Loops continued

1
Programming

• Loops (continued)

2
The for Statement

• Syntax
• for (ForInit ForExpression PostExpression)
• Action
• How it works
• Execute ForInit statement
• While ForExpression is true
• Execute Action
• Execute PostExpression
• Example
• int i
• for(i1 ilt20 i)
• cout ltlt "i is " ltlt i ltlt endl

3
Iteration Using the for Statement
ForInit
ForExpression
true
false
Action
PostExpression
4
N! (for)

• int number, factorial, n
• cout ltlt "Enter number "
• cin gtgt number
• factorial 1
• for(n1 nltnumber n)
• factorial n
• cout ltlt "The factorial of " ltlt number
• ltlt " is " ltlt factorial ltlt endl

5
2N (for)

• int number, result, n
• cout ltlt "Enter number "
• cin gtgt number
• result 1
• for(n1 nltnumber n)
• result 2
• cout ltlt "Two raised to the " ltlt number
• ltlt " power is " ltlt result ltlt endl

6
Iteration

• Key Points
• stopping criterion Make sure there is a
  statement that will eventually stop the loop
• initializationMake sure to initialize loop
  counters correctly
• side effect Loop counters should not be
  modified in the loop

7
The Lazy Student Problem

• Four students working together on HW1
• Problem will take 200 hours of work that can be
  done over the next few weeks
• The laziest student convinces the other three
  students to draw straws
• Each straw is marked with an amount
• The amount represents both the number of days and
  the numbers of hours per day that the student
  would work
• Example
• If the straw was marked three then the student
  who drew it would work for three hours per day
  for three days
• What are the best markings of the straws for a
  clever, lazy student?

8
Observations

• Need to find sets of numbers a, b, c, and d such
  that
• a2 b2 c2 d2 200
• Maximal legal number is 14 as 152 equals 225
• which is greater than 200
• Minimal legal number is 1
• No advantage to listing the combinations more
  than once
• Implication
• Generate the solutions systematically
• We will make sure that a lt b lt c lt d

9
Method

• Generate all possibilities for a where for each a
  possibility
• Generate all possibilities of b where for each b
  possibility
• Generate all possibilities for c where for each c
  possibility
• Generate all possibilities for d where for each d
  possibility
• Determine whether the current combination is a
  solution

10
Nested For Loop Solution

• int a, b, c, d
• for(a1 alt14 a)
• for(ba blt14 b)
• for(cb clt14 c)
• for(dc dlt14 d)
• if(aa bb cc dd 200)
• cout ltlt a ltlt " " ltlt b ltlt " " ltlt c
• ltlt " " ltlt d ltlt endl

11
The Do-While Statement

• Syntax
• do Action
• while (Expression)
• How it works
• Execute Action
• if Expression is true then execute Action again
• Repeat this process until Expression evaluates to
  false
• Action is either a single statement or a group of
  statements within braces

Action
true
Expression
false
12
N! (do-while)

• int number, factorial, n
• cout ltlt "Enter number "
• cin gtgt number
• factorial 1
• n 1
• do
• factorial n
• n
• while(n lt number)
• cout ltlt "The factorial of " ltlt number
• ltlt " is " ltlt factorial ltlt endl

13
2N (do-while)

• int number, result, n
• cout ltlt "Enter number "
• cin gtgt number
• result 1
• n 1
• do
• result 2
• n
• while (n lt number)
• cout ltlt "Two raised to the " ltlt number
• ltlt " power is " ltlt result ltlt endl

14
Maximum (do-while)

• int value //input value
• int max0 //maximum value
• do
• cout ltlt "Enter a value (-1 to stop) "
• cin gtgt value
• if(value gt max)
• max value
• while(value!-1)
• cout ltlt "The maximum value found is "
• ltlt " is " ltlt max ltlt endl

15
Waiting for a Reply

• char reply
• do
• cout ltlt "Continue(y/n) "
• cin gtgt reply
• //do something
• while(reply!'n')

16
Which Loop to Use?

• For loop
• Best for calculations that are repeated a fixed
  number of times using a value that is changed by
  an equal amount (usually 1) each time through the
  loop.
• While loop
• You want to repeat a segment of code without
  knowing exactly how many times it will be
  repeated.
• You are working with user input
• There are situations when the code segment should
  not be executed at all.
• Do-while loop
• The code segment should always be executed at
  least once.
• Otherwise, the situations when do-while loops are
  used are very similar to those when while loops
  are used.

