The for Loop and range()

Introduction

The for loop in Python iterates through elements of a sequence (list, string, range, etc.). It’s preferred when we know how many iterations we’ll make.

Basic Syntax

for element in sequence:
    # code block
    # executes for each element

Iterating through a list:

fruits = ["apple", "pear", "banana"]

for fruit in fruits:
    print(fruit)

# apple
# pear
# banana

Iterating through a string:

for letter in "Python":
    print(letter)

# P y t h o n (on separate lines)

The range() Function

range() generates a sequence of numbers for iteration.

range(stop) - from 0 to stop-1:

for i in range(5):
    print(i)

# 0 1 2 3 4 (does NOT include 5!)

range(start, stop) - from start to stop-1:

for i in range(2, 6):
    print(i)

# 2 3 4 5 (does NOT include 6!)

range(start, stop, step) - with step:

# Every 2
for i in range(0, 10, 2):
    print(i)
# 0 2 4 6 8

# Countdown
for i in range(5, 0, -1):
    print(i)
# 5 4 3 2 1

# Negative step
for i in range(10, 0, -2):
    print(i)
# 10 8 6 4 2

Converting to list:

print(list(range(5)))        # [0, 1, 2, 3, 4]
print(list(range(2, 7)))     # [2, 3, 4, 5, 6]
print(list(range(0, 10, 3))) # [0, 3, 6, 9]

Iterating with Index

Using range and len:

fruits = ["apple", "pear", "banana"]

for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")

# 0: apple
# 1: pear
# 2: banana

Using enumerate (recommended):

fruits = ["apple", "pear", "banana"]

for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

# 0: apple
# 1: pear
# 2: banana

# With different start
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")

# 1. apple
# 2. pear
# 3. banana

Iterating Through Dictionaries

student = {"name": "Ana", "age": 20, "grade": 9}

# Keys only (implicit)
for key in student:
    print(key)
# name age grade

# Keys explicitly
for key in student.keys():
    print(key)

# Values
for value in student.values():
    print(value)
# Ana 20 9

# Keys and values
for key, value in student.items():
    print(f"{key}: {value}")
# name: Ana
# age: 20
# grade: 9

break and continue Statements

break - exits the loop:

for i in range(10):
    if i == 5:
        break
    print(i)

# 0 1 2 3 4

continue - skips to next iteration:

for i in range(5):
    if i == 2:
        continue
    print(i)

# 0 1 3 4 (without 2)

The else Clause with for

Executes if loop finished normally (without break):

# With normal termination
for i in range(3):
    print(i)
else:
    print("Finished!")
# 0 1 2 Finished!

# With break
for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("Finished!")  # Doesn't execute!
# 0 1 2

Application - search:

numbers = [1, 3, 5, 7, 9]
target = 5

for number in numbers:
    if number == target:
        print(f"Found: {number}")
        break
else:
    print("Not found")

# Found: 5

Nested Loops

for i in range(3):
    for j in range(3):
        print(f"({i}, {j})", end=" ")
    print()  # New line

# (0, 0) (0, 1) (0, 2)
# (1, 0) (1, 1) (1, 2)
# (2, 0) (2, 1) (2, 2)

Multiplication table:

for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i*j:3}", end=" ")
    print()

#   1   2   3   4   5
#   2   4   6   8  10
#   3   6   9  12  15
#   4   8  12  16  20
#   5  10  15  20  25

Simultaneous Iteration with zip

names = ["Ana", "Bob", "Chris"]
ages = [20, 25, 30]

for n, a in zip(names, ages):
    print(f"{n} is {a} years old")

# Ana is 20 years old
# Bob is 25 years old
# Chris is 30 years old

Common Patterns

Sum of elements:

numbers = [1, 2, 3, 4, 5]
total = 0
for n in numbers:
    total += n
print(total)  # 15

# Or simply: sum(numbers)

Finding maximum:

numbers = [3, 7, 2, 9, 5]
maximum = numbers[0]
for n in numbers[1:]:
    if n > maximum:
        maximum = n
print(maximum)  # 9

# Or simply: max(numbers)

Filtering:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = []
for n in numbers:
    if n % 2 == 0:
        evens.append(n)
print(evens)  # [2, 4, 6, 8, 10]

Common Mistakes

1. Modifying list during iteration

# WRONG - unpredictable behavior!
lst = [1, 2, 3, 4, 5]
for x in lst:
    if x % 2 == 0:
        lst.remove(x)

# CORRECT - iterate through a copy
for x in lst[:]:
    if x % 2 == 0:
        lst.remove(x)

2. Confusion with range

# range(5) = 0,1,2,3,4 (doesn't include 5!)
for i in range(5):
    print(i)  # 0 1 2 3 4

# For 1-5:
for i in range(1, 6):
    print(i)  # 1 2 3 4 5

3. Not using enumerate

# Inefficient
for i in range(len(lst)):
    print(i, lst[i])

# Better
for i, elem in enumerate(lst):
    print(i, elem)

Key Points for Exam

• range(n) generates 0 to n-1
• range(a, b) generates a to b-1
• range(a, b, step) with step (can be negative)
• enumerate() for index + element
• zip() for parallel iteration
• break exits, continue skips
• else with for executes without break
• Don’t modify list during iteration!

Review Questions

1. What does list(range(2, 8, 2)) display?
2. When does the else block execute in for?
3. How do you iterate with both index and element?
4. What happens if you modify a list during iteration?