Arrays

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Arrays

• Dorian Miller

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Announcements

• Homework 3
• Due Thursday Sept 27, 1159pm EST
• Program 2
• Due Tuesday Oct 2, 1159pm EST

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Program 1 graded

• Program 1 results available
• See My Grades on BlackBoard
• Program 1 solution posted
• Purpose of program 1
• Simple JavaScript program
• Variables and computation
• HTML forms
• Practical debugging experience to fix program

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Range of scores

• Minor issues
• points 90-101
• Program needs improvement
• points 55-90
• Major problems with program
• less than 50 points

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Minor issues (points 90-101)

• Constants
• All numbers must be constants
• 200 common magic number
• Use of comments, missing or too few
• Missing, incorrect HTML tags

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Program needs improvement (points 55-90)

• Good strategy for partial credit
• Program runs (works with tester)
• Incorrect computation
• Output was not in form must be in form, not alert

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Major problems with program (less than 50 points)

• Program does not run
• Program produces no output
• Graded on HTML and Javascript (25 points)
• Late submission 0 points
• Edit web page after deadline
• Tester shows date and time of last file save

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Resources

• Office hours (or by appointment)
• Dorian MW 330-430
• Will Tu, Th 2-3pm
• Email questions to Will and Dorian
• Reading
• Online reading for each lecture listed on course
  schedule (next to slides)
• Lecture slides

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Collaboration

• Allowed to discuss
• lecture material
• recitation material
• Program specification
• Homeworks and programs completed independently
• Rule of thumb for discussions
• Discuss ideas
• Take a 1 hour break
• Independently implement code (shows significant
  understanding)

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Important Please stay after class

• Review honor code

Rule of thumb for discussions 1. Discuss ideas 2.
Take a 1 hour break 3. Independently implement
code (shows significant understanding)
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Practice tracing a program

• Trace a program (part of homework 3)
• Process by hand and paper
• Walk through program and predict output
• Useful for understanding and debugging program
• Typical exam question

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Trace the while loop

• What does this program do?
• var i 0
• var sum 0
• while (i
• sum sum i
• alert("i " i " sum " sum)
• i i5
• Final value of i and sum?
• i ???
• sum ???

• Values at alert
• list values to trace
• Each line, 1 pass through loop
• Variable value changes at assignment
• i sum
• ____ ____
• ____ ____
• ____ ____
• ____ ____
• ____ ____

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Trace the while loop

• What does this program do?
• var i 0
• var sum 0
• while (i
• sum sum i
• alert("i " i " sum " sum)
• i i5
• Final value of i and sum?
• i25 (i increased and condition false)
• sum 50 (did not change)

• Values at alert
• list values to trace
• Each line, 1 pass through loop
• i sum
• 0 0
• 5 5
• 10 15
• 15 30
• 20 50

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Trace the for loop

• What does this program do?
• var i 0
• var sum 0
• var i
• var sum 0
• for (i1 i
• sum sum i
• alert("i " i " sum " sum)
• Final value of i and sum?
• i ???
• sum ???

• Values at alert
• list values to trace
• Each line, 1 pass through loop
• Variable value changes at assignment
• i sum
• ____ ____
• ____ ____
• ____ ____
• ____ ____
• ____ ____

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Trace the for loop

• What does this program do?
• var i
• var sum 0
• for (i1 i
• sum sum i
• alert("i " i " sum " sum)
• Final value of i and sum?
• i 32 (changed but condition is false)
• sum 31 (Stays the same)

• Values at alert
• list values to trace
• Each line, 1 pass through loop
• Variable value changes at assignment
• i sum
• 1 1
• 2 3
• 4 7
• 8 15
• 16 31

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Big Picture

• Control structures
• If statements
• Loops
• This week
• Array data structures

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Simple program analogyControl structures

• Program Instructions
• While (need more)
• Take bread
• Get scoop of peanut butter
• Spread scoop on bread
• If (need strawberry)
• Get scoop of strawberry
• else
• Get scoop of grape
• Spread scoop on bread
• Done

• Control structure
• Selection
• Loops
• Related to program instructions

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Simple program analogyMaking a peanut butter and
Jelly sandwich

• Data until now
• Variables
• Single data elements
• Examples of single data
• Number of Scoops
• Slices of bread
• Sandwich as text

• Program Data
• PB scoops 5
• Jelly scoops 5
• Bread slices 10
• Sandwich ???

