Loops

1
Loops

• Chapter 3

2
In this chapter

• Flowcharts
• The difference between pre test and post test
  loops
• How to use relational operators in loop
  conditions
• Constructing counter controlled loops
• Constructing sentinel controlled loops
• Some applications of loops, including data input
  and validation, and computing sums and averages
• Using nested loops

3
flowcharts

• A flowchart is a diagram that uses special
  symbols to display pictorially the flow of
  execution within a program or program module.

4
Loop

• A loop is a repetition structure. A loop repeats
  a segment of code. The number of times the code
  segment is repeated is dependent on the exit
  condition. The exit condition can be at the
  beginning or end of the loop, dependant on the
  logic of how you want the loop to operate in
  relation to the exit condition. We will write
  three types of loops, the while, do-while, and
  for loop.

5
while loop

• int x 0
• while(xlt10) /exit condition/
• coutltltJim is my hero.ltltendl
• xx1 /counter adds to the value of x/
• The while loop is an all purpose loop. The exit
  condition is at the beginning, making this a
  pre-test loop. This loop is a counter controlled
  loop. If the counter (xx1)is missing the loop
  runs indefinitely because the value of x is never
  changed so the exit condition is never false.

6
do-while loop

• int x0
• do
• coutltltJim is my hero.ltltendl
• xx1/counteradds to the value of x/
• while(xlt10)
• The do while loop is a post test loop because the
  exit condition is at the end. Notice the
  semicolon after the exit condition. This is the
  only loop that is written with a semicolon. This
  is also a counter controlled loop. A do while
  loop will always run at least one time. This loop
  works well with menus.

7
for loop

• int i0
• for(i0ilt10i)
• coutltltJim is my hero.ltltendl
• /end for loop/
• The for loop is another counter controlled loop.
  Inside the parenthesis the i0 part gives the
  variable i an initial value of 0 The ilt10 part
  is the exit condition The i part is shorthand
  for ii1. This is called incrementing the
  variable. The variable is incremented at the
  closing curly brace. are curly braces.

8
sentinel controlled loop

• char passy
• while (passy)/ is a test for the same
  as/
• coutltltJim is my hero.ltltendl
• coutltltEnter y to continue or any key to
  end.ltltendl
• cingtgtpass)/store response from user/
• This loop repeats as long as the user enters the
  letter y. As soon as another letter is entered
  the loop ends.

9
do-while with sentinel value

• char passx
• do
• coutltltJim is my hero.ltltendl
• coutltltEnter y to continue or any key to
  end.ltltendl
• cingtgtpass)/store response from user/
• while (passy)
• The exit condition is at the end so this loop is
  guaranteed to run at least once. The loop is
  continued or ended by the actions of the user.

10
exit condition

• The exit conditions determine when a loop stops.
  There are six basic relational operators that we
  use in the exit condition
• means the same as -- while(passy)
• lt means less than -- while(xlt5)
• gt means greater than -- while(xgt0)
• ! means not equal to -- while(pass !q)
• lt means less than or equal to -- while(xlt5)
• gt means greater than or equal to -- while(xgt0)

11
nested for loops

• /The first column you see in the output is
  produced by the outer for loop.All
• other numbers come from the inner loop./
• includeltiostreamgt
• includeltstringgt
• using namespace std
• main()
• int i0
• int j0
• for(i0ilt10i)
• printf("d ",i)
• for(j0jlt5j)
• printf("d ",j)
• / end for(j/
• coutltltendl
• /end for(i/
• return 0

12
data validation

• We can validate the data by using it in our exit
  condition. What if we wrote a program that
  calculates the sum of an undetermined amount of
  numbers?

13
if statement nested in a loop

• includeltiostreamgt
• using namespace std
• main()
• int sum0
• int num0
• char pass'y'
• do
• if(numlt0)
• coutltlt"Enter a positive number to be
  summed."ltltendl
• cingtgtnum
• coutltlt"You entered " ltltnumltltendl
• if(numlt0)
• coutltlt"This number will not be added to the
  total"ltltendl
• num0

14
Output from previous slide