CSE 163

Lists and File Processing

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Announcements

• (Optional) Lesson 1.2 Review due tonight at 11:59pm on Gradescope
• Programming Practice 1 releases tomorrow (Thursday!)
• Homework 1: Processing releases Friday!
• Due next Friday, July 3rd.
• Attend office hours if you get stuck!

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Strings vs. Lists

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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Advanced Lists

• List methods can modify the contents of the list. Lists are mutable!
• Can use the in keyword to see if a list contains a value

words = ['I', 'love', 'dogs']

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if 'dogs' in words:

print('Found it')

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if 'cats' not in words:

print('Not there')

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• Function comments are the best way for others to know what your code does!
• Maintain good grammar
• Focus on what your function does, rather than how it does it?
• Ask yourself: If I came back to this code after 1-2 years, can I easily understand what this function does by reading my comment?
• Someone else should be able to understand, maintain, and develop with your code

• Function comments should be concise and provide a good explanation of what the function is doing. Some things to think about:
• What is the overall purpose of your function?
• Does your function take in any special parameters?
• What does it output? What does it return?

Function Comments

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Recap: Anatomy of a Function

6

def function_name(parameter, ...):

"""

What does this function do?

"""

#Function code

return value

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def keyword

Function name

Optional

parameters

Function code

colon

Indentation

Return statement

Docstring comment

Which function comment is “better”?

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

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• None is a special value in Python that represents the absence of a value
• You don’t have to know a lot about None for this course 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 (can lead to unexpected behavior or errors in your program)
• Check for None using the is keyword, instead of ==

None Value

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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File Processing

Line-by-line processing

Token processing

When would you use one over the the other?

with open(file_name) as f:

lines = f.readlines()

for line in lines:

# do something with line

with open(file_name) as f:

lines = f.readlines()

for line in lines:

tokens = line.split()

for token in tokens:

# do something with token

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Lesson Recap: Lists and Files

• Lists
• Introduced function to alter lists
• Lists are mutable! Strings are not!
• The in keyword
• Indexed-sequence (like strings) but can hold any value!
• For Java people – a lot like an ArrayList

• File Processing: Line vs. Token Processing

• File Paths
• Absolute vs. relative paths
• Make sure to use relative paths in Gradescope submissions!

• Documentation
• Doc-string (““ documentation ”” )

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