Loops

What is a Loop?

A way to repeat a block of code for a specific number of times.
i.e., A way to iterate over a collection of items.
bad: infinite loop

When to use loops?

When you want to add 2 to every number in a list.
When you want to print every item in a list.
When you want to sum all the numbers in a list.

Types of Loops

for loop
- for each item, do something.
while loop
- While some condition is true, do something.

Syntax

for item in collection:
    # do something

while condition:
    # do something

Example: For Loop

items = ["apple", "banana", "cherry", "date", "elderberry", "fig"]

for item in items:
    print(f"item: {item}!")

item: apple!
item: banana!
item: cherry!
item: date!
item: elderberry!
item: fig!

Example: For Loop, II

Sometimes we want the index of the item we are looping over. Use enumerate for this.

items = ["apple", "banana", "cherry", "date", "elderberry", "fig"]

for index, item in enumerate(items):
    print(f"index: {index}, item: {item}!")

index: 0, item: apple!
index: 1, item: banana!
index: 2, item: cherry!
index: 3, item: date!
index: 4, item: elderberry!
index: 5, item: fig!

Example: While Loop

import random

counter = 0
items = ["apple", "banana", "cherry", "date", "elderberry", "fig"]
results = []

while counter < 5:
    results.append(random.choice(items))
    counter += 1
print(results)

['fig', 'cherry', 'banana', 'elderberry', 'fig']

Example: While Loop, II

items = ["apple", "banana", "cherry", "date", "elderberry", "fig"]
counter = 0

while counter < 3:
    print(f"Item is {items[counter]}")
    counter += 1

Item is apple
Item is banana
Item is cherry

1-line loops

Sometimes you can write a loop in one line.
But only for loops, not while loops.

Example: 1-line loop

from datetime import datetime, timedelta

ds = [
    {"name": "John", "age": 25},
    {"name": "Jane", "age": 30},
    {"name": "Jack", "age": 35},
]
names = [d["name"] for d in ds]
date = datetime.now()
dates = []
while date < datetime.now() + timedelta(days=10):
    dates.append(date)
    date += timedelta(days=1)
print(names)
formatted_dates = [datetime.strftime(date, "%Y-%m-%d") for date in dates]
print("dates", formatted_dates)

['John', 'Jane', 'Jack']
dates ['2026-11-09', '2026-11-10', '2026-11-11', '2026-11-12', '2026-11-13', '2026-11-14', '2026-11-15', '2026-11-16', '2026-11-17', '2026-11-18', '2026-11-19']

Nested Loops

We can have loops inside loops.
This can be particularly useful when working with 2D data.
Be careful, though, as it can be hard to read.
Make your naming conventions clear.

Nested Loops, 2d items

2-dimensional items:

items = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

objs = [
    {"name": "John", "cities": ["New York", "Los Angeles", "Chicago"]},
    {"name": "Jane", "cities": ["San Francisco", "Seattle", "Portland"]},
]

Nested Loops, 2d items

items = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

objs = [
    {"name": "John", "cities": ["New York", "Los Angeles", "Chicago"]},
    {"name": "Jane", "cities": ["San Francisco", "Seattle", "Portland"]},
]
for i1, row in enumerate(items):
    print(f"Row {i1}")
    for i2, item in enumerate(row):
        print(f"Item {i2}: {item}")

Row 0
Item 0: 1
Item 1: 2
Item 2: 3
Row 1
Item 0: 4
Item 1: 5
Item 2: 6
Row 2
Item 0: 7
Item 1: 8
Item 2: 9

Break, Continue, Else

break: stop the loop.
continue: skip the rest of the loop and go to the next iteration.
else: run when the loop is done without break.

Example: Break

Stop the loop when we find the item “date”.

items = ["apple", "banana", "cherry", "date", "elderberry", "fig"]
new = []

for item in items:
    if item == "date":
        break
    new.insert(0, item)
print(new)

['cherry', 'banana', 'apple']

Example: Continue

items = ["apple", "banana", "cherry", "date", "elderberry", "date", "fig"]
new = []

for item in items:
    if item == "date":
        continue
    new.insert(0, item)

Example: Else

Run the else block if the loop completes without breaking.

items = ["apple", "banana", "cherry", "date", "elderberry", "date", "fig"]
new = []

for item in items:
    if item == "date":
        new.insert(0, item)
    new.append(item)
else:
    print("Done", new)

Done ['date', 'date', 'apple', 'banana', 'cherry', 'date', 'elderberry', 'date', 'fig']