.
various “extended” sets from 128 to 255

8. Variable Is a logical name for a container
(an actual piece of computer memory for values)
Has a type associated with it
tells the computer how to interpret the bits
Must be declared before use:
int i; float result;
int i=0; char initial=’K’;

9. Variable Declaration: Examples
int myID;
myID

10. Variable Declaration: Examples
int myID;
char myInitial = ’J’;
Single “forward quotes” or apostrophe (’) rather than “back quotes” (‘)

11. Variable Declaration: Examples
int myID;
char myInitial = ’J’;
myInitial

12. Variable Declaration: Examples
int myID;
char myInitial = ’J’;
char myInitial = 74 ;
myInitial

13. Variable Declaration: Examples
float commission = 0.05;
short int myHeight = 183; /* cm */
long int mySalary = ;
long float chanceOfADate = 3e-500;
double chanceOfA2ndDate = 1.5e-500;

14. Variable Declaration: Examples
float commission = 0.05;
short int myHeight = 183; /* cm */
long int mySalary = ;
long float chanceOfADate = 3e-500;
double chance_of_a_2nd_date = 1.5e-500;
“Keywords”

15. Keyword ...has a special meaning in C ...is “case-sensitive”
...cannot be used as variable names
Examples:
int, char, long, main, float, double, const, while, for, if, else, return, break, case, switch, default, typedef, struct, etc. (see D&D 2/e Appendix A or D&D 3/e page 545, Figure 15.4)

16. Variable Declaration: Examples
float commission = 0.05;
short int myHeight = 183; /* cm */
long int mySalary = ;
long float chanceOfADate = 3e-500;
double chanceOfA2ndDate = 1.5e-500;
“Identifiers”

17. Identifier
...is a series of characters consisting of letters, digits and underscores ( _)
...cannot begin with a digit
...must not be a keyword
...is “case-sensitive”
Examples:
sUmoFA, x1, y2, _my_ID_, Main (careful!)

18. not to be confused with ==
Assignment
Puts a specified value into a specified variable
Assignment operator: =
<variable name> = <expression> ;
not to be confused with ==

19. Assignment: Examples float x = 2.5 ; char ch ; int number ;
ch = ’\n’ ;
number = ;
/* current value of number is 9. */
number = number * 2;
/* current value of number is now 18. */

20. Assignment Value must have a type assignable to the variable
Value may be automatically converted to fit the new container
Example:
various.c

21. various.c #include <stdio.h>
/* Do various assignment statements */
int main() {
int integer;
char character;
float floatingPoint;
integer = 33;
character = 33;
floatingPoint = 33;
integer = 'A';
character = 'A';
floatingPoint = 'A';
integer = 33.33;
character = 33.33;
floatingPoint = 33.33;
integer = floatingPoint;
floatingPoint = integer;
return 0; }
various.c

22. Constant Variables ...are variables that don’t vary
...may not be assigned to.
...must be initialized
const float Pi = ;
const int classSize = 100;

23. Constant Variables: Examples
const int myID = 192;
myID = 666; /* Error! */
const int passMark = 80;
short char pAsSgRaDe = ’P’;
const float pi = ; /* oops */
const double golden_ratio = ;

24. Example: Constants #include <stdio.h>
/* Converts an angle in degrees to radians. */
const float PI = ;
int main() {
float angleInDegs;
float angleInRads;
printf("Enter angle in degrees:");
scanf("%f", &angleInDegs);
angleInRads = PI/180*angleInDegs;
printf("%f\n", angleInRads);
return 0; }
Converts an angle from degrees to radians
output “Enter angle in degrees”
input angleInDegrees
angleInRadians =  / 180 * angleInDegrees
output angleInRadians

25. “Global” constant variable
Example: Constants
#include <stdio.h>
/* Converts an angle in degrees to radians. */
const float PI = ;
int main() {
float angleInDegs;
float angleInRads;
printf("Enter angle in degrees: ");
scanf("%f", &angleInDegs);
angleInRads = PI/180*angleInDegs;
printf("%f\n", angleInRads);
return 0; }
“Global” constant variable
“Local” variables
more on this later...

26. Topics Type Variables Keword and Identifiers Assignments
Constant Variables

27. Reading King D&D: Kernighan & Ritchie Chapter 2, 2.4 – 2.8
Chpater 7, 7.1 – 7.3
D&D:
Chapter 9, 9.1 – 9.5, 9.11
Kernighan & Ritchie
2.4 – 2.8, 2.10, 2.12