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Loops continued

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cout 'Two raised to the ' number ' power is ' result ... Implication. Generate the solutions systematically. We will make sure that a = b = c = d ... – PowerPoint PPT presentation

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Title: 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)