1. Intro to Programming & Algorithm Design
4/23/2017
Intro to Programming & Algorithm Design
Loops
NG Assg
Labs
This presentation can be viewed on line in a file named:
ch05.IntrotoProg.ppt
Copyright 2003 by Janson Industries

2. 4/23/2017
Objectives
Explain
Advantages of loops
Different types of loops
Common loop mistakes
Show how to implement the various loop types in Java

3. 4/23/2017
Why Loops?
If a set of instructions needs to be repeated many times, best to use a loop rather than repeating the instructions
Less coding
Fewer mistakes
For example, to keep executing with an unknown number of data records, you can't do it without a loop

4. Why Loops? Example Here's pseudocode for a program to double a number
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Here's pseudocode for a program to double a number
To double another number we have to rerun the program
For example...
Module main
Declare Integer numToDouble, result
Display “Enter number to double and press Enter”
Input numToDouble
result = numToDouble * 2
Display “2 X ”, numToDouble, “ = ”, result
End Module

5. To double another number, have to issue the java command again
We want program to just ask for another number...

6. 4/23/2017
Why Loops? Example
With a loop, we could have the instructions run again
And again, and again, and....
Module main
Declare Integer numToDouble, result
BeginLoop
Display “Enter number to double and press Enter”
Input numToDouble
result = numToDouble * 2
Display “2 X ”, numToDouble, “ = ”, result
EndLoop
End Module

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Why Loops?
This is why loops have to check a control variable to see if they should process the statements again
How and when the check is done is the difference between the different types of loops
Condition-controlled
Count-controlled

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Why Loops?
Condition-controlled loops use a true/false condition to determine how many times to loop
While executes when condition true
Do While executes when condition true
Do Until executes when condition false
Count-controlled loops execute a specific number of times
For

9. 4/23/2017
Why Loops?
Condition-controlled loops also differentiated by when the condition is checked
While is a pre-test loop
Do While is a post-test loop
Do Until is a post-test loop

10. Loops While loop – pre-test
Checks the condition first and only executes if condition true
: : :
While hair = “dirty”
Lather
Rinse
Display "Is hair dirty or clean"
Input hair
End While

11. Loops
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While
Condition check done first
If check true, executes statements
Lather and rinse will not be executed if condition is false
Input…
Display…
Is hair = “dirty”
True
Lather
Rinse
False
False

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Loops
In While, Do While, and Do Until loops, the programmer explicitly manipulates the control variable(s)
In java, while loop syntax is
Keyword while
Condition (in parenthesis)
Statements to execute (in braces)
while (condition) {
statements to execute }

13. Simple pre-test loop example
Notice loop runs 4 times even though check is "counter < 4"
Control variable (counter) defined outside of loop and manipulated inside loop

14. Loops Do While – post-test Do Until – post-test
Checks condition at end and executes if condition true
Do Until – post-test
Checks condition at end and executes if condition false
Loops Do
: :
While hair = “dirty” Do
: :
Until hair = “clean”

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Loops
Do While condition test done at end of loop
Lather and rinse will always be executed at least once whether the condition is true or not but only continues if true
Lather
Rinse
Is hair = “dirty”
True
False

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Loops
Do Until
Condition test done at end of loop
Lather and rinse will always be executed at least once whether the condition is true or not but only continues if false
Lather
Rinse
Is hair = “clean”
False
True

17. Do While Loops In java, do while loop syntax is Keyword do
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Do While Loops
In java, do while loop syntax is
Keyword do
Statements to execute (in braces)
Keyword while
Condition (in parenthesis)
do {
statements to execute
} while(condition)

18. Answer: if numberOfTimesToLoop = 0
Simple post-test loop example - Notice loop runs 4 times (just like pretest)
What condition(s) would result in different output for the different loop types?
Answer: if numberOfTimesToLoop = 0

19. Do Until Loops In java, there is no Do Until
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Do Until Loops
In java, there is no Do Until
Closest you can come to a Do Until is use a Do While and negate the condition
do {
statements to execute
} while(!condition)

20. For Loop A little complicated
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For Loop
A little complicated
Does a lot of stuff very compactly
Most for statements allow the programmer to specify
A loop control variable
An initial value for the variable
An evaluation condition
A Boolean expression with the loop control variable
A variable update value

