Lecture 4

More Loops & Nested Loops

Summer 2025

Adrian Salguero

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Announcements

• Homework 1 is available, due Sunday, July 6th at 11:59pm
• Coding Practice 1 will be released later today
• Due Sunday, July 6th at 11:59pm
• Additional practice on material covered in class
• Not eligible for resubmission!
• No class on Friday, July 4th. 🎇🎆

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Lecture Overview

• Loops Review
• Basics of Lists
• For-each versus index looping
• Nested Loops
• Practice Problems!

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Review: Looping = repeat the same code

• Looping is a way for us as programmers to control how often a specific code block (i.e. the loop body) runs.
• Each time a loop body runs, values used may be different (not always)
• You can loop on different sequences of values
• Lists → [1, 2, 3, 4]
• Strings → “CSE160”
• Range object → represents a sequence of numbers
• Looks like a list but doesn’t behave like a list

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for fahr in [30, 40, 50, 60, 70, 80]:

cent = (fahr - 32) / 9.0 * 5

print(fahr, cent)

print("All done")

Loop body�is indented

Lecture Overview

• Loops Review
• Basics of Lists
• For-each versus index looping
• Nested Loops
• Practice Problems!

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List Basics

• A List is a collection of values/elements
• Values can be of different data types (Python does not restrict you, but other programming languages will!)

• Declaring an empty list
• list_name = [ ]
• Declaring a non-empty list
• list_name = [1, 2.15, “hi”, True]

• You can get a specific element in a list by using its index
• Index starts at 0
• If you reference an index that doesn’t exist → ERROR!

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list1 = [2, 4, 6, 8, 10]

list1[3]

list1[5]

→ ERROR

len(list1) = 5

Lecture Overview

• Loops Review
• Basics of Lists
• For-each versus index looping
• Nested Loops
• Practice Problems!

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For-each loop versus For-loop using index

• For-each loop is what we’ve used so far, but there is another way of looping through a sequence when it is a list
• For-each loop
• Iterator (“dummy”) variable refers to the element in the sequence/list
• For-loop with indexing
• Iterator(“dummy”) variable refers to the index (position) of the element
• For this example: let’s use list1 = [2, 4, 6, 8, 10]

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for elem in list1:

print(elem)

for elem in range(len(list1)):

print(list1[elem])

For-each loop

For-loop with indexing

Looping over Strings using Indexing

course = "CSE161"

for i in range(0, len(course)):

print(course[i])

​

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Can we update this string using indexing?

NO!

We will discuss why in the future.

When should I use for-each? Indexing?

• Loop through each element in a list and obtain the value to use
• Either one!
• Loop through each element in a list and update values in specific positions of the list
• Use index looping since you will need the index to update the list value
• Simple example: Loop through a list and double each value

Still unsure? Ask yourself these questions

1. Do I need to know the index/position of these elements?
2. Does my sequence even allow for indexing? (Some sequences don’t!)
3. Do I need to modify my sequence in any way?
1. If it’s a list, most likely you’ll need to loop using the index

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Lecture Overview

• Loops Review
• Basics of Lists
• For-each versus index looping
• Nested Loops
• Practice Problems!

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Nested Loops!

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for i in range(3):

for j in range(2):

print("i is", i)

print("j is", j)

print("*************************")

print("All done!")

​

​

​

i is 0

j is 0

i is 0

j is 1

*************************

i is 1

j is 0

i is 1

j is 1

*************************

i is 2

j is 0

i is 2

j is 1

*************************

All done!

​

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Output

Another Nested Loop

for i in range(2):

print("Outer", i)

for j in range(2,4):

print(" Inner", j)

print(" Sum", i + j)

print("Outer", i)

​

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Output

Outer 0

Inner 2

Sum 2

Inner 3

Sum 3

Outer 0

Outer 1

Inner 2

Sum 3

Inner 3

Sum 4

Outer 1

​

​

Another Nested Loop

for i in range(2):

print("Outer", i)

for j in range(2,4):

print(" Inner", j)

print(" Sum", i + j)

print("Outer", i)

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loop body:�3 statements

“nested”�loop body:�2 statements

i does not update until nested loop completely finishes running

Lecture Overview

• Loops Review
• Basics of Lists
• For-each versus index looping
• Nested Loops
• Practice Problems!

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Test your Knowledge

for i in [0,1]:

print(i)

print(i)

i = 5

for i in range(10, 0):

print(i)

​

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for i in range(0, 2):

print("Outer", i)

for i in range(2, 4):

print(" Inner", i)

print("Outer", i)

​

Be careful with which variables you use in your loops → don’t risk overwriting the iterator variable with another loop!

More Loop Practice

Suppose you are given the following two lists:

• dept = ["CSE", "ECE", "MATH"]
• course_num = [100, 110, 160]

​

Write code that prints out all possible combinations of department names with course numbers. For example, your code should output exactly:

CSE 100, CSE 110, CSE 160, ECE 100, …, MATH 160

Note: Yes…including the spaces between the department and course number. Commas are optional, you can print each on a new line.

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More Loop Practice

You are asked to review some code that prints all the numbers starting from 1 through to 50. The code is given below:

for tens_digit in [0, 1, 2, 3, 4]:

for ones_digit in [1, 2, 3, 4, 5, 6, 7, 8, 9]:

print(tens_digit * 10 + ones_digit)

​

However, when you run this code the output is incorrect! How would you change it so the output is correct without rewriting the for loops from scratch?

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Some Fixes for the Previous Problem

for tens_digit in [0, 1, 2, 3, 4]:

for ones_digit in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]:

print(tens_digit * 10 + ones_digit + 1)

​

for tens_digit in [0, 1, 2, 3, 4]:

for ones_digit in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]:

print(tens_digit * 10 + ones_digit)

​

for ones_digit in [1, 2, 3, 4, 5, 6, 7, 8, 9]:

print(ones_digit)

for tens_digit in [1, 2, 3, 4]:

for ones_digit in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]:

print(tens_digit * 10 + ones_digit)

print(50)

​

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TODOs

• Homework 1 due Sunday, July 6th by 11:59pm
• Coding Practice 1 due Sunday, July 6th by 11:59pm

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