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CB 3.7,3.8 - Iteration in Python

Advik Garg, Srijan Atti, Akhil Singamneni, Aashray Rajagopalan / October 2023 (2894 Words, 17 Minutes)

Introduction

In this lesson, we will explore the various ways to create loops in Python. Loops are essential for repetitive tasks and are a fundamental concept in programming. We will cover different types of loops, advanced loop techniques, and how to work with lists and dictionaries using loops.

For Loops

Used to iterate over a sequence (such as a list, tuple, string, or range)
Executes a block of code for each item in the sequence

# APCSP Pseudo-Code: Iterating Over a List of Fruits

fruits ← ["apple", "banana", "cherry"]

FOR EACH fruit IN fruits:
    DISPLAY fruit
END FOR

# Example 1: Simple for loop
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

apple
banana
cherry

While Loops

Used to execute a block of code as long as a condition is true

# APCSP Pseudo-Code: Using a While Loop to Count and Display Numbers


i ← 1


WHILE i ≤ 5:
    DISPLAY i
    i ← i + 1
END WHILE

# Example 2: Simple while loop
i = 1
while i <= 5:
    print(i)
    i += 1

Looping with Lists and Dictionaries

Used to iterate through the elements of lists and dictionaries

# APCSP Pseudo-Code: Loop Through a List

numbers ← [1, 2, 3, 4]
FOR EACH num IN numbers:
    DISPLAY num
END FOR

# APCSP Pseudo-Code: Loop Through a Dictionary
person ← {"name": "aashray", "age": 15, "city": "San Diego"}
FOR EACH key, value IN person:
    DISPLAY key, ":", value
END FOR

# Example 3: Loop through a list
numbers = [1, 2, 3, 4]
for num in numbers:
    print(num)

# Example 4: Loop through a dictionary
person = {"name": "aashray", "age": 15, "city": "San Diego"}
for key, value in person.items():
    print(key, ":", value)

1
2
3
4
name : aashray
age : 15
city : San Diego

Popcorn Hack 1

Use a loop to get X amount of inputs. Then use a loop to find the type of each value.
Extra Challenge: If an input is a number, make the corresponding value in the dictionary a number.

X = int(input("Enter the number of inputs: "))

sample_dict = {}

for i in range(X):
    key = input("Enter key for input {}: ".format(i+1))
    value = input("Enter value for input {}: ".format(i+1))
    
    if value.replace('.', '', 1).isdigit():
        if '.' in value:
            value = float(value)
        else:
            value = int(value)
    
    sample_dict[key] = value

for key, value in sample_dict.items():
    print(f"{key}: {type(value).__name__}")

Looping with Index Variable

You can use the range function to create a loop with an index variable.

# APCSP Pseudo-Code: Loop Through a List Using Index

lst ← [4, 6, 7, 2]
FOR i IN RANGE(LENGTH(lst)):
    DISPLAY "Index: " + STRING(i)
    DISPLAY "Element: " + STRING(GET_ELEMENT(lst, i))
END FOR

# Example 5: Loop with an index variable

lst = [4, 6, 7, 2]

for i in range(len(lst)): # Loop for the number of elements in the list
    print('Index: ' + str(i)) # Print the index
    print('Element: ' + str(lst[i])) # Print the element

Index: 0
Element: 4
Index: 1
Element: 6
Index: 2
Element: 7
Index: 3
Element: 2

Nested If Statements

You can nest conditional statements inside a for loop to execute different code based on conditions.

# APCSP Pseudo-Code: For Loop with Nested If Statements

numbers ← [1, 2, 3, 4, 5]
FOR EACH num IN numbers:
    IF num MOD 2 EQUALS 0:
        DISPLAY num, "is even"
    ELSE:
        DISPLAY num, "is odd"
    END IF
END FOR

# Example 6: For loop with nested if statements
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        print(num, "is even")
    else:
        print(num, "is odd")

is odd
is even
is odd
is even
is odd

Popcorn Hack 2

Use the input() function to append a range of integers from a list
Use a nested if statement to only print numbers in the list that are evenly divisble by 3

nums = []
length = int(input("how many numbers?: "))

for x in range(length):
    x = int(input("Enter a number"))
    
    nums.append(x)

for i in nums:
    if i % 3 == 0:
        print(i, "is divisible by 3")
    else:
        print(i, "is not divisible by 3")

is not divisible by 3
is not divisible by 3
is divisible by 3
is not divisible by 3
is not divisible by 3

Try/Except

Using a try and except block inside a loop can handle errors gracefully.
Very useful for production code, even in frontend webapps
- Ex: Giving an error page instead of dumping critical information on the webpage

# APCSP Pseudo-Code: Handling Errors in a For Loop

numbers ← [1, 2, "three", 4, 0, "five"]
FOR EACH item IN numbers:
    TRY:
        DISPLAY 10 / item
    CATCH ZeroDivisionError:
        DISPLAY "Division by zero"
    CATCH TypeError:
        DISPLAY "Type error"
    END TRY
END FOR

numbers = [1, 2, "three", 4, 0, "five"]

for item in numbers:
    try:
        print(10 / item)
    except ZeroDivisionError: #Type of error: Dividing by Zero
        print("Division by zero")
    except TypeError: #Type of error: Dividing by something that isn't a number
        print("Type error")

