xiaojuan cai computational thinking 1 lecture 8 loop structure xiaojuan cai （蔡小娟）...

Xiaojuan Cai Computational Thinking 1

Lecture 8

Loop Structure

Xiaojuan Cai（蔡小娟）

[email protected]

Fall, 2015

mailto:[email protected]


Objective

• To understand the concepts of loops:

• definite loops: for

• indefinite loops: while

• To understand interactive loop, sentinel loop

and end-of-file loop

• To understand the basic ideas of Boolean

algebra


Roadmap

• For loops

• Indefinite loops

• Interactive, sentinel, file, nested loops

• Boolean algebra

• Other loop patterns:

• post-test, loop and a half


For loops: review• Definite loop, syntax

for <var> in <sequence>:

<body>

• Semantics

• The index variable <var> takes on each successive value in

<sequence>, and execute <body> once for each value

• Note: there might be infinite for loops, for example

for x in a:

a.append(x)


Roadmap

• For loops



• Boolean algebra




Motivating example• Write a program computing the average of any

size of numbers entered by the user.

• Algorithm pattern – accumulator (average1.py)Input the count of the numbers, n

Initialize sum to 0

Loop n times

Input a number, x

Add x to sum

Output average as sum/n


Indefinite loop• What if we do not know how many numbers?

• The indefinite or conditional loop

keeps iterating until certain

conditions are met.

• while <condition>:

<body>

condition?

body

yes

no


Indefinite loop• i = 0

while i <= 10:

print i

i = i + 1

• The same output as this for loop:

for i in range(11):

print i


An infinite loop

• What should you do if you’re

caught in an infinite loop?

• First, try pressing control-c

• If that doesn’t work, try control-alt-

delete

• If that doesn’t work, push the reset

button!


Interactive loops

• Average without knowing the exact

number.

set moredata to “yes”

while moredata is “yes”

get the next data item

process the item

ask user if there is moredata


Interactive loops

• Algorithm1 (average2.py)initialize sum to 0.0

initialize count to 0

set moredata to “yes”

while moredata is “yes”

input a number, x

add x to sum

add 1 to count

ask user if there is moredata

output sum/count


Sentinel loops

• A sentinel loop continues until reaching

a special value that signals the end.

• This special value is called the sentinel.get the first data item

while item is not the sentinel

process the item

get the next data item


Sentinel loops• Algorithm 2

initialize sum to 0.0

initialize count to 0

input a number, x

While x != sentinel

add x to sum

add 1 to count

input another number, x

output sum/count

• sentinel = a negative numbers (average3.py)

• sentinel = the empty string “” (average4.py)


File loops• If the input size is large, you make a typo

then you need to start over again in an

interactive loop.

• File loops – read data from a file

(average5.py)

open a file f

for line in f.readlines():

process the item


File loop + EOF sentinel• EOF sentinel loop (average6.py)

open a file f

line = f.readline()

while line != “”

process the item

line = f.readline()

•What if there is a blank line in the file?

•Blank line = ‘\n’, EOF = “”


Nested loop

• What if there are several data in one

line (separated by commas)?open a file f

line = f.readline()

while line != “”

for xStr in string.split(line, ","):

sum = sum + eval(xStr)

count = count + 1

line = f.readline()


Design nested loops• Design the outer loop without

worrying about what goes inside

• Design what goes inside, ignoring the

outer loop.

• Put the pieces together, preserving

the nesting.


Roadmap

• For loops



• Boolean algebra




Boolean expressions• So far we’ve used Boolean expressions to compare two

values <expr><relop><expr>

• We need more expressive expression.

• For example, two points are the same iff their x

coordinates and y coordinates are same.

• This code is akward:if p1.getX() == p2.getX():

if p1.getY() == p2.getY():

else:

else:


Boolean expressions• <expr> and <expr>

• <expr> or <expr>

• not <expr>

P Q P and QT T TT F FF T FF F F

P Q P or Q

T T TT F TF T TF F F

P not PT FF T


Compound expressions• Consider a or not b and c

• How should this be evaluated?

• The order of precedence, from high to

low, is relop, not, and, or.

• Use parentheses!

(a or ((not b) and c))


Example: pingpong game• Player A: a

• Player B: b

• Winning condition:

• One of the score is larger than or equal to 11

• Win by at least 2 points

(a >= 11 or b >= 11) and abs(a-b) >= 2


Boolean algebra

• and has properties similar to multiplication

• or has properties similar to addition• 0 and 1 correspond to false and true.

Algebra Boolean algebra

a * 0 = 0 a and false == false

a * 1 = a a and true == a

a + 0 = a a or false == a


Boolean algebra• True or a == True

• False and a == False

• Distributed:

a or (b and c) == (a or b) and (a or c) a and (b or c) == (a and b) or (a and c)

• Double negatives cancel out:

not(not a) == a

• DeMorgan’s laws:not(a or b) == (not a) and (not b)not(a and b) == (not a) or (not b)


Built-in types and bool• response[0] == "y" or "Y" will always evaluate to

True. Why?

• Python 2.7 has bool type with two values: True and False

• Older versions use 1 and 0

• Zero int/float/long is interpreted as False, while non-zero

int/float/long is True.

• An empty sequence is interpreted as False while any

non-empty sequence is True.


Short-circuit

• x and y

• First evaluate x, if x is False, return False

without evaluate y!

• x or y

• First evaluate x, if x is True, return True

without evaluate y!

• shortCircuit.py


Flexibility• ans = raw_input("What flavor [vanilla]? ")

if ans != "": flavor = ans

else: flavor = "vanilla"

• ans = raw_input("What flavor [vanilla]? ")

if ans: flavor = ans

else: flavor = "vanilla"

• ans = raw_input("What flavor [vanilla]? ")

flavor = ans or "vanilla"

• flavor = raw_input("What flavor [vanilla]? ") or "vanilla"


Roadmap

• For loops



• Boolean algebra




Post-test loops

• Input validation

• The program asks for inputs

until a valid value being entered.

• repeat input a numberuntil number is >= 0

• Python doesn’t have such built-in

statement to do this

condition?

body

yes

no


Solutions• Setting number to -1

number = -1

while number < 0:

number = input("Enter a positive number: ")

• Infinite loop + break statementwhile True:

number = input("Enter a positive number: ")

if x >= 0: break

• Break statement makes Python immediately

exit the enclosed loop


Loop and a half• If we need to print some message when the input is

negative.

• While loop:number = -1while number < 0: number = input("Enter a positive number: ") if number < 0: print "The number was not positive“

• Break statementwhile True: number = input("Enter a positive number: ") if x >= 0: break else: print "The number was not positive."


Loop and a half

• The loop exit is in the middle of the loop

body.

• The use of break is mostly a matter of style

and taste.

• The logic of a loop is hard to follow when

there are multiple exits (break statements).


Conclusion• Decision structures allow a program to execute

different sequences.

• if, if-else, if-elif-else statements

• Boolean expressions: <, <=, >, >=, ==, !=

• Bool type: True, False

• Exception handling makes programs more

“bulletproof”.

• Algorithm design: correct, efficient, and

understandable

xiaojuan cai computational thinking 1 lecture 8 loop structure xiaojuan cai （蔡小娟）...

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