Module 3

1
Module 3

• Data Types and Expressions,
• Input and Output

2
Recall from Last Module

• Variables represent memory locations with names
  that you make up
• Three basic data types so far integral, floating
  point, and character
• Different types can use different amounts of
  memory
• Well talk some more now about data types

3
Floating Point Numbers

• They have both an integer part and a fractional
  part, with a decimal in between
• They also have an exponent

Sign Exponent Mantissa
4
Strings of Characters

• C/C both support a data type called stringa
  string is a sequence of characters
• While characters use a, strings use hey
• Examples Hello, a, 678
• The empty string has no characters, and is
  written as

5
Modifiers to Data Types

• Integers will allow the modifiers long and short
• short int is equivalent to short
• long int is equivalent to long
• Floating point double allows the modifier long
• Eg long double static_tension
• The consequence is possibly increased (or
  decreased, for short) amount of memory used
• This is system dependent!

6
Modifiers, Continued

• For all standard data types, one can use the
  modifier const will turn the variable into a
  constant
• When we use this, we can set the value of the
  variable exactly once
• Thereafter, we can use that value, but can never
  change it
• EG const char first_letter

7
Assigning Values to Variables

• Syntax
• An expression is something that evaluates to a
  particular value
• It usually is a combination of numbers,
  variables, and arithmetic operations
• Eg income total_trans busns_cost
• We use the operator, called gets

Variable Expression
8
Compilers and Expressions

• When the expression is encountered
• First it is evaluated
• Then the result is stored in the memory location
  named on the left
• Lets walk through an example or two of this on
  the board

9
Setting the first value

• When we give a value to a variable that
  previously had none, we say we are initializing
  the variable
• We can do this when we declare the variable if we
  know what value we want it to start with
• Eg int Joe_age 67

10
Mixed types

• In general, it is considered an error to try to
  put a result of one type into a variable of
  another
• Integral types allow type promotion
• Floating point types allow type promotion
• No types allow demotion without help
• Can you think of some examples of each? Why does
  this make sense?

Char lt Short lt Int lt Long
Float lt Double lt Long Double
11
Example of Promotion
include ltiostream.hgt int main( ) // What gets
printed? int x 3, y 4 char z 'Z' x
z cout ltlt "X is" ltlt x ltlt endl return 0
12
Operators Put To

• The put to operator is used with cout to
  produce output
• The thing to the left must be a streama source
  of data
• The right hand side can be any simple type or
  string

cout ltlt X is ltlt x ltlt endl
13
Operators Get From

• Likewise for output there is the gtgt or get
  from operator
• Here the stream is on the left side getting
  something from a stream and saving it into a
  variable
• To use these, you must use the iostream.h header
  file

14
Addition, Subtraction, Multiplication

• These operators operate just like you are used
  towe use , -, and to symbolize these
  operations
• Note that the minus sign can be used as a unary
  operator to make a single number or variable
  negative, EG

x y (-5)
15
Division Operator

• The division (/) operator doesnt always operate
  the way you are used to
• Integer division must give an integer result!
• This is to avoid type mismatches
• So what happens is dependent on the type

16
Real Numbers Versus Integers in Division
7/3 2.33333333 in real numbers 7/3 2
remainder 1 in integer math So 7/3 ? 2
for an integer in C And 7 3 ? 1 for an
integer in C This second operator is called
the modulus operator We read this as 7 mod 3
17
Increment/Decrement Operators

• The increment () and decrement (--) operators
  are unary operatorsthey take only one variable
• They can be used on the left or right of a
  variable to add or subtract one from it
• Which side they appear on may have an effect on
  the results!

18
Prefix Increment
Code for each int x 3, y 4
int x 3, y 4 x y
x y Results stored in
memory X 5
X 4 Y 5
Y 5 Thus when the value is extracted from Y
is indicated by the position of the operator
19
Precedence of Operators

• You need to know the precedence of basic
  operators for this course! (Well, not all)
• The precedence determines which thing is done
  first
• For the math operators, you should know

Precedence
Associativity Unary -, , -- right to
left , /, left
to right , -
left to right
right to left
20
Associativity

• The associativity of an operator tells us which
  direction to combine operators of equal precedence

x y z 4 m done first done
next done last
21
Parentheses

• Parentheses can be used to change the usual order
• Anything inside parentheses is done first
• Evaluate the following
• (7 (10 5) 3) 4 9)
• ((- 5 5) 5 10)

22
Type Casting

• Occasionally we need to explicitly change from
  one type to another
• Doing this is called type castingit can only be
  done when C actually knows how to do it
• Usually done when you are throwing information
  away!

23
Type Casting Continued

• Our compiler seems to accept either of two
  different ways of expressing this
• X int(Y)
• X (int) Y
• What are the results here if X is an integer and
  Y
• is a floating point value?

24
Something Funny About ?

• We use the equals sign to mean many things
• We have to restrict meanings in programming
  languages
• Thus we can not use for comparison of values
  (we will use in C)
• Likewise, when we talk about functions, we will
  have to restrict our meanings there

25
Input From the User
int main( ) int x 0 cout ltlt Please input
an integer ltlt endl cin gtgt x cout ltlt Please
input another integer ltlt endl cin gtgt x
26
Values Stored in Variables

• What happened to the values stored in x?
• We have a choice of persistence or overwriting
• C overwrites old values
• The mailbox with only one letter
• When an old value is gone, it is really gone!
• There is no way to recover past values of
  variables

27
Accumulation Operators

• If you want the effect of a running total, we
  have special operators , , /, and -
• The effect of (for example)

CODE EQUIVALENT Sum
purchase Sum Sum purchase
28
Legal Code, Bad Style

• The following is a legal C statement
• What passes through an assignment operator is
  what will be passed to the previous
• This is a mess to read or documentdont do it!

variance myValue overAll result