CS 17700 Recitation Slides

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CS 17700Recitation Slides

• Week 10

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Announcements

• Reminder Exam 2 will be on Tuesday (Nov 4) from
  630-730 PM
• PHYS 112 for students in lab sections 0101-0701
• FRNY G140 for students in lab sections 0801-1201
• Covers Chapters 8-13
• Evening consulting hours will be from 730 to
  900 PM that day

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Announcements

• Do not cheat! We DO check for suspiciously
  similar files, and students who cheat on an
  assignment will receive a zero on it.
• Before you turn in an assignment, check to make
  sure that all your code is in the file with the
  required name (ex. "user_name.html"), and delete
  any unneeded files in your lab/project folder.

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Questions?
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Conditional Execution

• An if statement is a control statement that
  provides conditional execution
• control statements control the execution of other
  JavaScript statements
• conditional execution lets your program react
  differently (execute different code) in different
  situations

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Conditional Repetition

• A while loop is a control statement that provides
  conditional repetition
• Conditional repetition is similar to conditional
  execution, but the statements inside the loop can
  be executed more than once
• Many problems involve repeating a task until some
  condition is met (or fails to be met)

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While Loops

• General form of a while loop
• The statements between the curly brackets are
  called the loop body
• As long as the boolean test evaluates to true,
  the loop body statements are executed and the
  test re-evaluated

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While Loop Example

• Let's say that we want to choose a random number
  between 1 and 100 until we get one that is less
  than 20

num1 20 num2 RandomInt(1, 100) document.writ
e(num2 "ltbr/gt") while(num2 gt num1) num2
RandomInt(1, 100) document.write(num2
"ltbr/gt") document.write("ltbr/gtThe number
"num2" is less than 20.")
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Avoiding Redundancy

• In the previous code, we gave num2 a random value
  in two different places (this is redundant)
• Want to avoid redundant code
• it makes code needlessly repetitive
• it can lead to errors if you try to change the
  code

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Avoiding Redundancy

• To avoid redundant code
• "prime" the loop with default values
• make sure that your default values dont make
  your code work incorrectly!
• make sure your default values will cause the loop
  body to be executed at least once
• define a boolean "flag" that is used in the
  boolean test instead of other variables

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Avoiding Redundancy Examples

• Priming the loop
• Defining a flag

num1 20 num2 50 //50 is greater than 20,
so loop will run gt1 times while(num2 gt num1)
num2 RandomInt(1, 100) alert("The
number " num2 " is less than 20.")
num1 20 done false while(done false)
num2 RandomInt(1, 100) if(num2 lt num1)
done true //change the flag so the loop
will stop alert("The number " num2 "
is less than 20.")
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Loop Tests

• How humans think "do x until y happens"
• ex. choose a random number until you get a number
  less than 20
• How while loops work "do x while z happens"
• ex. choose a new random number while the current
  one is greater than or equal to 20
• The loop test will check for z, which is usually
  the opposite of y
• ex. while (rand_num gt 20)

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Counter-Driven Loops

• A while loop can also be used to repeat a task a
  certain number of times
• The boolean test is based on a counter variable,
  which is changed (usually incremented by one) in
  the loop body
• General form

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Counter-Driven Loops Example

• num_kittens parseFloat(prompt("How many kittens
  do you want?"))
• num_ducks parseFloat(prompt("How many ducks do
  you want?"))
• count 0
• while( count lt num_kittens )
• document.write("ltimg src'kitten.jpg' /gt")
• count count 1
• count 0
• while( count lt num_ducks )
• document.write("ltimg src'daffy.jpg' /gt")
• count count 1

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Counter-Driven Loops

• Note the counter doesn't have to start at zero,
  nor do you have to add or subtract one to it each
  time
• Other counter-driven loops
• start at 1000, divide by 3 each time
• start at 1, add 2 each time
• etc.

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Infinite Loops

• If the boolean test always evaluates to true, the
  while loop will run forever
• Infinite loops are (usually) bad
• in JavaScript, they can freeze your browser
• in other programming languages, they can freeze
  your whole computer
• To prevent infinite loops, make sure some part of
  the loop test changes inside the loop

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Computer Science

• Computer Science is the study of computation and
  problem solving
• Major themes in CS
• Theory - design and mathematical analysis of
  algorithms, understanding the capabilities and
  limitations of computers
• Software design, development, testing and
  maintenance of software
• Hardware - development of devices to execute
  programs

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Encryption Algorithms

• Private Key Encryption
• each pair of users agrees on a private key
• a message is encrypted with the private key by
  the sender (A), and decrypted with the same
  private key by the receiver (B)
• problem how do A B securely agree on a private
  key?

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Encryption Algorithms

• Public Key Encryption
• Each user has a public and private key, and gives
  their public key out to other users
• A encrypts with B's public key, and B decrypts
  with their private key
• only B can read the message, but can't be sure
  that it came from A
• even better A encrypts with A's private and B's
  public keys, and B decrypts with B's private and
  A's public keys
• only B can read the message, and it definitely
  came from A

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Software Engineering

• Software engineering is "industrial strength"
  programming (programming on a large scale)
• Involves teams of programmers and stages of
  development
• Coordination among programmers/teams and rigorous
  code testing are essential to successful projects

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Software Engineering

• Stages in the Software Life Cycle
• Requirement Analysis and Specification Figure
  out what the customer needs and write it down
  with lots of detail
• Design Design the software system conceptually
  (break it down into distinct pieces and figure
  out which algorithms to use)
• Implementation Write code. Teams may work
  independently and integrate their modules to
  achieve the finished program
• Testing The code is tested both as independent
  modules (unit testing) and as a whole. Redesign
  and reimplementation are done as needed to fix
  problems.
• Operation and Maintenance The system must be
  installed and maintained over time (updates, etc).

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Artificial Intelligence

• AI attempts to create programs that exhibit
  human-like characteristics (the ability to
  reason, think, etc)
• Expert Systems programs that encapsulate expert
  knowledge in a specific domain
• ex. medical expert systems capture the knowledge
  of a number of doctors, and can be used to
  diagnose illnesses as if the patient were being
  treated by all those doctors in person
• Neural Computing design of programs that mimic
  the biological brain

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Questions?