COP 3035
Lecture File 04
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Iteration: Looping
–
–
–
–
Using while loops
Major loop types and major loop tasks
Multiple Termination Conditions
Simulation/Modeling and introduction to
using pseudo-random numbers
Copyright Ann Ford Tyson
1
The Four Control Structures

FLOW OF CONTROL
– Sequential
one after another, in order
– Selection or Branching (if, if-else, if-elif)
can choose between different statements
– Iteration or Looping (while, for)
repeat instructions over and over
– Subprogram (function)
perform a task when called
2
Iteration

Iteration (looping): control structure
which allows statements to execute
repeatedly

Iteration statements in Python
– while
– for
3
while loop
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

most general, can implement ANY form
of loop structure needed
SYNTAX
true or false
while <expression> :
<stmt block>
loop body
ACTION
while the expression is true, execute the
statement block
4
Flow of control: while
false
evaluate
expression
true
execute loop body
EXIT
5
while characteristics p.1

what would happen if the expression
was false when the loop was first
encountered?
6
while characteristics p.1a

what would happen if the expression was
false when the loop was first encountered?
the loop body would never execute
7
while characteristics p.2

what would happen if the expression
never became false ?
8
while characteristics p.2a

what would happen if the expression
never became false ?
"infinite loop"
9
infinite loop characteristics
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
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may see same output over and over
may see computer seem to "hang"
to stop (Windows PC):
– click on the usual X box
– control-alt-delete
– may need to restart
10
Did you hear about the computer
scientist found dead in the
shower?
Sure! They thought he had been dead for days. And in his hand, he
was clutching a shampoo bottle with these directions:
lather
rinse
repeat
Is that an
algorithm?
11
Overview of Major Loop Types
loops
count-controlled
iteration counter
sentinel
event-controlled
flag
12
Overview of Major Loop Tasks
loops
counting
event counter
summing
running total
13
first loop example
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sentinel-controlled loop
executes until a particular value is read
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event-controlled loop
(more general term)
executes until some event occurs
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first example: sentinel-controlled, eventcontrolled, sentinel is 'Y' or 'N'
14
while: sentinel-controlled
# ask user for Y or N and recover from
# data entry errors
# first do a priming read
response = input ("Enter Y or N -> ")
while (response != 'Y' and response != 'N'):
response = input ("Please try again. Enter Y or N -> ")
15
first example: questions
How many times does the loop body
execute?
zero or as many as needed
What is the value of response after the
loop exits?
complement of expr must be true, hence
(response == 'Y') or (response == 'N')
16
first example: what if questions p.1
what if we changed the and to or ?
17
first example: what if questions p.1a
what if we changed the and to or ?
infinite loop
18
first example: what if questions p.2
what if we forgot the priming read?
19
first example: what if questions p.2a
what if we forgot the priming read?
response would be uninitialized when
the test expression is encountered
20
second loop example
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count-controlled loop
executes a specified number of times
a counter controls the number of iterations

example: print the numbers from 1 through
100 and their squares, as in the next table

notice in the code:
– loop body executes 100 times
– count is 101 when loop exited
21
second loop example p.2

Number
======
1
2
3
4
. . .
98
99
100
Square
======
1
4
9
16
. . .
9604
9801
10000
# 5 blanks
# between
22
while: count-controlled
# print table heading once
print ("Number Square")
print ("====== ======")
count = 1
# initialize counter
while count <= 100:
square = count ** 2
print ("%5d%11d" % (count, square))
count = count + 1
# update counter
23
Now, let's look at the program

In lecture we will now switch to IDLE
and examine this program running

This program is available from the class
web site
countContrLoop.py
24
second example: what if questions p.1
what if we forget to increment
the counter inside the loop?
25
second example: what if questions p.1a
what if we forget to increment the counter
inside the loop?
infinite loop, count stays 1
26
second example: what if questions p.2
what if we used "count < 100"
instead?
27
second example: what if questions p.2a
what if we used "count < 100" instead?
OFF BY ONE error; through 99 only
28
Loops: two major semantic
(logic) errors to watch for !