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Examples of list of data

• List of multiple data items
• UNC NCAA championships
• 1957, 1982, 1993, 2005
• Names of seven dwarfs
• Grumpy, Sneezy, Doc, Happy, Dopey, Bashful,
  Sleepy

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Wrong way

• Variables are not good solution to store
  championships
• Example Wrong way to store list
• var year1 1957
• var year2 1982
• var year3 1993
• var year4 2005
• Many items difficult to manage as individual
  variables
• How about 100 or 1000 data items?

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Arrays

• An array is a list of values that can be
  represented by one variable
• Example
• var wins 1957, 1982, 1993, 2005
• Syntax
• wins is a variable
• Square brackets indicates list of array items
• Items separated by comma

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Fixed array

• var wins 1957, 1982, 1993, 2005
• Fixed
• Values not intended to change in program
• Example Years of championship are fixed
• Set at design time when program is created

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Array index

• Array index position of data in list
• First index 0
• Called zero offset
• Last index number of items -1
• Example
• First item, index 0 has value 1957
• Last item, index 3 has value 2005
• 0 1 2 3
• var wins 1957, 1982, 1993, 2005

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Array access

• Variable wins refers to entire list
• var wins 1957, 1982, 1993, 2005
• Access one array item
• Get the value stored in the list at the index
  position
• Square brackets indicate position of list item
• winsindex
• Example of array access
• wins0 is 1957
• wins1 is 1982
• wins2 is 1993
• wins3 is 2005

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Array access example

• Read the value of championship
• var wins 1957, 1982, 1993, 2005
• Store the final win value in variable last
• var last wins3
• Display value of first win
• document.write(First win wins0)

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Array length

• Array could be any length
• var wins 1957, 1982, 1993, 2005
• How many array elements? 4
• wins.length
• Syntax
• Period operator
• Length property, number of items in array
• Example
• var numberOfItems wins.length

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Display elements in array

• Display elements in array
• var wins 1957, 1982, 1993, 2005
• document.write(wins0
  )
• document.write(wins1
  )
• document.write(wins2
  )
• document.write(wins3
  )
• Write a for loop to display elements
• First index is 0
• Last index is wins.length -1

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Solution

• Details
• Index offset by one
• Start index 0
• Last index i (wins.length-1)
• var wins 1957, 1982, 1993, 2005
• var i
• for(i0 i
• document.write(winsi "
  ")

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Errors

• Out of bounds access
• Access array index outside valid range
• Valid range 0 to array.length-1
• Results undefined
• Example undefined values
• var wins 1957, 1982, 1993, 2005
• wins-1 is undefined
• wins4 is undefined

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Review concept of memory

• Review Memory
• Cell stores value
• Each cell has address
• Memory Stores data

address
9278 9279 9280 9281 9282 9283
cell
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Declaration
var jellyScoops

• Declaration
• Creates variable in program
• Reserves memory cell
• Value starts undefined
• Why address 9279?

9278 9279 9280 9281 9282 9283
jellyScoops
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undefined
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Array in memory

• var wins 1957, 1982, 1993, 2005
• Array elements use consecutive memory locations

wins
9278 9279 9280 9281 9282 9283
1957
1982
1993
2005
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Array in memory

• Win is a reference variable
• Reference variable stores the memory location
• winsindex refers to memory location wins
  index

wins
9278 9279 9280 9281 9282 9283
1957
1982
1993
2005
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Dynamic Array

• Array values not known at design time
• Example compute average grade
• List of assignment scores
• User determines number of scores
• User enters scores

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Create a new array

• Create a new array with 5 elements
• var scores new Array(5)
• Syntax
• new operator creates an array object
• Array is the array object
• (5) the number of elements in array
• new and Array are keyword words

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New Array

• Initial array values
• null value that stands for nothing
• null is keyword
• Check initial value
• var scores new Array(5)
• if (scores0 null)
• alert(Yes, the value is null)

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Write array value

• Write one array value
• Assign value like any variable
• Store integer 99 in array index 0
• scores0 99

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Set Initial array values

• What is a reasonable initial value for scores?
• Initial value ???
• var scores new Array(5)
• Write a for loop to initialize score array
• Start index ???
• End index ???

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Solution

• Set initial value
• 5 array elements
• Start index 0
• Last index 4
• var scores new Array(5)
• var i
• for(i0 i
• scoresi0

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Exercise

• Say these scores are entered into the array
• scores 33, 74, 20, 51, 60
• The average is
• Sum 33 74 20 51 60
• Average sum / 5
• Write a for loop for compute the average

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Solution Compute average

• var scores 33, 74, 20, 51, 60
• var i
• var sum0
• for(i0 i
• sum sum scoresi
• document.write("sum " sum "
  ")
• document.write("average " sum/scores.length)

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Important Please stay after class

• Review honor code