1. ITEC113 Algorithms and Programming Techniques
C programming: Variables, Expressions Part I

2. Objectives To understand what variables are
initialization/garbage values
naming conventions
To learn about the frequently used data types
To understand the components of
an assignment Statements
arithmetic expressions
To learn about
frequently used operators,
operator precedence

3. VARIABLES int x; char gender; float avg; float sum=0; char name[10];
Variables are basic data objects manipulated in a program.
Each variable has to be declared before use.
Each variable has a name and a data type.
You can give initial value (variable initialization) on variable declaration.
Examples:
int x;
char gender;
float avg;
float sum=0;
char name[10];
int *fp;

4. VARIABLES
Variable declaration allocates a cell in the main memory whose size is determined by the data type
For example for int 4 bytes are used, for double 8 bytes are used
When the variable is created in the main memory it contains garbage value
This is due to the existence of 1’s and 0’s in the memory. 1 means high voltage, 0 means low voltage.
It is a good idea to initialize variables before first usage.
A variable name is the symbolic representation of the memory location that is allocated on variable declaration

5. Rules on Variable Names:
DO NOT use reserved words as variable names
(e.g. if, else, int, float, case, for, …).
The first character has to be a letter or underscore. It can not be a numeric digit.
The second and the other characters of the name can be any letter, any number, or an underscore “_”.
Examples 
Some valid names:
  my_name, m113_1, salary, bluemoon , _at
Some invalid names:
my name, my-name , 1stmonth , salary! , guns&roses ,

6. Tradition on Variable Names:
These are NOT rules but you can increase the quality of your program by using them!
Select related and meaningful names indicating tasks of the variables.
Do not use variable names that exceed 8 characters.
Use small case letters for variable names.
 Upper case letters are mostly used in the names of symbolic constants.

7. Variable Declaration:
Variable declaration is used to introduce the system to the variables that the programmer decides to use on the rest of the program.
On variable declaration,
variable name,
data type
are declared.
Also you can give the initial value of the variable on its declaration.
Example : int k ;
int m=15;
float fnumber= 1.75;
char ch=’w’ ;

8. Data Types of the Variables :
A variable data type specifies:
The kind of value a variable can store
The set of operations that can be applied to the variable
There are 3 main different data types and their derivations for declaration in ANSI–C.   
Main Data types
Derived Data Types
integer
short, long
float
Double
char

9. Data Types and Sizes : (Continued)
Integers (int) :
Integers are all numeric values that have no fractional or decimal components.
Integer numbers may be positive or negative.
Examples : –
C compiler allocates 4 bytes (32 bits) to an integer (int) variable.
An integer variable can store values in the range
–32, through 32,767
Derived Integers :
short, long and unsigned are data types derived from int, and used to keep integer values.
The sizes of long and short is differentiated from int.
The data type unsigned is used only with positive integers.

10. Data Types and Sizes : (Continued)
The sizes of long and short is differentiated from int.
The data type unsigned is used only with positive integers.
Data Types
Bytes Used
int
4 Bytes
short
2 Bytes
double
8 Bytes
unsigned

11. Data Types and Sizes : (Continued)
Real Numbers :
C compiler uses float and double data types for storing real numbers.
The float data type requires 4 bytes and has a precision of seven digits
This means after the decimal point you can have seven digits
Example:
The double data type requires 8 bytes and has a precision of fifteen digits
Example :  

12. Data Types and Sizes : (Continued)
We can use Scientific Notation to represent real numbers that are very large or very small in value.
The letters e or E is used to represent times 10 to the power.
Example:
1.23 x is represented in C as e5 or e+5 or 1.23E5
1 x is represented in C as 1e-9

13. Data Types and Sizes : (Continued)
Character : ( char )
Characters constants are usually used enclosed single quotes
Example: ‘A’ , ‘7’,
Only one byte of memory location is used by a character variable.
In ASCII code is used to represent uniquely any one of the available 255 characters
Example:
A is represented by decimal 65 or
8-bit binary

14. Data Types and Sizes : (Continued)
Categories of characters :
Alphabetic Letters :
( Upper case : ‘A’ , ‘B’, …….. ‘Z’ )
( Lower case : ‘a’ , ‘b’, …….. ‘z’ )
Numeric digits
( ‘1’,’2’,’3’,…………,’9’,’0’ )
Special Characters
( blank, ‘+’,’#’,’-‘,’_’,……..)
Control Characters
( ‘\n’ , ‘\t’ , ……. )

15. Assignment Statements
The ‘=‘ sign is an assignment operator.
Assignment statements replace old values of the variables with the new ones
An assignment statement assigns a value or a computational result to a variable.
Example
cnt = 1; sum = 0;
ch = ‘Y’;
  sum = sum + 1; avg = sum / cnt;
stores values 1 and 0 to cnt and sum.
stores character ‘Y’ to ch
stores computational results to sum and avg

16. Expressions
Arithmetic Expressions involve arithmetic operators such as *,+,-,/,%:
Example : a * 5 + b % 4
Relational Expressions involve relational operators that compare two values such as >,<,== etc:
Example: a > b
Logical Expressions involve the logical and and or operators && and || and are used to combine relational expressions:
Example: ( a > b && c == 7 )

17. Arithmetic Expressions
In the Assignment Statement:
M = a * 5 + b % 4 ;
The expression to the right of the assignment operator ( = ) involves an arithmetic operation that combines arithmetic operands with arithmetic operators.
The most commonly used arithmetic operators are:
Addition (+) Operator
Subtraction (-) Operator
multiplication (*) Operator
division (/) Operator
remainder (%) Operator
For real or integer numbers
For integer numbers only

18. Operator Precedence Rules
Arithmetic expressions inside parentheses are executed first (left to right).
Unary operators ( minus signs and plus signs) are executed before multiplications, divisions and remainder operations.
Additions and subtractions are executed last.
Operators
Associativity
Priority Level
( , )
Left to Right
Highest
+ , - ( unary )
Right to Left
* , / , %
+ , -
Lowest
parentheses
-ve and +ve signs
Mult. Div., and mod.
Add and subtract

19. Operator Precedence Rules:Examples
? =3 + 5* 4
Evaluated as 3 + (5*4) and the result is 23
? = 8 / (5 – 2)
Evaluated as 8 / 3 and the result is 2
? = % 5
Evaluated as 8 + (12%5) and the result is 10
? = 6 * 5 / 2 + 2
Evaluated as ( (6*5) /2) + 2 and the result is 17
? = 9 – * 6
Evaluated as 9 – 4 + (2*6) and the result is 17
? = * (3 + 4)
Evaluated as 1 + (2 * (3+4)) and the result is 15
? = 5 * % 4
Evaluated as (5*2) + (9 % 4) and the result is 11
? = 5 * 2 % ( 7 – 4)
Evaluated as (5 * 2) % (7 – 4) and the result is 1

20. That’s it for now! Next Lecture: More on variables, data types and expressions