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

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

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;

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

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 ,

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.

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’ ;

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

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 : 13 7 –6 208 1024 C compiler allocates 4 bytes (32 bits) to an integer (int) variable. An integer variable can store values in the range –32,768 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.

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

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: 3.14159 534.322344 0.3333333 0.1234567 The double data type requires 8 bytes and has a precision of fifteen digits Example : -3738.7878787878 3.141592653589790 0.123456789123456 

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 10 5 is represented in C as 1.23e5 or 1.23e+5 or 1.23E5 1 x 10 -9 is represented in C as 1e-9

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 0 1 0 0 0 0 0 1

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’ , ……. )

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

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 )

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

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

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 ? = 8 + 12 % 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 – 4 + 2 * 6 Evaluated as 9 – 4 + (2*6) and the result is 17 ? = 1 + 2 * (3 + 4) Evaluated as 1 + (2 * (3+4)) and the result is 15 ? = 5 * 2 + 9 % 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

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