21. For Loop Pseudo code example: ctr = 1 to 3 step 1 Declare Integer ctr
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Pseudo code example:
Declare Integer ctr
For ctr = 1 to 3 step 1
statements to be repeated
End For
Defines the loop control variable ctr
ctr = 1 to 3
Initializes ctr to 1 and defines the max value for ctr
step 1
defines the increment value as 1

22. For Loop If you don’t specify a step value the default is 1
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For Loop
If you don’t specify a step value the default is 1
Declare Integer ctr
For ctr = 1 to 2
Lather
Rinse
End For

23. For Loop Executes a defined number of times In this case twice
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For Loop
Executes a defined number of times
In this case twice
Also called a definite loop, counted loop, counter controlled loop
for ctr
=1 to 2
True
Lather
Rinse
False
False

24. Java For Loop The for keyword Then in parenthesis
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The for keyword
Then in parenthesis
Loop control variable definition (optional) and initialization
Semicolon
The loop condition evaluation
The step increment definition
Then in braces the statements to be repeated
for (int ctr = 1; ctr < 3; ctr = ctr + 1) { statements to be repeated }

25. Java For Loop Example int ctr = 1 ctr < 3 ctr = ctr + 1
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Example
for (int ctr = 1; ctr < 3; ctr = ctr + 1) {
statements to be repeated }
int ctr = 1
Creates the variable ctr
Initializes ctr to 1
ctr < 3
Defines the condition to check
ctr = ctr + 1
Defines the increment as 1

26. For Loop If the loop is going to execute a fixed number of times
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For Loop
If the loop is going to execute a fixed number of times
For loop easier than while or do while
Instead of statements to
Define the control variable
Define the limit
Increment the control variable
And the do or do while keywords
You use the for statement to do all those things

27. Notice how much less code is needed

28. For Loop To truly understand the For loop, you have to understand
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For Loop
To truly understand the For loop, you have to understand
The generally accepted interpretation of the pseudocode and/or flowchart text
When all the "pieces" of the for statement are performed

29. 4/23/2017
For Loop
If the pseudocode had specified 0 to 99 (0 and 99 are inclusive)
It means the statements should be executed 100 times
Once when it was 0
Once when it was 1
Etc, etc, etc
Once when it was 99

30. 4/23/2017
For Loop
To get the for loop to work the same way with java, could specify any of these
int ctr = 0; ctr <100; ctr = ctr + 1
int ctr = 0; ctr <= 99; ctr = ctr + 1
int ctr = 1; ctr <101; ctr = ctr + 1
int ctr = 1; ctr <= 100; ctr = ctr + 1

31. For Loop Why? Because the for does "int ctr = 1" first and only once
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For Loop
Why? Because the for does
"int ctr = 1" first and only once
Then repeatedly
The condition is checked
If condition is true
Statements are executed
Ctr is incremented by one
When the condition is false
The statement after the End For is executed

32. For Loop int ctr = 1 Is ctr < 3 Perform Statements ctr = ctr +1
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For Loop
int ctr = 1
Is ctr
< 3
Perform
Statements
ctr = ctr +1
True
False

33. For Loop In the pseudocode 1 to 2 means
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In the pseudocode 1 to 2 means
Do the statements while the control variable is equal to 1 or 2
This means
The variable is initially set to 1
Condition is checked
The statements are performed
The variable is incremented to 2
The variable is incremented to 3
The statement after the End For is performed

34. Sentinel Values
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Sometimes you want a loop to execute until a user enters a value the means “stop the loop”
Called a sentinel value
While or Do While best for this
Declare Integer userInput, result
Input userInput
While (userInput > 0)
result = userInput * 2
Display result
End While

35. Back To Our Original Problem
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Back To Our Original Problem
Want to run doubling app until user indicates to stop
In addition, we want to add non-logical (aka physical) requirements to design
User interface
Formatting

36. Back To Our Original Problem
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Back To Our Original Problem
External Design (XD)
Enter number you would like to double and press Enter. 4
2 x 4 = 8 -1
OK, you entered a value <= 0, ending execution

37. Final flow chart and pseudocode

38. 4/23/2017
Notice design does not worry about peculiarities of the programming language

