COMP 14 Introduction to Programming

1
COMP 14Introduction to Programming

• Jingyu Yan
• February 14, 2005

2
Announcements

• Mid-Term Exam - March 9
• covers Chapters 1-5
• look at previous mid-term, homework exercises,
  in-class exercises, words in bold print in
  textbook
• This Wednesday
• Please bring your laptop

3
Loops

• Allow us to repeat statements some number of
  times
• Must use a loop control variable
• controls how many times to loop
• 4 Parts to Every Loop
• initialization - set loop control variable before
  condition
• condition - when to stop
• update - change to the loop control variable
• body - actions to repeat

4
Typical Uses of Loops

• Repeat a section of code a specified number of
  times - counter-controlled
• Repeat a section of code (reading input) until
  the a specific value is read - sentinel-controlled
  
• Repeat a section of code (reading input) until a
  valid value is entered - input validation
• Repeat a section of code (reading from a file)
  until the end of the file is reached -
  EOF-controlled
• Repeat a section of code until a boolean variable
  becomes false - flag-controlled

5
Questions

• What type of while loop should be used?
• Read every line in a file until the number -99 is
  read.
• Read every line in a file where the length of the
  file is unknown.
• Make sure that the user enters a lowercase
  letter.
• Print the even numbers between 0 and 50.

sentinel-controlled
EOF-controlled
input validation
counter-controlled
6
The while Loop
while (expression) statement
7
The while Loop

• final int LIMIT 3
• int count 0
• while (count lt LIMIT)
• System.out.println (count)
• count
• System.out.println (All done!)

boolean condition
Output 0 1 2 All done!
initialization
loop body
update
8
In-Class Exercise

• Write a while loop that prints
• 2
• 4
• 6
• 8

int i 2 while (i lt 8) System.out.println
(i) i 2
int i 1 while (i lt 4) System.out.println
(i2) i
9
In-Class Exercise

• Write a while loop that prints the odd numbers
  between 0-100.

int i 0 while (i lt 100) if ((i 2) 1)
System.out.println (i) i
int i 1 while (i lt 99) System.out.println
(i) i2
10
Question

• What is the loop control variable in the
  following while loop?
• int count 0, num
• num Integer.parseInt(keyboard.readLine())
• while (num lt 0)
• System.out.println (num)
• num Integer.parseInt(keyboard.readLine())
• count

num
11
Today in COMP 14

• The for loop
• The do...while loop
• break and continue
• Nested loops
• Textbook Reference Ch 5 (pgs. 229-247)

12
The for Loop

• Specialized form of while loop
• Simplifies the writing of counter-controlled
  loops
• Basic Form

for (initialization condition update)
statement(s)
13
The for LoopExecution

• initial statement executes
• loop condition evaluated
• If loop condition evaluates to true, execute for
  loop statement and execute update statement
• Go back to step 2 and repeat until loop condition
  is false

14
The for LoopSyntax
for ( initialization condition update )
loop body
15
for vs. while

• A for loop is functionally equivalent to the
  following while loop structure

initialization while ( condition ) loop
body update
16
The for LoopExample
Output 0 1 2 All done!

• final int LIMIT 3
• int count
• for (count0 countltLIMIT count)
• System.out.println (count)
• System.out.println (All done!)

boolean condition
initialization
update
loop body
17
Comparing while and for

• While Loop
• final int LIMIT3
• int i 0
• while (i lt LIMIT)
• System.out.println (i)
• i
• System.out.println
• (All done!)

For Loop final int LIMIT3 int i for (i0
iltLIMIT i) System.out.println
(i) System.out.println ("All done!")
18
The for Loop

• Like a while loop, the condition of a for
  statement is tested prior to executing the loop
  body
• The body of a for loop will execute zero or more
  times
• It is well suited for executing a loop a specific
  number of times that can be determined in advance
• Anything you can do in a for loop, you can do in
  a while loop

19
The for Loop

• Does not execute if initial condition is false
• Update expression changes value of loop control
  variable, eventually making it false
• If loop condition is always true, result is an
  infinite loop
• for statement ending in semicolon is empty does
  not affect program

for (count0 countltLIMIT count)
20
Examples
Hello Hello Hello Hello Hello
for (i1 ilt5 i) System.out.println
("Hello") System.out.println ("")
for (i1 ilt5 i) System.out.println
("Hello") System.out.println ("")
Hello Hello Hello Hello Hello
for (i1 ilt5 i) System.out.println ("")

21
Multiples.java Example

• Ask the user for a positive number
• Ask the user for a limit
• Print all of the multiples of the positive number
  up to limit
• What happens if the user enters a negative number?

22
In-Class Exercisefor vs. while Loops
Output 0 3 6 9 12 15 18

• final int MAX 20
• int i
• for (i 0 iltMAX i3)
• System.out.println (i)
• Predict the output
• Translate this to a while loop.

final int MAX 20 int i 0 while (i lt MAX)
System.out.println (i) i 3
23
The dowhile Loop

• Statements executed first, then expression
  evaluated
• Statement(s) executed at least once then
  continued if expression is true
• Basic Form

do statement(s) while (expression)
24
The do...while LoopSyntax
do loop body while ( condition )
The loop body is executed once initially, and
then the condition is evaluated
The loop body is executed repeatedly until the
condition becomes false
25
dowhile Loop(Post-test Loop)
do statement(s) while (expression)
26
The do...while Loop

• Like a while loop, but its condition is at the
  end of the loop
• Loop body always executes at least once
• Must also be checked for termination (not an
  infinite loop)
• Anything you can do with a do...while loop, you
  can do with a while loop

27
The do...while LoopExample

• final int LIMIT 3
• int count 0
• do
• System.out.println (count)
• count
• while (count lt LIMIT)
• System.out.println (All done!)

