CS 177 Recitation

1
CS 177 Recitation

• March 29, 2007

2
Announcements

• Project4 due next Wednesday!
• Exam2 results now available on WebCT
• Question 17 is problematic, so any answer will be
  accepted (grades will be adjusted)
• Key to Exam2 will be posted on website
• Will revisit exam2 later today
• Project3 grades posted on WebCT
• How to access Project3 grading sheet
• Login to lore.cs.purdue.edu
• Your grading sheet (.html) is located at
  /u/u98/cs177/2007Spring/Grades/Project3/

3
Objects

• What is an object?
• Something which has data and operations
• Could an integer be considered an object?
• Yes.
• What are its data and operations?
• Data ? integer number
• Operations ? , -, /, , gt, lt, ,

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Objects

• Think of a car as an object.
• What are its data?
• Color, model, year, horsepower, mileage, etc
• What are its operations?
• Start engine, steer, accelerate, brake, change
  gears, etc
• How about the data for a string?
• The sequence of characters that are enclosed in
  quotes
• How about the operations for a string?
• Convert to uppercase, lowercase, access a
  particular character, search for a substring,
  find first occurrence of a character

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Object Oriented Programming

• What is object-oriented programming?
• A programming approach in which the problem is
  broken down into objects
• Example A library management system could have
  objects for books, librarians, readers, orders,
  etc.
• What makes object-oriented programming so useful?
• Large systems can be broken into objects, and the
  objects can be implemented by different teams and
  later brought together
• Objects can be reused in other applications
• Why is an object an abstract representation?
• It does not represent all possible data and
  operations
• Example A car object would not represent the
  pizza leftovers on the back seat
• We only represent what is necessary to solve the
  problem

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Properties and Methods

• What are properties of objects?
• The data or characteristics of objects (like
  variables)
• Example For a string object, the length is a
  property
• str Strings rock!
• str.length would give us 13
• What are methods (functions) of objects?
• The operations that can be performed on the
  objects
• str MaKe Me LOwEr
• lowstr str.toLowerCase()
• lowstr is now make me lower
• None of the methods for strings actually modify
  the string, they simply return a new string

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Properties and Methods

• What are some properties that we already know?
• document.myForm.myInput.value
• document.images.myImage.src
• What is a method that we know?
• document.write()
• In the above examples, what is the object?
• document
• So how can we access properties or methods?
• ltObject namegt.ltProperty or Method namegt

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String Manipulations

• str remains the same Foo 2 You!

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Example 1
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More String Manipulations

• What does the charAt() method do?
• Given an integer, it returns the character at
  that index in the string
• str Clark Kent
• ch str.charAt(3) (ch now holds r)
• ch str.charAt(5) (ch now holds )
• Note for a string of length n, the first
  character is at index 0, and the last is at n-1.
• What does the substring() method do?
• Given a beginning index and an ending index, it
  returns the part of the total string from the
  beginning index up to, but not including, the
  ending index.
• str My TA is AWESOME!
• sub str.substring(9,16) (sub now holds
  AWESOME)
• sub str.substring(4,12) (sub now holds A is
  AWE)
• Note first index is included in the substring,
  but the last is not.

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Example 2

• Using charAt() and substring(), write a function
  Capitalize(str) that receives a string and
  returns the same string with the first letter
  capitalized and all others lowercase.

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More string methods

• What does the search() method do?
• Given either a string or list of characters, it
  returns the index of the beginning of the first
  occurrence of the string or any character
  specified in the list.
• str I think therefore I am
• i str.search(I)
• (i 0)
• i str.search(f)
• (i 13)
• i str.search(here)
• (i 9, index of h)
• i str.search(\klmn\)
• (i 5, index of n)
• i str.search(\the\)
• (i 2, not i 8!)
• i str.search(you)
• (i -1, there is no you in str!)

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General searching
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Example 3

• Write JavaScript code which removes the all
  characters a from the string str
• str banana
• result
• i 0
• while(i lt str.length)
• if (str.charAt(i) ! a)
• result result str.charAt(i)
• i i 1
• str result
• What would result be here?
• bnn

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Example 4

• Write JavaScript code to look through the string
  str and find the index of the first occurrence of
  help, but is not case-sensitive (i.e. it would
  also find HeLP).
• str IsohHeLpelPmonhEllpious
• str str.toLowerCase()
• index str.search(help)
• What would index be here?
• 4