39. Create a Raptor FC and run to verify we have the correct logic
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Create a Raptor FC and run to verify we have the correct logic

40. Final java class looks like this
import java.io.IOException;
import java.util.Scanner;
public class DoubleWithWhile {
public static void main(String[] args) {
// Variable and object needed to read from command line
Scanner keyboard = new Scanner(System.in);
// Variables needed to hold input, intermediate values, and results
int numToDouble = 0;
int result = 0;
// Prints out a blank line and instruction
System.out.println(" ");
System.out.print("Enter number would you like to double and press Enter. ");
// Priming read
numToDouble = keyboard.nextInt();
// Start of loop and check for sentinel value
while (numToDouble > 0){
// Calculates and displays result
result = numToDouble * 2;
System.out.println("2 X " + numToDouble + " = " + result);
4/23/2017
Final java class looks like this

41. // Prints out a blank line and instruction System.out.println(" ");
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// Prints out a blank line and instruction
System.out.println(" ");
System.out.print("Enter number would you like to double and press Enter. ");
// Reads next user input
numToDouble = keyboard.nextInt();
// End of loop }
// Quitting message
System.out.print("OK, you entered a value <= 0, ending execution. ");
// End of method
// End of class

42. With a loop, problem solved

43. Loops Lots of reasons to use loops Accumulating Data validation
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Lots of reasons to use loops
Accumulating
A shopping cart of items
Totals
Data validation
Declare Integer empAge
Display "Enter employee age"
Input empAge
While (empAge > 95 OR empAge < 16)
Display "Enter a valid age between 16 & 95"
End While

44. Totaling in a Loop Module main Declare Real cost, totalCost
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Module main
Declare Real cost, totalCost
Display “Enter first item cost"
Input cost
While (cost > 0)
totalCost = totalCost +cost
Display “Enter next item cost or zero to end"
End While
Display “The total cost for all items is $“ ,
totalCost
End Module

45. Can keep count of how many items are entered and display
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Module main
Declare Real cost, totalCost
Declare Integer counter = 0
Display “Enter first item cost"
Input cost
While (cost > 0)
counter = counter + 1
totalCost = totalCost +cost
Display “Enter next item cost or zero to end"
End While
Display “The total cost for the “ , counter
, “ item(s) is $“ , totalCost
End Module
Can keep count of how many items are entered and display

46. Nested Loops Loops within loops
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Nested Loops
Loops within loops
Example, a report that reads and prints a file's contents and has the following format:
Report header
For each page
A page header with page number
For next 33 items
Print line item
A page footer with page number
Report footer

47. Exercise XD Report will look like Report Header Page Header on Page #
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Report Header
Exercise XD
Report will look like
Page Header on Page #
##################################################
Page Footer on Page #

48. Exercise XD Can be a variable number of pages Report Footer
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Page Header on Page #
##################################################
Page Footer on Page #
Exercise XD
Can be a variable number of pages
Report Footer

49. 4/23/2017
Nested Loops
When a file is read, system returns a boolean value to indicate if there is anymore data
Called an "end of file" indicator
Indicator abbreviated as eof
eof has a boolean value of true or false
Can be used to control looping

50. Nested Loops Inner Loop Outer Loop Module main
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Nested Loops
Module main
Declare Integer pageCtr = 1, lineCtr = 0
Print Report Header and Skip to next page
pageCtr = pageCtr + 1
Read record From file
While (not eof)
Print Page Header and pageCtr
While (lineCtr < 33 AND not eof)
Print item From record
lineCtr = lineCtr + 1
End While
lineCtr = 0
Print Page Footer and pageCtr
Skip to next page
Print Report Footer
End Module
Inner
Loop
Outer
Loop

51. Exercise Design (SFC) an application that:
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Design (SFC) an application that:
Gets customer account data from a file that has account number, customer name, and balance due
For each customer:
Print the account number and name
Then for each month for the next 10 months
Print the customer’s projected balance
Assume:
No finance or interest charge
Customer makes no new purchases
Customer pays off the balance with equal monthly payments of 10 percent of the original balance due

52. Algorithm 1. Read first record from the file 2. While not eof
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Algorithm
1. Read first record from the file
2. While not eof
a. Display account number, customer name
b. Then 10 times
i. Subtract 10% from balance due
ii. Display balance due
c. Read next record from the file