10.0
5.0
Type error
2.5
Division by zero
Type error

Popcorn Hack 3

Create a for loop that uses a try and except statement for an AttributeError
Use integers and a list to create scenarios where the loop will either print something expected or print an error message
CHALLENGE: Try using the math module for this error

import math

numbers = [2, 5, -2 , 8]

for i in numbers:
    try:
        square_root = math.sqrt(i)
        print(f"The square root of {i} is: {square_root}")
    except AttributeError:
        print(f"Error: Attribute error encountered for: {i}")
    except ValueError:
        print(f"Error: Value error encountered for item: {i}")

The square root of 2 is: 1.4142135623730951
The square root of 5 is: 2.23606797749979
Error: Value error encountered for item: -2
The square root of 8 is: 2.8284271247461903

Continue and Break

Continue statement skips the current iteration
Break statement exits the loop prematurely

# APCSP Pseudo-Code: For Loop with Continue and Break

numbers ← [1, 2, 3, 4, 5]
FOR EACH num IN numbers:
 
    IF num EQUALS 3:
        CONTINUE
    IF num EQUALS 5:
        BREAK 
    DISPLAY num
END FOR

# Example 8: For loop with continue and break
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num == 3:
        continue  # Skip the number 3
    if num == 5:
        break  # Exit the loop when 5 is encountered
    print(num)

1
2
4

Nested For Loops

You can also put for loops within for loops
Allows for looping an exponential amount of times

# APCSP Pseudo-Code: Nested Loops for Group Names

groups ← [["advik", "aashray"], ["akhil", "srijan"]]

FOR EACH pair IN groups:
    FOR EACH person IN pair:
        DISPLAY person + " is cool"
    END FOR
    DISPLAY pair[0] + " and " + pair[1] + " love to code code code"
END FOR

groups = [['advik', 'aashray'], ['akhil', 'srijan']]

for pair in groups:
    for person in pair:
        print(person + ' is cool')
    print(pair[0] + ' and ' + pair[1] + ' love to code code code')

advik is cool
aashray is cool
advik and aashray love to code code code
akhil is cool
srijan is cool
akhil and srijan love to code code code

(OPTIONAL) Popcorn Hack 4

Create a nested for loop that iterates over a dictionary that has:
- A name for each key
- A list for each value containing stuff like age, grade, etc.
Break/continue if certain conditions are met
Have fun! If you want to, relate it to a theme!

student_info = {
    'Imaad': [15, 'A'],
    'Tanay': [16, 'B'],
    'Arnav': [15, 'C'],
}

for name, details in student_info.items():
    print(f"Student: {name}")
    age, grade = details  

    if age > 18:
        print(f"{name} is above 18 years old.")
        break  

    if grade == 'A':
        print(f"{name} has an 'A' grade.")
        continue  

Student: Imaad
Imaad has an 'A' grade.
Student: Tanay
Student: Arnav

Iteration via Recursion

A technique where a function calls itself
Can be used to recreate loops until a certain condition is met

# APCSP Pseudo-Code: Recursion for Factorial Calculation

FUNCTION factorial(n):
    IF n EQUALS 0:
        RETURN 1
    ELSE IF n LESS THAN 0:
        RETURN "undefined"
    ELSE IF TYPEOF(n) EQUALS "float":
        RETURN "not solvable without gamma function"
    ELSE:
        RETURN n TIMES factorial(n - 1)
    END IF
END FUNCTION

result ← CALL factorial(5)
DISPLAY "Factorial of 5 is", result

# Example 9: Recursion for factorial calculation
def factorial(n):
    if n == 0: #Conditions to stop the recursion
        return 1 # 0! is 1
    elif n < 0:
        return "undefined" # Undefined for negative numbers 
    elif isinstance(n, float):
        return "not solvable without gamma function" # Only accept integers
    else:
        return n * factorial(n - 1) #Function calling itself

result = factorial(5)
print("Factorial of 5 is", result)

Factorial of 5 is 120

Homework

Add student names w/ grades to a dictionary until the user doesn’t want more students
- Prompt for user input for all of these
Use a nested if/else statement in a for loop
- Get the highest score in the dictionary
- Add all students who passed into a new list (add student names, not their scores)
Bonus: Use a try/except for any scores that aren’t integers

students = {}

def is_valid_grade(grade):
    try:
        int_grade = int(grade)
        return True
    except ValueError:
        return False

while True:
    name = input("Enter student name (or 'exit' to stop): ")
    if name.lower() == 'exit':
        break

    grade = input("Enter student grade: ")
    while not is_valid_grade(grade):
        print("Invalid grade. Please enter a valid integer grade.")
        grade = input("Enter student number grade(Don't include percent sign): ")

    students[name] = int(grade)

highest_score = float('-inf')
passing_students = []
failing_students = []

for name, grade in students.items():
    if grade > highest_score:
        highest_score = grade

    if grade >= 60:
        passing_students.append(name)
    else:
        failing_students.append(name)

print("Student grades:", students)
print("Highest score:", highest_score)
print("Passing students:", passing_students)
print("Failing students:", failing_students)

Student grades: {'Imaad': 98, 'Tanay': 97, 'Arnav': 95, 'Test': 30}
Highest score: 98
Passing students: ['Imaad', 'Tanay', 'Arnav']
Failing students: ['Test']