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Exam2 Revisit

• Statistics
• Total  grade  number  percent   75-100  
    A       8       7   65- 74     B  
      41       34   55- 64     C      
  45       38   45- 54     D       19    
    16  below 45     F       7      
  5
• Average 61, highest 91, lowest 28
• These are statistics before grade update
  (everyone will get 3pts for question 17)

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Exam2 Revisit

• Questions that only 20-30 people got right
  1, 5, 6, 9, 13, 16, 21, 22, 29
• Now review each one. Answers rendered green.
• 1. Which of the following is not an advantage of
  von Neumann architecture over its predecessors?
• Data is stored in memory for CPU to fetch when
  executing programs
• Stored program
• (c) I/O devices allow for interaction with the
  user
• (d) CPU executes by loading program instructions
  from memory and executing them in sequence

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Exam2 Revisit (cont.)

• 5. Given the definition of the function check()
  below, which of the following statements is FALSE
  regarding check()?
• function check()
• input prompt("Please input a positive number
  ", "")
• input parseFloat(input) // if input is
  initially , after parseFloat, input is NaN
• if(input ! "")
• document.write("Input is valid!")
• else
• document.write("Input is invalid!")
• (a) the function does not check whether the input
  is a number
• (b) the output will be Input is invalid! if
  nothing is entered in the prompt
• (c) the function does not check whether the
  number is positive
• (d) the function uses prompt correctly

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Exam2 Revisit (cont.)

• 6. What does the following function do? Assume
  parameters x and y are both positive integers.
• function guessWhat(x, y)
• var res, dif
• if(x lt y) dif y - x
• res Math.random() dif x
• else dif x - y
• res Math.floor(Math.random() dif) y
  
• return res
• (a) if x is no greater than y, return a random
  number in (x, y otherwise return a random
  integer in y, x
• (b) if x is no greater than y, return a random
  number in x, y) otherwise return a random
  integer in y, x-1
• (c) if x is no greater than y, return a random
  number in x, y) otherwise return a random
  integer in y, x
• (d) if x is no greater than y, return a random
  number in (x, y otherwise return a random
  integer in y1, x

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Exam2 Revisit (cont.)

• 9. Suppose that a TA is handing back grade sheets
  for a programming project to her students. The TA
  walks up to the first student, asks for her name,
  and then proceeds to search through the stack of
  gradesheets one-by-one from the beginning. How
  many sheets must the TA look through in the worst
  case scenario where each students grade sheet
  turns out to be the last sheet in the stack by
  the time she finishes? Give the exact answer
  assuming there are N students. That is, she had
  to compare N names for the first student.
• (a) N(N1)/2 (i.e. N (N-1) (N-2)
  1)
• (b) N2
• (c) N
• (d) N!

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Exam2 Revisit (cont.)

• 13. Identify the TRUE statement(s) regarding
  encryption
• I. Private key encryption requires the sharing of
  a private key
• II. Public key encryption does not require the
  sharing of any key
• III. Public key encryption allows the receiver to
  authenticate the sender
• IV. Almost all secure communications on the
  Internet use private key encryption
  (answer a)
• 16. In project 2, you were asked to create an
  object which is used to represent an element in
  the periodic table. What are the two things that
  define an object?
• (a) Variables and Constructors
• (b) Properties and Methods
• (c) Methods and Constructors
• (d) Properties and Variables

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Exam2 Revisit (cont.)

• 21. What are the values of the variables s and t
  after executing the following code fragment? Be
  careful.
• var s 20
• var t 30
• if (s gt 100) // condition not satisfied!
• s 40 // this is not executed!
• t 20
• if (t gt 20) // condition not satisfied!
• s 80 // this is not executed!
• t 10
• (a) s 40, t 20
• (b) s 20, t 10
• (c) s 80, t 10
• (d) s 20, t 30

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Exam2 Revisit (cont.)

• 22. What will be the value of the variable z
  after the following code fragment is executed? Be
  very careful!
• var x 20
• var y 20
• var z 10
• if (x lt y)
• z x y - 40
• else
• if (x 10) // asssignment here,
  not equivalence! Condition satisfied!
• z x10 - y - 60
• else
• if (x y)
• z 4x - 2y
• Answer b, variable z x10 y 60 20!

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Exam2 Revisit (cont.)

• 29. What will the following code output?
• if (A gt a) // condition not satisfied
• document.write("Maybe Right")
• else
• if (z gt Z) // condition satisfied
• document.write("Maybe Wrong")
• else
• document.write("Correct")
• (a) There will be no output.
• (b) Correct
• (c) Maybe Wrong
• (d) Maybe Right

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Questions?