Infinite Loop
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Off by One
29
More Assignment Operators
<variable>
<op>=
<expr>
interpreter expands to
<variable> = <variable> <op> <expr>
where <op>= is
+=
-=
*=
/=
//=
%=
30
More Assignment Examples

i=1
j=3

i += 1

i *= j

j *= i + 1
# execute in sequence
translation: i = i + 1
effect: i  2
translation: i = i * j
effect: i  6
translation: j = j * (i + 1)
effect: j  21
31
Nested Loop Example:
flag-controlled + sentinel-controlled
# ask user to enter a Fahrenheit
# temperature, convert it to Celcius, then
# repeat as long as user wishes
fahrenTemp = 0
celTemp = 0
response = ""
keepGoing = True
# create variables needed
32
Nested Loop Example p.2
# assume user wants to convert at least
# one temperature
while keepGoing:
# get a temp and convert it
fahrenTemp = int(input("Enter a temperature in Fahrenheit -> "))
celTemp = int (5.0 / 9.0 * (fahrenTemp - 32))
33
Nested Loop Example p.3
print ("In Celcius that is ", celTemp)
# ask if user wants to convert again
print ()
response = input ("Do Again? Enter Y or N -> ")
while (response != 'Y' and response != 'N'):
response = input ("Please try again." +
" Enter Y or N -> ")
# reset flag
keepGoing = (response == 'Y')
# end of while (keepGoing) loop
34
Summing and Counting Loops
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Task: read scores from user and
calculate mean score, where
mean =
sum of scores / number of scores
Example data (mean == 443/5 == 88)
77
95
82
89
100
-1
88.6 as a float
# sentinel, signal to stop
35
Fundamental Technique: keep a
cumulative total (running sum)
let sum store running total
let value be what is read
general summing loop pseudocode
set sum to zero
while ( have more values )
read a value
add it to sum
36
Fundamental Technique: keep a
cumulative count (running count)
let count store running count
let value be what is read
general counting loop pseudocode
set count to zero
while ( have more values )
read a value
add 1 to count
37
Program example (p. 1)
# create constants and variables
QUIT_CODE = -1
score = 0
numScores = 0
sum = 0
mean = 0
38
Program example (p. 2)
# priming read
score = int(input ("Enter a score (-1 to quit): "))
while score != QUIT_CODE:
sum = sum + score
numScores += 1
score = int(input ("Enter a score: "))
# calculate mean and print
mean = sum / numScores
print ("Mean is: ", mean)
39
What if we used this loop instead?
# skip priming read and rearrange
while ( score != QUIT_CODE) :
score = int(input ("Enter a score: "))
sum = sum + score
numScores += 1
sentinel would be used as the last score
sum and mean would be wrong
40
Finding the maximum and
minimum in a data set
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Why? Statistics, image processing
(error detection, smoothing, etc.), game
list of high scores, etc.
Task: Read integers from user which
represent the number of yards a football
player has gained in the last 5 games
he has played
Find the highest and lowest yardage
(ESPN will pay for this program!)
41
More about the task
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Example data
87
55
194
210
7
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We can look at a short list and see max (210)
and min (7). How does a computer do this?
42
Algorithm
set min to 1st value
set max to 1st value
while ( have more values )
read a new value
compare it to min, and reset min
if needed
compare it to max, and reset max
if needed
43
Max/Min example (p. 1)
# create constants and variables
NUM_GAMES = 5
yards = 0
max = 0
min = 0
count = 1
44
Max/Min example (p. 2)
# read 1st value & initialize min & max
yards = int(input("Enter a yardage: "))
min = yards
max = yards
45
Max/Min example (p. 3)
while ( count < NUM_GAMES ) :
# process next value
how many
iterations?
yards = int(input("Enter a yardage: "))
if ( yards < min ) :
min = yards
elif ( yards > max ) :
max = yards
why?
count += 1
46
Max/Min example ( p. 4)
# output final results
print ("Lowest was: ", min)
print ("Highest was: ", max)
print ("Now send bill to ESPN!")
47
Multiple Termination Conditions
and Simulation
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Multiple Termination Conditions
– while ((some condition is true) and
(a second condition is true))
– alternatives: more conditions, using
combinations of and, or, not, etc.
48
Loop Exercise
Multiple Termination Conditions
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We will now switch over to this exercise
and work on it live in class
loopExerciseStarter.py
loopExerciseSoln.py
49
Simulation

Simulation
– use a computer program to simulate or
model a real-world process
• flight simulators, playing golf, football on a
computer and other games
• model behavior of a business, world economy,
beach erosion, hurricane tracking, etc.
– can explore and test theories which cannot
be tested in "real world"
• ask what if questions, such as, what if we raise
the prime interest rate by 1% ?
50
Random Numbers
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Random numbers: Examples
– dice roll
– lottery numbers generated by the actual
number drawing
– no discernable order or pattern
51
Pseudo-Random Numbers p.1
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Pseudo-random numbers
– appear "random" to user
– e.g. simulate a dice roll in a program
– computer-generated "easy-pick" lottery
numbers
– random plays and shuffling on a music
player (like an iPod)
– generated by an algorithm, so not truly
random; there is a pattern
52
Programming/Mathematics
Humor: Random Numbers
The generation of random numbers is
too important to be left to chance.
-- Robert Coveyou
53
Pseudo-Random Numbers p.2