Output 0 1 2 All done!
initialization
loop body
update
boolean condition
28
Comparing while and do...while

• while Loop
• final int LIMIT3
• int count 0
• while (count lt LIMIT)
• System.out.println (count)
• count
• System.out.println
• (All done!)

• do...while Loop
• final int LIMIT3
• int count 0
• do
• System.out.println
• (count)
• count
• while (count lt LIMIT)
• System.out.println
• ("All done!")

29
while vs. do...while
i 11 while (i lt 10) System.out.print (i
" ") i 5 System.out.println()
blank line
i 11 do System.out.print (i " ") i
5 while (i lt 10) System.out.println()
11
30
In-Class ExerciseThe do...while Loop

• int x 0, y 0
• do
• System.out.println (xy)
• if (y lt x)
• y 2
• x
• while (x lt 5)
• Predict the output of the loop

• x y
• 0 0
• Output

2
1
4
2
3
4
5
0
0
4
6
16
31
break Statements

• Used to exit early from a loop
• Used to skip remainder of switch structure
• Can be placed within if statement of a loop
• If condition is met, loop exited immediately

32
break Example
int i for (i0 ilt5 i) System.out.print
("i ") if (i 2) break System.out.p
rintln (i)
i 0 i 1 i
33
continue Statements

• Used in while, for, and do...while structures
• When executed in a loop, the remaining statements
  in the loop are skipped proceeds with the next
  iteration of the loop
• When executed in a while/dowhile structure,
  expression evaluated immediately after continue
  statement
• In a for structure, the update statement is
  executed after the continue statement then the
  loop condition executes

34
continue Example
int i for (i0 ilt5 i) System.out.print
("i ") if (i 2) continue System.ou
t.println (i)
i 0 i 1 i i 3 i 4
int i0 while (ilt5) System.out.print ("i
") if (i 2) continue System.out.prin
tln (i) i
i 0 i 1 i i i ... (infinite loop)
35
Nested Control Structures

• Provides new power, subtlety, and complexity
• if, ifelse, and switch structures can be placed
  within while loops
• for loops can be found within other for loops
• each time through the outer loop, the inner loop
  goes through its full set of iterations

36
Nested Control Structures Example

• for (int row 1 row lt 5 row)
• for (int star 1 star lt row star)
• System.out.print()
• System.out.println()

Output

• Can't use the variable row outside the outer for
  loop.
• Can't use the variable star outside the inner
  for loop.

37
Flips.java Example

• Flip a coin 5 times
• choose a random number
• Print the total number of heads and tails

38
Loop Features
39
Choosing a Loop Structure

• while
• you don't know how many times to execute the loop
  body
• do...while
• you don't know how many times to execute the loop
  body, but it's at least once
• for
• you know exactly how many times to execute the
  loop body

40
Loops

• 4 parts of every loop
• initialization
• condition
• loop body
• update

• Order
• while and for
• initialization
• condition
• loop body
• update
• do...while
• initialization
• loop body
• update
• condition

41
Program 3

• Rock-Paper-Scissors-Spock-Lizard
• variation on Rock-Paper-Scissors
• Have the computer choose one of the 5 shapes.
• Hint Math.random
• Ask the user to choose one of the 5 shapes or 'q'
  to quit.
• Decide who wins.
• After the user quits, print out a tally of the
  user wins, losses, and ties.

42
Program 3

• You must print the title of the game.
• You must make sure the user knows what to enter
  to choose a particular shape.
• Allow the user to enter invalid input and prompt
  them for valid input.
• You do not have to use letters for selecting the
  shapes.
• Start early!

43
Program 3Break Program into Smaller Steps

• Print out welcome message, options, and
  directions to user
• Write a loop that allows the user to enter a
  character and end the loop if the user enters 'q'
  (or 'Q')
• Write code that will print out the shape (rock,
  paper, ...) based on what the user entered
• Write code that will make the computer choose a
  shape
• Write code that will print out what shape the
  computer chose

44
Program 3Break Program into Smaller Steps

• Write code that decides if the computer or the
  user won
• Write code that will keep up with the number of
  wins, losses, and ties for the user
• Write code that will print out the number of
  wins, losses, and ties after the user chooses to
  quit

45
Math.random Example

• Print a random number in -1, 0, 1.

double randomNum int sample012 //
Math.random() will return 0.0, 1.0) randomNum
Math.random() // sample012 will be 0, 1,
2 sample012 (int) (randomNum 3) //
print out -1, 0, 1 System.out.println(sample01
2 - 1)
System.out.println ((int) (Math.random() 3) -
1)
46
In-Class ExerciseRandom

• Write a code fragment to choose a random number
  in 0, 1, 2, 3.
• Print 'A' if the number chosen is 0, 'B' if the
  number is 1, 'C' if the number is 2, and 'D' if
  the number is 3.
• Hint a switch statement would be appropriate here

sample (int) (Math.random() 4)
47

• int sample
• // sample will be 0, 1, 2, 3
• sample (int) (Math.random() 4)
• switch (sample)
• case 0
• System.out.println("A")
• break
• case 1
• System.out.println("B")
• break
• case 2
• System.out.println("C")
• break
• case 3
• System.out.println("D")
• break

48
Next Time in COMP 14

• Programming Session
• Please bring your laptop