Arrays

1

• Arrays

2
Lists of Variables

• Sometimes, it becomes necessary to order
  variables in a list
• Temperature on April 1 90
• Temperature on April 2 87
• Temperature on April 3 84
• in order to spot trends.

3
Lists of variables

• All of the variables and values are of the same
  type integers, strings, whatever.
• Their order is important.
• Their place in the list is important
• May be shortened as
• Temp1 90
• Temp2 87
• Temp3 84

4
Track the order as a separate variable

• Called an index
• Temp1 90
• Temp2 87
• Temp3 84
• x 2
• Temp x ?
• Name index value. index can change!

5
Arrays

• A difficult concept at first int z5
• Means z0, z1 , z2, z3, z4 are
  separate integer variables
• Starts at 0.
• An array with 5 elements is indexed (or
  subscripted) from 0 to 4.
• in C int z 5

6
Array example

• int x
• int z 10
• for (x 0 x lt 9 x x1)
• zx x
• What is the value of z4? 4!
• in fact
• z0 0 z1 1 z2 2 ... to z9
  9

7
Arrays do not have to be numbers

• int x
• string myArray 5
• myArray0 hello"
• myArray1 we don't"
• myArray2 have"
• myArray3 a test"
• myArray4 next week"
• for (x0 x lt 4 x)
• cout ltlt myArrayx
• cout ltlt endl

8
What are arrays good for?

• They are confusing just to read
• int a 5
• EmployeeInfo 1021
• Temperature x

9
What are arrays good for?

• They are hard to track context awareness is
  difficult
• Whats going on in this code?
• int x, RunningTotal 0
• int Sum 5
• for (x 0 x lt 4 x)
• RunningTotal RunningTotal x
• Sum x RunningTotal

10

• Sum0 0
• Sum1 0 1
• Sum2 0 1 2
• Sum3 0 1 2 3
• Sum4 0 1 2 3 4 10

11
What are arrays good for?

• Think of the use of lists, that are
• ordinal (ordered in sequence)
• item positions are important
• lists can be anything strings, names, numbers,
  ages, costs, addresses

12
Using three arrays to keep information

• string lastName 5
• string firstName 5
• int age 5
• e.g.
• lastName 0 Truman
• firstName 0 Harry
• age 0 175

13
array items are correlated by index
14
continued

• lastName 1 Garcia
• firstName 1 Jerry
• age 1 55
• lastName 2 Smith
• firstName 2 John
• age 2 24

15
continued

• lastName 3 Mouse
• firstName 3 Mickey
• age 3 75
• lastName 4 Buckley
• firstName 4 Mike
• age 4 39

16
array items are correlated by index
table
record
field
17
What is this?

• Information that is grouped by index
• Kept in arrays
• e.g. lastName1, firstName1, and age1 go
  together to form one profile for person 1
• Is a DATABASE.
• Were creating our own database.

18
printing out the info note index

• int index 0
• cin gtgt index
• if (index gt0) (index lt4)
• cout ltlt Item ltlt index ltlt endl
• cout ltlt First Name firstName index ltlt
  endl
• cout ltlt Last Name lastName index ltlt
  endl
• cout ltlt Age age index ltlt endl

19
just imagine

• Each array contains thousands of items
• More arrays to hold soc. sec. , birthdate, pay
  scale, years of service
• Program ways to enter data as well as display.
• A full-scale Database Management program.

20
Changing array values - age

• void changeAge( int indexToChange )
• int newAge
• cout ltlt Enter age "
• cin gtgt newAge
• age indexToChange newAge
• note
• array index is a parameter passed into the method
• nothing is returned, array is changed directly
  within the method.

21
Changing array values first name

• void changeFirstName( int indexToChange )
• string userInput
• cout ltlt "Enter First Name "
• cin gtgt userInput
• firstName indexToChange userInput

22
Changing array values last name

• void changeLastName( int indexToChange )
• string userInput
• cout ltlt "Enter Last Name "
• cin gtgt userInput
• lastName indexToChange userInput

23
printing out the info note index

• for (int index0 indexlt4 index)
• cout ltlt First Name ltlt firstName index
  
• cout ltlt Last Name ltlt lastName index
• cout ltlt Age ltlt age index ltlt for
  index ltlt x