Chapter 4: The for Loop — One by One, In Order¶

Definite vs Indefinite Loops¶

	while	for
Type	Indefinite loop	Definite loop
When	While condition is True	Until end of sequence
Count	May not know before running	Fixed by sequence length

for iterates through a sequence (list, string, range, etc.) from start to finish, one element at a time.

for Basic Structure¶

for variable in sequence:
    code to execute

for i in [1, 2, 3, 4, 5]:
    print(i)

i takes each value from the list one by one.

range() — Creating Number Sequences¶

range(stop)           # 0 to stop-1
range(start, stop)    # start to stop-1
range(start, stop, step)  # start to stop-1, stepping by step

list(range(3))           # [0, 1, 2]
list(range(1, 5))        # [1, 2, 3, 4]
list(range(1, 10, 2))    # [1, 3, 5, 7, 9]
list(range(5, 0, -1))    # [5, 4, 3, 2, 1]

for i in range(5):
    print(i, end=" ")

0 1 2 3 4

Lists¶

A list is an ordered collection of values.

animals = ["dog", "cat", "lion", "python", "fox", "wolf"]
print(animals[0])    # dog   (first item, index starts at 0)
print(animals[2])    # lion
print(animals[-1])   # wolf  (last item)

for animal in animals:
    print("Hello,", animal)

Hello, dog
Hello, cat
Hello, lion
Hello, python
Hello, fox
Hello, wolf

Iterating Over Strings¶

Strings are sequences too. You can extract one character at a time.

for ch in "PYTHON":
    print(ch * 2, end="")

PPYYTTHHOONN

`_` — When You Don't Need the Value¶

Use _ when only the iteration count matters and the variable value is unused.

for _ in range(3):
    print("Hello!")

Hello!
Hello!
Hello!

Nested for Loops¶

You can place a for loop inside another for loop.

for i in range(3):       # outer: 3 times
    for j in range(4):   # inner: 4 times
        print("*", end="")
    print()

****
****
****

Each time the outer loop runs once, the inner loop runs from start to finish.

# Full multiplication table
for i in range(2, 10):
    for j in range(1, 10):
        print(f"{i} × {j} = {i*j}", end="\t")
    print()

break and continue¶

They work the same way as in while loops.

# break: exit loop immediately when condition is met
for i in range(10):
    if i == 5:
        break
    print(i, end=" ")

0 1 2 3 4

# continue: skip the rest of this iteration
for i in range(10):
    if i % 2 == 0:
        continue
    print(i, end=" ")

1 3 5 7 9

len() — Getting the Length¶

A = [10, 20, 30, 40, 50]
print(len(A))    # 5

for i in range(len(A)):
    print(i, A[i])

Use range(len(A)) when you need both the index and the value.

The `in` Operator — Check Membership¶

print(3 in [1, 2, 3, 4, 5])    # True
print(6 in [1, 2, 3, 4, 5])    # False
print("a" in "apple")           # True

Combined with conditionals:

fruits = ["apple", "banana", "grape"]
target = input("Find fruit: ")
if target in fruits:
    print(target, "is available!")
else:
    print(target, "is not available.")

Introduction to Algorithms¶

Greedy Algorithm — Making Change¶

Use the largest denomination first, as many as possible.

amount = int(input("Change amount: "))
coins = [500, 100, 50, 10, 1]
result = []

for coin in coins:
    count = amount // coin
    amount = amount % coin
    result.append((coin, count))

for coin, count in result:
    if count > 0:
        print(f"{coin}: {count} coins")

Change amount: 1234
500: 2 coins
100: 2 coins
50: 0 coins
10: 3 coins
1: 4 coins

Brute Force — Exhaustive Search¶

Try every possible combination.

# Find two-digit numbers whose digits sum to 10
for n in range(10, 100):
    ones = n % 10
    tens = n // 10
    if ones + tens == 10:
        print(n, end=" ")

19 28 37 46 55 64 73 82 91

Practice Missions¶

Mission 1: Star Pyramid (for version)¶

*
**
***
****
*****

n = 5
for i in range(n):
    # Write your code here
    pass

Mission 2: Find the Minimum¶

Without using min(), find the smallest value in a list using a for loop.

numbers = [34, 7, 23, 32, 5, 62]
# Write your code here

# Minimum: 5

Mission 3: FizzBuzz¶

Print numbers 1 to 30, but: - Multiples of 3 → "Fizz" - Multiples of 5 → "Buzz" - Multiples of 15 → "FizzBuzz"

for i in range(1, 31):
    # Write your code here
    pass

# 1 2 Fizz 4 Buzz Fizz 7 8 Fizz Buzz 11 Fizz 13 14 FizzBuzz ...

Mission 4: Greedy — Minimum Coins¶

Calculate the minimum number of coins for a given amount.

amount = int(input("Amount: "))
coins = [500, 100, 50, 10, 1]
total_count = 0
# Write your code here

# Amount: 1730
# Minimum coins: 7 (500×3, 100×2, 10×3)

Mission 5: Brute Force — PIN Cracker¶

Print all 4-digit PINs (0000–9999) whose digits sum to a given value.

target = int(input("Digit sum: "))
# Use nested for loops or range(10000)

# Digit sum: 5
# 0005 0014 0023 ... 5000

Key Summary¶

Concept	Description
`for variable in sequence:`	Iterate through sequence one element at a time
`range(n)`	Numbers from 0 to n-1
`range(a, b, step)`	Numbers from a to b-1, stepping by step
List	`[val1, val2, ...]` — ordered collection, index starts at 0
String iteration	Can extract one character at a time
`_`	When the loop variable is not needed
Nested for	for inside for — 2D iteration
`len()`	Length of a sequence
`in`	Check if a value exists in a sequence → bool
`break`	Exit loop immediately
`continue`	Skip this iteration, move to next
Greedy algorithm	Make the best choice at each step
Brute force	Try all possible combinations