workshop_4.md

Workshop 4 — Loops

Prerequisites

• Workshop 1: variables, data types, type casting, f-strings, operators
• Workshop 2: input(), string methods, booleans, truthiness
• Workshop 3: if/elif/else, and/or/not, in, chained comparisons, conditional expressions

1. Why loops

Suppose you want to print three numbered lines. You could write three print() calls — but then four lines means a fourth call, and a hundred lines means a hundred. A loop lets you repeat work without repeating code:

count = 1
while count <= 3:
    print(f"Line {count}")
    count += 1
# Line 1
# Line 2
# Line 3

while means "as long as this condition is True, run the block again". The condition is the same kind of condition you wrote in Workshop 3.

2. The three parts of a while loop

Every working while loop has three parts: a starting value, a condition, and a change that moves the condition toward False:

countdown = 3
while countdown > 0:
    print(countdown)
    countdown -= 1
print("Go!")
# 3
# 2
# 1
# Go!

• Starting value: countdown = 3
• Condition: countdown > 0
• Change: countdown -= 1

The print("Go!") is not indented, so it is outside the loop and runs once, after the loop finishes.

3. The infinite loop

Forget the change and the condition never turns False:

count = 1
while count <= 3:
    print(count)      # 1, forever — count never changes
# press Ctrl+C to stop it

count stays 1, so count <= 3 stays True and the loop never ends. If your program hangs and prints forever, press Ctrl+C and look for the missing change.

4. for and range

When you already know how many times to repeat, for with range() says it more directly than while:

for i in range(3):
    print(i)
# 0
# 1
# 2

range(3) produces the numbers 0, 1, 2. The loop variable i takes each of them in turn, and the block runs once per value. You don't manage the counter yourself — there's no starting value to set and no change to remember.

5. range's three forms

Call	Numbers produced
range(5)	0 1 2 3 4
range(2, 6)	2 3 4 5
range(1, 10, 3)	1 4 7

With one argument it counts from 0. With two it counts from the first up to the second. With three, the last argument is the step.

The stop value is always excluded. range(5) stops before 5, so it gives five numbers starting at 0. To count 1 to 3, ask for range(1, 4):

for number in range(1, 4):
    print(number)
# 1
# 2
# 3

6. for over a string

for isn't only for numbers. Given a string, the loop variable takes each character in turn:

for letter in "cat":
    print(letter)
# c
# a
# t

7. Accumulating a total

A variable created before the loop survives across iterations, so you can build up a result:

total = 0
for number in range(1, 5):
    total += number
    print(f"added {number}, total is now {total}")
print(f"Final total: {total}")
# added 1, total is now 1
# added 2, total is now 3
# added 3, total is now 6
# added 4, total is now 10
# Final total: 10

Note that total = 0 is before the loop. Put it inside and it resets to 0 on every pass, and the final answer is just the last number.

8. Counting with a condition inside the loop

Everything from Workshop 3 works inside a loop body. Here an if decides whether this particular pass counts:

vowels = 0
for letter in "programming":
    if letter in "aeiou":
        vowels += 1
print(f"vowels: {vowels}")
# vowels: 3

The loop visits every character; the if chooses which ones matter.

9. break

break leaves the loop immediately, skipping the remaining iterations:

for number in range(1, 10):
    if number == 4:
        break
    print(number)
print("done")
# 1
# 2
# 3
# done

range(1, 10) would have gone up to 9, but the loop stops the first time number == 4. Note that 4 is never printed — break happens before the print().

10. continue

continue skips the rest of this pass and carries on with the next one:

for number in range(1, 6):
    if number == 3:
        continue
    print(number)
# 1
# 2
# 4
# 5

The loop does not end — only 3 is missing. Keep the difference clear: break ends the loop, continue ends one pass.

11. Nested loops

A loop body can contain another loop. The inner loop runs fully for every pass of the outer one:

for row in range(1, 4):
    for col in range(1, 4):
        print(f"{row} x {col} = {row * col}")
    print("---")
# 1 x 1 = 1
# 1 x 2 = 2
# 1 x 3 = 3
# ---
# 2 x 1 = 2
# 2 x 2 = 4
# 2 x 3 = 6
# ---
# 3 x 1 = 3
# 3 x 2 = 6
# 3 x 3 = 9
# ---

The outer loop runs 3 times, the inner one 3 times per outer pass, so the inner print() runs 9 times. The print("---") is indented at the outer level, so it runs once per row.

12. while vs for

Use	When
for	You know how many repeats: a count, or every character of a string
while	You repeat until something becomes true, and can't say how many passes that takes

Asking the user for input is the classic while case — you have no idea how many attempts they will need:

password = ""
while password != "python":
    password = input("Password: ").strip()
print("Welcome!")

password = "" before the loop makes the first check False, so the loop runs at least once. The input() inside is the change that can turn the condition False.

What's next

See workshop_4/ for runnable examples, and your homework in python-127-homework-4.