17
A simpler lazy student problem

• three straws a, b, c
• minimum value 1
• maximum value 3
• a lt b lt c
• 1 1 1
• 1 1 2
• 1 1 3
• 1 2 2
• 1 2 3
• 1 3 3
• 2 2 2
• 2 2 3
• 2 3 3
• 3 3 3

• for (a1 alt3 a)
• for (ba blt3 b)
• for (cb blt3 c)
• cout ltlt a ltlt b ltlt c ltlt endl

18
How to Stop a Loop

• Known number of iterations before the loop stops
  (for)
• Test for a user-controlled condition before or
  after each iteration (while, do-while)
• Use the break command.

19
break

• The break command is the same as the one used
  previously in switch.
• break leaves the current loop immediately. It is
  recommended that break be used for situations
  where the loop needs to be terminated immediately
  (e.g., due to user intervention or if a fatal
  error occurs).

20
Maximum (while with break)

• int value0 //input value
• int max0 //maximum value
• while(true)
• cout ltlt "Enter a value (-1 to stop) "
• cin gtgt value
• if(value gt max)
• max value
• if(value-1)
• break
• cout ltlt "The maximum value found is "
• ltlt " is " ltlt max ltlt endl

• MAX

21
Common Loop Errors

• while(balance ! 0.0)
• balance balance - amount
• This will lead to an infinite loop!
• for(n1 nltcount n)
• cout ltlt "hello" ltlt endl
• "hello" only printed once!

22
Common Loop Errors (with correct indentation and
curly braces)

• while(balance ! 0.0)
• // do nothing infinitely !!
• balance balance - amount
• This will lead to an infinite loop!
• for(n1 nltcount n)
• // do nothing n times!!
• cout ltlt "hello" ltlt endl
• "hello" only printed once!

23
Common Loop Errors

• while(balance ! 0.0)
• balance balance - amount
• balance may not become equal zero due to
  numerical inaccuracies
• int power
• while(power lt 1000)
• cout ltlt "Next power of N is " ltlt power ltlt endl
• power n
• Be sure to initialize to 0 a variable used for
  sums
• Be sure to initialize to 1 a variable used for
  products

24
Nested Loops

• Nested loops are loops within loops. They are
  similar in principle to nested if and if-else
  statements.
• Many applications require nested loops.

25
Nested Loops

• // Find the average score on 8 lab assignments
• int n, lastlab8
• double avg, score, tscore
• char resp
• do
• tscore 0
• for(n1 nltlastlab n)
• cout ltlt "Enter students score for lab " ltlt n
  ltlt " "
• cin gtgt score
• tscore score
• avg tscore/double(lastlab)
• cout ltlt "The average score is " ltlt avg ltlt endl
• cout ltlt "Enter another student (y/n)? "
• cin gtgt resp
• while(resp'y' resp'Y')

26
Diamond Pattern

• Print out the following diamond pattern

27
Diamond Pattern

• Subproblem
• print out the upper half
• print out the lower half
• Print out upper half
• row 1 print 4 spaces, 1 star
• row 2 print 3 spaces, 3 stars
• row 3 print 2 spaces, 5 stars
• row 4 print 1 space, 7 stars
• row 5 print 0 spaces, 9 stars
• Algorithm Refinement
• row 1 print (5-row) spaces, (2row - 1) stars
• row 2 print (5-row) spaces, (2row - 1) stars
• row 3 print (5-row) spaces, (2row - 1) stars
• row 4 print (5-row) spaces, (2row - 1) stars
• row 5 print (5-row) spaces, (2row - 1) stars

28
Diamond Pattern

• int row, space, star // loop counters
• for(row1 rowlt5 row) //top half
• for(space1 spacelt5-row space)
• cout ltlt " "
• for(star1 starlt2row-1 star)
• cout ltlt ""
• cout ltlt endl
• for(row4 rowgt1 row--) //bottom half
• for(space1 spacelt5-row space)
• cout ltlt " "
• for(star1 starlt2row-1 star)
• cout ltlt ""
• cout ltlt endl

29
Multiplication Table

• // Program to output the
• // multiplication table
• int i //Outer loop counter
• int j //Inner loop counter
• for(i1 ilt10 i)
• for(j1 jlt10 j)
• cout ltlt ij ltlt " "
• cout ltlt endl