CS149D Elements of Computer Science

1
CS149D Elements of Computer Science
Ayman Abdel-Hamid Department of Computer
Science Old Dominion University Lecture 16
10/24/2002
2
Outline

• Variables and Data types
• Assignment statement
• Arithmetic operators
• Increment/Decrement operators

3
Variables Names

• Program variables correspond to memory spaces
  reserved for storage
• A variable name is called an identifier
• An identifier in C can be up to 255 characters
  long
• Can not begin with a digit (Invalid 1First)
• Can not contain blanks (Invalid num elements)
• Can not contain a hyphen, underscore is OK
  (Invalid num-elements)
• Special symbols are not allowed (Invalid
  cost, cost!)
• Reserved words can not be used as identifiers
  (Invalid int, const, float)
• A reserved word is a word that has a special
  meaning in C. It can not be used as a
  programmer-defined identifier
• C is case sensitive, so count is different than
  Count
• To declare a variable, need to identify its data
  type

4
Data Types

• C Data types (built-in data types)
• Integers
• Floating-point numbers
• Characters
• And more
• The data type determines how data is represented
  in the computer and the kind of processing that
  the computer can perform on it
• The number of bytes that a data type occupies in
  memory is system dependent

5
Variable Declaration

• DataType identifier
• int x
• DataType identifier, identifier,
• int x,y,z
• The initial value stored in a variable is not
  know unless you initialize it with the
  declaration
• int x 10
• Can mix declaration with declaration/initializatio
  n
• int x10, y, z 5
• Use const keyword to indicate that the value of
  this variable can not change
• const float PI 3.141593f

6
Numeric Data Types

• Integer numbers
• short, int, long Can use unsigned keyword
  (unsigned short)
• Numbers with fractions
• float, double, long double (double is double
  precision)
• Can use scientific notation float x 5.0e6
  which means x ? 5 106
• const float x 2.3 //x is considered a double
  constant
• const float x 2.3f //or const float x 2.3F
• Number of bytes and range of values is system
  dependent. In Microsoft VC

The textbook lists 2 bytes for int on a Borland
Turbo C 3.0 compiler See Table 2.2 on page 29
7
Scientific Notation

• Rewrite the floating-point number as a mantissa
  times a power of 10
• Mantissa absolute value greater than or equal to
  1.0 and less than 10.0
• 25.6 ? 2.56 101
• Letter e is used to separate mantissa from
  exponent
• 25.6 ? 2.56e1
• Precision number of digits allowed for decimal
  portion of mantissa
• Exponent range number of digits allowed for for
  exponent
• Float double long double
• Precision 6 15 19 see Table 2.2

8
Alphanumeric Data Types

• Single alphanumeric character
• char x a char y 1 is different than int
  y 1
• Most programming languages use ASCII to represent
  the English alphabet and other symbols (1
  byte/character)
• Sequence of characters
• string name CS149 Lecture

9
Assignment statement

• Identifier expression
• Where expression can be a constant, another
  variable, or the result of an operation
• int x, y
• x 10 // x ? 10, x is assigned the value of 10
• y x
• x x 10
• Multiple assignments x y z 0
• Look out for the data types of both sides of the
  assignment
• int a
• a 12.8 // actually a is assigned 12
• Numeric conversion with possibility of
  information loss

10
Arithmetic Operators1/3
Operators Unary operator One operand Binary
operator two operands Unary Plus
Addition - Unary minus - Subtraction Multiplica
tion / floating-point division or integer
division (no fractional part) modulus operator
(remainder of integer division) Examples Area_tria
ngle 0.5 base height Y -x
11
Arithmetic Operators2/3

• Examples of integer division and modulus
• 7/2 3 72 1
• 2/7 0 27 2
• 7.0/2.0 3.5 5 2.3 Error (both operands
  must be integers)
• Note that the result of integer division is a
  truncated result, not a rounded result. 5/3
  1 5.0/3.0 1.66666
• Operation between different types is a mixed
  operation
• Value with lower type is converted or promoted to
  the higher type
• int sum 18, count 5
• float average
• average sum/count // average assigned 3.0 and
  not 3.6 (Why?)
• The correct way to compute average would be
• average (float) sum / (float) count //
  explicit type casting

12
Arithmetic Operators3/3
Practice problems page 34
int a 27, b 6 float c c a/(float)b
int b 6 float a, c 18.6 a (int) c/b
13
Increment and Decrement Operators

• Unary operators for incrementing and decrementing
  variables , --
• Can be used in a prefix (count) or postfix
  (count) position
• int x
• x is equivalent to x x 1 is equivalent to
  x
• How about
• x y 3
• x y 3 //is the value assigned to x the same
  in both cases NO
• Will see what is the difference next lecture when
  we talk about operator precedence and compound
  arithmetic expressions