CSE 163

Introduction to Python II

Strings and Lists

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Suh Young Choi

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🎶 Listening to: Across the Spider-Verse Soundtrack�💬 Before Class: Read any good books lately?

This Time

• Python Crash Course - Day 2
• Strings and String Functions
• Slicing
• Lists
• Documentation
• None

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Last Time

• Python Crash Course - Day 1
• Hello world
• Variables
• Expressions
• Types + Casting
• Loops
• Conditionals
• Functions (parameters + returns)

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Quick Announcements!

• Still waiting on course website ☹
• CSE Support is helping out ☺

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• Office hours have begun!—schedule will be posted on Ed/course website.

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• Reminders:
• Checkpoints are shorter coding and conceptual problems
• Learning Reflections are guided study notes/reflections on the weekly material
• Take-Home assessments are longer programming exercises

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Recap: Lesson

Strings

• Text data that has indices for characters
• Has functions to transform strings (returns new ones!)
• Slicing semantics

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Lists

• A lot like strings (indexed-sequence), but can hold any value!
• For Java people—a lot like an array

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Documentation

• Doc-string (“”” documentation “””)
• Type annotations

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None

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String Methods vs. len

Call functions on strings using syntax:

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This is not true for len (or other built-in Python functions):

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This asymmetry is annoying, but you have to get used to it!

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s.function(params)

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len(s)

Strings

vs.

Lists

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s = 'hello world'

# Length

len(s) # 11

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# Indexing

s[1] # 'e'

s[len(s) - 1] # 'd'

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# Looping

for i in range(len(s)):

print(s[i])

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for c in s:

print(c)

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l = ['dog', 'says', 'woof']

# Length

len(l) # 3

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# Indexing

l[1] # 'says'

l[len(l) - 1] # 'woof'

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# Looping

for i in range(len(l)):

print(l[i])

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for word in l:

print(word)

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Function comments

Function comments are the best way for others to know what your code does!

• Remember grammar?
• Focus on what your function does, rather than the how

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def sum_what(nums):

“””

Takes in a list of

numbers and returns the

sum of those numbers.

“””

total = 0

for i in nums:

total += i

return total

def sum_how(nums):

“””

Takes in a list of numbers,

then calculates the sum of

all numbers using a for loop

to iterate through all numbers.

“””

total = 0

for i in nums:

total += i

return total

Anatomy of a Function

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function �header

parameters

function �body

function call

Anatomy of a Function (updated!)

function �header

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parameters

function �body

return statement

function call

None

• None is a special value in Python that represents the absence of a value
• You don’t have to know a lot about None except:
• It can cause your program to crash if you ask a None value to do something
• You might have cases where you return None
• Check for None using the is keyword, instead of ==

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def increment(x):

if x < 0:

return None

else:

return x + 1

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if increment(-1) is None:

print('Failed')

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What are you still curious about?

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Group Work:

Best Practices

When you first working with this group:

• Introduce yourself!
• If possible, angle one of your screens so that everyone can discuss together

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Tips:

• Starts with making sure everyone agrees to work on the same problem
• Make sure everyone gets a chance to contribute!
• Ask if everyone agrees and periodically ask each other questions!
• Call TAs over for help if you need any!

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Next Time

Quiz Section

• Course Infrastructure
• Testing & Debugging Process
• Style

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Lesson

• More list functions
• File I/O

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Before Next Time

• Go to section tomorrow!
• Complete the Lesson 3 reading before Friday!

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