53. Exercise XD Output will look like… Account Number: #######
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Output will look like…
Account Number: #######
Customer: Xxxxx Xxxxxx
Balance over next 10 months
########
Account Number: ########

54. Exercise What's input and what variables will you need to hold them?
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Exercise
What's input and what variables will you need to hold them?
acctNum for Account number
custName for Customer name
balanceDue for Balance due
Based on calculation what variables will you need to hold intermediary and results?
paymentAmt for 10 percent of the balance

55. Exercise So in flow chart, what's first? Then what?
Variable definition
Then what?
Priming read
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56. Exercise
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Then what?
While that checks not eof

57. Exercise
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Then what?
Print headers

58. Exercise
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Then what?
Calc paymentAmt

59. Exercise Then what? For loop that runs 10 times to print balances
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Then what?
For loop that runs 10 times to print balances
For loop is nested within the while loop

60. Exercise
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Then what?
Calc balanceDue and print it out

61. Exercise What happens when last balance printed? Blank line
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Exercise
What happens when last balance printed?
Blank line
Read next record
Check for end of file

63. Having an incorrect comparison
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Loop Gotchas
Having an incorrect comparison

64. 4/23/2017
Loop Gotchas
Not initializing the control variable – in java, should always initialize

65. Not changing the control variable
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Loop Gotchas
Not changing the control variable

66. Loop Gotchas The last example resulted in an infinite loop
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Loop Gotchas
The last example resulted in an infinite loop
The condition is never false
Loop statements continually executed
Can also happen because of a bad/trivial condition (always true)
1 == 1
End program execution by pressing and holding Ctrl key and then pressing C key (Crtl + C)

67. 4/23/2017
Loop Exercise Assg 1
Program will calculate the number of bills needed for a dollar amount (until 0 entered)
For instance,
Program prompts user
What is the dollar amount?
User enters 58
Program displays
2 twenty(ies), 1 ten(s), 1 five(s) and 3 one(s)
20 is the largest denomination, no 2 dollar bills.

68. Loop Exercise Attack this in two parts Design everything else first
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Loop Exercise
Attack this in two parts
How to calculate the bills
Everything else
Design everything else first
Answer 1

69. Bills Exercise Continued
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Bills Exercise Continued
How to figure out the number of bills a little trickier
First: you need variables to keep track of the number of bills for each denomination (bill counters)
Couple of ways to do calc
if value > than largest denomination
Add one to that bill counter
Subtract denomination from amount and check again
else if value > second largest

70. Bills Exercise Continued
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Bills Exercise Continued
So for example, if 58 (dollars) entered
Check if dollars is bigger than 20
Yes
Add one to 20 counter
Subtract 20 from dollars, check again No
if value > than 2nd largest denomination
Add one to that bill counter
Subtract denomination from dollars and check again
else if ….
Answer 2

71. Bills Exercise Continued
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Bills Exercise Continued
Alternatively
Divide the amount by the biggest denomination
twentiesCtr = dollars/20
This assumes dolls is an integer so that the decimal remainder will be truncated
Calc the remainder
dollars = dollars – (twentiesCtr * 20)
Divide the amount by the 2nd biggest denomination
etc.
Alt Answer

72. Bills Exercise Continued
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Bills Exercise Continued
Let’s prove that it works by creating a Raptor FC
Answer 3

73. 4/23/2017
Non-graded Assg 1
Using nested loops, write a program to display the outer loops counter value as follows
Don’t use math to generate numbers like 4,444 or 55,555
Send file, topic Ch05Assg1
Need a hint? 1 22
333
4444
55555

74. Points to Remember Loops are control structures that repeat actions
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Loops are control structures that repeat actions
Makes computer programs more efficient
Four general types
While
DoWhile
DoUntil
For
Loops characterized as pretest or posttest

75. Points to Remember All require Can be nested Beware the infinite loop
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Points to Remember
All require
A loop control variable
Which must be initialized and updated
A condition that tests the control variable against a sentinel value
Can be nested
Beware the infinite loop

76. Assignments Non-Graded Graded Ch05Assg1 Chap 5 labs 4.1-4.4
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Assignments
Non-Graded
Ch05Assg1
Chap 5 labs
Graded
Chap 5 lab 4.5