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Iterations 3.8 | Introduction

Iterations or Loops are super important when coding in Python or Javascript. They essentially allow you to repeat tasks over and over again for as long as you want as much as you want.

There are 4 types of loops that will be covered:

For loops
While loops
Do/While loops
Infinite loops

For Loops:

Used to iterate over a sequence
repeat a code a certain number of times,
“for” goes through each item

# Example 1: Simple for loop in Python
groceries = ["milk", "bread", "eggs", "juice", "yogurt"]
for grocery in groceries:
    print(grocery)

#Example 2: Simple for loop in JavaScript
let groceries = ["milk", "bread", "eggs", "juice", "yogurt"];
for (let i = 0; i < groceries.length; i++) {
    console.log(groceries[i]);
}

%%js
console.log("hello")

<IPython.core.display.Javascript object>

While Loops:

keeps running as a given condition is True
this loop starts with count = 0
as long as condition is met, of count being less than 5, it will keep running Inside Loop:
- prints current value of count
- count gets increased by one ever iteration

# Example While Loop in Python
count = 0
while count < 5:
    print(count)
    count += 1

# Example While Loop in JavaScript

let count = 0;

while (count < 5) [
    console.log(count);
    count++;
]

Do-While Loops:

executes code block at least one before checking condition
guarentees that loops is run at least one time, no matter the condition

Infinite Loops:

a loop that never stops running because condition never becomes False
this is the same code as previous while loops, but you never update count so it always remains at 0
because it is at 0, the while loop runs forever

# Example of Infinite Loop in Python
count = 0
while count < 5:
    print(count)
    # Forgetting to update `count` makes it run forever

%%js
# Example of Infinite Loop in JavaScript

let count = 0;

while (count < 5) {
    console.log(count);
    # Forgetting to update the 'count' causes an infinite loop
}

<IPython.core.display.Javascript object>

Looping with Dictionaries

Iterate through the elements of lists and dictionaries

person = {"name": "Arya", "age": 15, "city": "San Diego"}
for key, value in person.items():
    print(key, ":", value)

How This Works: Dictionary Definition: The dictionary person has three key-value pairs:

“name”: “Arya” “age”: 15 “city”: “San Diego” FOR EACH key, value IN person::

FOR EACH means the loop will go through each element in the dictionary. In Python, dictionaries store key-value pairs. For example, “name” is a key, and “Arya” is its corresponding value. The .items() method is used to get both the key and the value in each iteration. Loop Execution:

First Iteration: key = “name” value = “Arya” The program displays: name : Arya Second Iteration: key = “age” value = 15 The program displays: age : 15 Third Iteration: key = “city” value = “San Diego” The program displays: city : San Diego DISPLAY key, “:”, value:

For each pair, it prints the key, followed by a colon :, and then the value. In Python, this is done using print(key, “:”, value).

Looping with Index Variable

using range()

fruits = ['apple', 'banana', 'cherry']
for i in range(len(fruits)):
    print(f"Index {i}: {fruits[i]}")

How it works:

range(len(fruits)) generates numbers from 0 to 2 (since len(fruits) is 3).
The loop runs 3 times (one for each index), and in each iteration:
- i takes the value of the index.
- fruits[i] gives the element at that index.

Nested If Statements

when an if statement is placed inside another if, elif, or else statement

age = 20
has_ticket = True

if age >= 18:
    print("You are old enough to watch the movie.")
    if has_ticket:
        print("You can enter the theater.")
    else:
        print("You need a ticket to enter.")
else:
    print("You are too young to watch this movie.")

You are old enough to watch the movie.
You can enter the theater.

Explanation:

if checks if age variable is over 18, prints false if not
if if statement is checked then moves on to next if statement and checks has_ticket
if has_ticket true then prints statment, if false prints another statement

APCSP Pseudo-Code: Nested Loops for Group Names

groups = [['arya', 'shawn'], ['aarav']]

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

Try/Except

try:
    num = int(input("Enter a number: "))  # Code that might raise an exception
    result = 10 / num  # Could raise ZeroDivisionError if num is 0
    print("Result:", result)
except ValueError:
    print("That's not a valid number!")
except ZeroDivisionError:
    print("You can't divide by zero!")
else:
    print("Operation was successful!")