Pseudo-random numbers
– Typically generated in a sequence
r0, r1, r2, r3 …
– Each value depends on previous value
rN = some calculation using rN-1
– first value is seed
54
Example Program: A Lottery Game
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player starts with $10
player bets $1 each play
player picks a number between 1 and 999
computer draws a number (same range)
using a pseudo-random number generator
if they match, player wins $500
play until either player is broke or has won
once
55
Pseudocode for main loop
initialize balance and won flag
while (have money) and (haven't won yet)
decrement balance
pick a number
draw a number
if they match
then
set won flag to true
print a win message
else
print a try again message
56
Important function usage

import random
this gives us access to the random module in
Python

module
a group of standard Python library functions

random.randint(1, MAX)
calls this function and generates a pseudorandom number between 1 and MAX,
inclusive
57
Now, let's look at the program

In lecture we will now switch to IDLE
and examine the lottery game program

This program can be printed from the
class web site
lotteryGame.py
58
Next…
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more about loops
for loop
efficiency issues
comparing real numbers
choosing a loop for a
particular task
59
Interest Example
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Invest money in an account paying an
annual rate of interest compounded
monthly. How long, in months, until the
amount doubles?

Example data:
1000.00
5.25
original amount
annual interest rate
60
Examine this program
interest.py

In lecture we will now switch to IDLE
and examine this program while it runs
61
Interest Example Questions p.1

What do we know is true after the
loop exits?
62
Interest Example Questions p.1a

What do we know is true after the
loop exits?
balance >= goalBalance
the logical complement is true
63
Interest Example Questions p.2

What would happen if we used this
instead?
– while balance != goalBalance :
64
Interest Example Questions p.2a

What would happen if we used this
instead?
– while balance != goalBalance :

Probable infinite loop
65
General Programming Principle

Never compare real numbers for exact
equality
– avoid use of == != <= >=
with real numeric types (float)

we would use import math and
math.fabs() to make "equality"
comparisons
66
Example

If x and y are floats,
import math
if (math.fabs(x-y) < 0.0001):
#fabs – float absolute

or some other criterion value, treat them
as equal
67
Interest Example Questions p.3

Why declare goalBalance and monthlyRate?
Why not just use this?
VERY inefficient
items in blue are repeated for
every iteration

while (balance
< GOAL_FACTOR * originalAmt) :
balance = balance + ((annualRate /
MONTHS_PER_YEAR) / 100.0 * balance)
month = month + 1
68
Efficiency Note

Do not put code inside a loop which
should not be repeated

Example: a calculation which only
needs to be done once, as in the
previous example
69
for loop

Usage: count-controlled loops
a counter controls the number of
iterations
70
Examples

Using a list of values

for variable in [value1, value2, etc.]:
stmt1
stmt2
stmt3

for num in [0, 1, 2, 3, 4]:
print (num) # prints 0,1,2,3,4
71
Examples

Using the range function

for num in range(5): # prints 0,1,2,3,4
print(num)
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Using start value, end limit, step value
for num in range (1, 10, 2):
print (num) # prints 1,3,5,7,9

Also can count down (negative step)
72
Examine this program
userSquares2.py

In lecture we will now switch to IDLE
and examine this program while it runs
73
recall this loop example:

Number
======
1
2
3
4
. . .
98
99
100
Square
======
1
4
9
16
. . .
9604
9801
10000
# 5 blanks
# between
74
while: count-controlled
# print table heading once
print ("Number Square")
print ("====== ======")
count = 1
# initialize counter
while count <= 100:
square = count ** 2
print ("%5d%11d" % (count, square))
count = count + 1
# update counter
75
using for
# print table heading once
print ("Number Square")
print ("====== ======")
for count in range(1, 101) :
square = count ** 2
print ("%5d%11d" % (count, square))
initialization and update do not
require separate statements
76
Nested for loops
How would we generate this
multiplication table?
1
2
3
4
5
...
9
1
1
2
3
4
5
...
9
2
2
4
6
8
10
...
18
3
3
...
6
9
...
12
15
...
27
9
9
18
27
36
45
...
81
77
Examine this program
nestedFor.py

In lecture we will now switch to IDLE
and examine this program while it runs
78
Odds and Ends…
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# you can use strings with a list
for name in ["amy", "bob", "carl", "denise"]:
print (name)
# you can mix types in a list
for item in ['A', 45, "fred"]:
print (item)
# you cannot use characters with range, as in
# for ch in range('A', 'Z')
# print (ch)
# produces a syntax error
79
Choosing a loop structure p.1
Is the loop count-controlled?
for usually best
Is the loop event-controlled (flag, sentinel,
other conditions being tested)?
while
Multiple termination conditions?
while
80
Next …
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Testing and Debugging
More programming, more Python!
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81