Lab 2

1
Lab 2

• posted, due 2/10
• Lab 3 will be posted 2/5
• Must be submitted via UB Learns according to the
  instructions on the web site
• There are no emails involved in the submission

2
How do we use numbers?

• Counting
• Measuring
• Non-numerical designations
• Finally, they can designate a state unrelated
  to the number (1 if by land, 2 if by sea)

3
So computers

• Must deal with each type of number in its own
  way
• Must store those numbers in their own way
• Must make those numbers available, though,
  through a common interface
• That interface, is the VARIABLE

4
Variables

• x, y, z
• i, j, k
• TotalCountOfNewEmployees
• Geekish_Designations_0123
• They Stand for Something, can be named anything
  except reserved C words

5
Variable Types (primitive, common)

• Integers meant to be a whole number (used for
  counting) C int
• Floating Pt. meant to be a detailed number
  (used for measuring) called a double
• Character has no meaning other than to be read
  by a human C char
• Strings of Characters have no meaning other
  than to be read by a human. Not native to C,
  but libraries exist to help
• Boolean reflects a state of true or false
  C bool

6
x 14

• Could mean 14 (not likely)
• Could mean 13.999995
• Could mean 14.000001
• Could mean 20 (honestly), might not be base-10,
  more later
• Could mean Fourteen, which has no meaning other
  than to be read by humans

7
Typing a variable clarifies what it does and
its precision

• int x
• x 14
• Therefore, 14 means 14
• double x
• x 14.0000
• Therefore, x has meaning, 14 to 4 places

8
note

• Can be initialized at the declaration
• int x 14 // type name initValue
• Can only be typed once
• Value can be reset (thats why its called a
  variable).

9
floating point?

• 14.6567
• 128.1231
• 154367.001423
• 3.14159
• 987987465986543.
• adequate precision
• cout truncates for space

10
chars and strings

• include ltiostreamgt
• include ltstringgt
• using namespace std
• char Id 'm' // chars are single-quoted
• string name "mikeb" // strings are double
• int main()cout ltlt Id ltlt endlcout ltlt name ltlt
  endl

11
New Types

• Boolean true or false (really means presence or
  absence), used to signify a condition, or
  state of being.
• the bool type can have values true, false, 1,
  or 0
• bool X,Y, Z
• X true (same as 1, means X is present)
• Y false (same as 0, means Y is NOT
  present)
• Z X OR Y (what is Z?, i.e. does Z signify
  some presence?)
• Z X AND Y (what is Z?)

12
A smoke, water, burglar alarm

• Boolean variables W, B, S, A
• W is true if water
• B is true if burglar
• S is true if smoke
• A means sound the alarm
• A W and B and S
• Does W and B and S make sense?
• A W or B or S

13
example code

• include ltiostreamgtinclude ltstringgtusing
  namespace stdchar Id 'm'string Name
  "mikeb"bool Flag falseint x,y,zdouble
  a14.0123,b,c
• int main( )x true // lets try 1,
  1, 1cout ltlt x ltlt endlcout ltlt Id ltlt
  endlcout ltlt name ltlt endlcout ltlt flag ltlt
  endlcout ltlt a ltlt endl

14
once a variable is designed

• you can get it from the operator (if you want)
• int x
• x0
• cout ltlt enter x
• cin gtgt x

15
The "if..else" statement

• if ( a b )
• // do this
• else
• // do that

comparison
16
"logical" comparisons

• if ( a b )
• if ( a ! b )
• if ( a lt b )
• if ( a gt b )
• if ( a lt b )
• if ( a gt b )

17
demo
18
Legality

• Variable value and variable type must be the
  same.
• Deterministic behavior - All computers yield the
  same result with the same variables

19
Computers DO NOT completely check our programs

• Called Compilation
• Must be successful before a program can run
• But not all errors are caught in compilation

20
Compilation
will it work?

21
Illegality

• Compiler Errors however, some languages and
  compilers allow mis-designations of variables
• Indeterminate results
• Run-time Errors

22
Illegal 1 mis-defined variables

• int y
• y 14.0001
• char q
• q abcdef
• double z
• z hello

23
Why Type Variables

• Deterministic behavior of variables and the
  numbers they represent.
• New definition of Legality We will never
  perform mathematical or logical operations on
  variables of different types.

24
Illegal 2 mixing variables in same equation

• int x
• double y
• char z
• x 12
• y 16.001
• z x y

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Illegal 3 - Operation wrong for variable type

• char Q, R, S
• Q R / S (/ divide)