Lecture 5: Loops

Craig Zilles (Computer Science)

September 26, 2019

https://go.illinois.edu/cs105fa19

CS 105

Are you sitting next to someone to talk to for the clicker

questions?

Today I'm using: pythontutor.com

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Big Picture (Muddiest Points)• I felt that overall, this chapter is harder than the

previous chapters so I was very confused in general.• I didn't completely understand the concept of using

while and for loops. I will be honest, the more we are learning, things are coming more clearer about coding, but everything makes more sense after lecture.

• It felt like I could never get the answer on my own, but when I pressed 'show answer' and saw what to do it made sense. What can I do so I skip that step?

• in the participation activites, why does the answer sometimes have indents and other times it does not?

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Today1. Conditionals

• if, elif, else• Nesting

2. Looping: conditions, body, iteration3. for Loops (definite loops)4. range5. while Loops (indefinite loops)6. break and continue7. Loop Patterns

• Count, Sum• Find, Find Best

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Design of conditionals• Conditionals are for executing blocks 0 or 1 times

• Why sometimes do we make multiple if else statements but other times we make one big if, elif and else statement?

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Shape of "decision tree": if w/o elseAsked my TA to send email to all students in the class that didn't take Exam 0.• Step 1: make a set of all students that took Exam 0• Step 2: check each student in class if in the set

if student in exam0_takers:send_email(student)

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Shape of "decision tree": if w/elseCompany sends recruiting invitations to students with Python in their resume, sends 'nack' email to others

if 'python' in resume.lower():send_invitation(student)

else:send_polite_decline(student)

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Choosing between many alternativesFinal exam location based on first letter of netid:

[a-j] Loomis 100[k-o] DCL 1320[p-z] English 214

first_char = netid.lower()[0]

if first_char <= 'j':

location = 'Loomis 100'

elif first_char <= o:

location = 'DCL 1320'

else:

location = 'English 214'

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NestingWhen to use elif and when to use else? I think there should be only 1 else in the whole program but I saw:

if sales_type == 2:if sales_bonus < 5:

sales_bonus = 10else:

sales_bonus = sales_bonus + 2else:

sales_bonus = sales_bonus + 1

Can I change the first 'else' into elif?9

Nesting• CS likes composition; indent + indent = nesting

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Code Block A

Code Block B(Execution determined by condition1)

if condition1:

Code Block E(Same indentation as A)

if condition2:

Code Block C(Execution determined by condition2)

Which is not a sequence of blocks?

A) A, EB) A, B, EC) A, B, D, ED) A, B, C, D, E

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Code Block A

Code Block B(Execution determined by condition1)

if condition1:

Code Block E(Same indentation as A)

if condition2:

Code Block C(Execution determined by condition2)

Code Block D(Same indentation as B)

Loops• What I found most confusing was how to identify a loop

and when do use one• The loops in general confuse me. I never know how

python knows how to execute which line of code in which order.

• I do not understand the nested loops and still do not understand the difference between the for and while loops.

• I am having a hard time understanding what a loop variable is and its purpose.

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Loop concepts• Loops are for repeating code 0 or more times (iterations)

loop conditionloop body(one or more statements)

post-loop stuff

1. Evaluate condition2. If True, (execute body and go back to step 1)3. Do stuff after the loop

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For loops (definite loops)• I am still confused on when to use a while loop vs. a for

loop.

• Primary use of for loops:• Do some action for each element of a collection

for student in ['Cesar', 'Jessica', 'Sydney']:

print('{0} is awesome!'.format(student))

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How many lines will be printed?course_times = {'CS 105':'F9-11', 'CS 125':'MWF11-12'}

for course in course_times:

print(course, 'meets', course_times[course])

A) there is an errorB) 1C) 2D) 3E) 4

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How many lines will be printed?things = [22, [33, 44], 55, [66]]

for thing in things:

print(str(thing))

A) there is an errorB) 2C) 3D) 4E) 5

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Announcements

• Be sure to complete Spatial Post-Test

• Quiz 1 live now; finish by Saturday night

• Exam 1 next week; will be quite similar to Quiz 1

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Unpacking• the concept of unpacking!!

• Extracts a collection into many variables, e.g., • Number of vars on left must equal len(collection)

first, last, age = ['Craig', 'Zilles', 27]

• Can be used to swap values: cute trick…x = "started as x"y = "started as y"x, y = y, x

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Enumerate• When do we use enumerate? Can you go over what this

does?

• Use it when to get index when iterating thru collection

cities_by_pop = ['New York', 'Los Angeles', 'Chicago']

message = '{0} is number {1} by population'

for index, name in enumerate(cities_by_pop):

print(message.format(name, index + 1))

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What is the contents of new_list?orig_list = [3, 7, 22, 90]

new_list = []

for index, value in enumerate(orig_list):

if (index % 2) == 0:

new_list.append(value)

A) [3, 7]B) [3, 22]C) [3, 7, 22, 90]D) [7, 90]E) [22, 90]

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range statements

• Useful for generating (finite) sequences of numbers:• E.g., the numbers from 0 to 9

range(10) # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

range(3, 8) # [3, 4, 5, 6, 7]

range(-3, 11, 3) # [-3, 0, 3, 6, 9]

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Using range to repeat fixed # of times

for i in range(5): # note that i not used

print('CS 105 is lit!')

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How many lines of output will this print?A) 0 –or– there is an errorB) 1C) 4D) 5E) 6

Using range to get every other char

orig_str = 'The quick brown fox jumped ...'

new_str = ''

for index in range(0, len(orig_str), 2):

new_str += orig_str[index]

print(new_str)

Teqikbonfxjme . is printed

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Loop nesting• I think nested loops were the most confusing part of the

text. I find them incredibly difficult to read and understand what is happening in them. The challenge problems based around them were pretty hard for me.

• Muddiest point is nesting loops and how to implement continue and break commands

• I would love to go over more how to follow the code in nested loops because sometimes I do not understand what gets read in what order.

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Nested Loops• Print out the intersection of two lists

list1 = [‘lemon’, ‘orange’, ‘lime’]

list2 = [‘banana’, ‘lemon’]

for thing1 in list1:

for thing2 in list2:

if thing1 == thing2:

print(thing1)

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While loops (indefinite loops)• I was confused on while loops, specifically when they

should be used.

• Roughly: used when you can't anticipate # of iterations

• What does sentinel mean? Does it close out a loop?

• Computer scientists use it to mean 'stopping symbol'

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Example while in its natural habitatnew_list = []

while True:

val = input("Enter a value or 'q' to quit\n")

if val == 'q':

break

new_list.append(val)

print(str(new_list))

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