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

1
2
3
4
5

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.

sample_dict = {"name": "eshaan", "age": 15, "city": "San Diego"}

for i in sample_dict:
    print(i)
    print(type(i))

name
<class 'str'>
age
<class 'str'>
city
<class 'str'>

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")

1 is odd
2 is even
3 is odd
4 is even
5 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 = [10, 20, 30, 40, 50]

#Use the input() function to append a range of integers from a list
for i in range(4):
    nums.append(int(input("Enter number:")))

print(nums)

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

[10, 20, 30, 40, 50, 24, 48, 44, 41]
10 is not divisible by 3
20 is not divisible by 3
30 is divisible by 3
40 is not divisible by 3
50 is not divisible by 3
24 is divisible by 3
48 is divisible by 3
44 is not divisible by 3
41 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
numbers = []

nums = [0, 1, 2, 3, 4, 5]

for num in nums:
    try:
        numbers.append(10 * (num-4))
    except AttributeError:
        print("error")

for num in nums:
    try:
        print(10 / num)
    except ZeroDivisionError:
        print("error")

error
10.0
5.0
3.3333333333333335
2.5
2.0

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!
people = {}

# Code here

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 = {}
    

while True:
    studentname = str(input("Enter name (or 'q' to quit):"))
    if studentname.lower() == 'q':
        break
    studentgrade = int(input("Enter grade:"))
    students[studentname] = studentgrade
    
highestscore = 0
passingstudents = []

# Find the highest score and create a list of passing students
for studentname, studentgrade in students.items():
    if studentgrade > highestscore:
        highestscore = studentgrade
    if studentgrade >= 60:
        passingstudents.append(studentname)

print("Student dictionary:", students)
print("Highest score:", highestscore)
print("Students who passed:", passingstudents)


Student dictionary: {'Eshaan': 99, 'John': 48, 'Jim': 85}
Highest score: 99
Students who passed: ['Eshaan', 'Jim']