For Loops

Python For Loops: A Comprehensive Guide

For loops are a fundamental control flow statement in Python, used to iterate over a sequence (like a list, tuple, string, or range) and execute a block of code for each item in that sequence. They are incredibly powerful for automating repetitive tasks.

1. Basic Syntax

The basic syntax of a for loop in Python is:

for item in sequence:
  # Code to be executed for each item
  # This block is indented

• for keyword: Starts the loop.
• item: A variable that takes on the value of each element in the sequence during each iteration of the loop. You can choose any valid variable name.
• in keyword: Connects the item variable to the sequence.
• sequence: The iterable object (list, tuple, string, range, etc.) that you want to loop through.
• : (colon): Marks the end of the for statement and the beginning of the loop's code block.
• Indentation: Crucially important in Python! The code block that will be executed for each item must be indented (usually 4 spaces).

2. Looping Through Lists

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

for fruit in fruits:
  print(fruit)

Output:

apple
banana
cherry

In this example, the fruit variable takes on the value "apple" in the first iteration, "banana" in the second, and "cherry" in the third. The print(fruit) statement is executed for each of these values.

3. Looping Through Strings

Strings are also iterable, meaning you can loop through their characters:

message = "Hello"

for char in message:
  print(char)

Output:

H
e
l
l
o

4. Looping Through Tuples

Tuples work similarly to lists:

coordinates = (10, 20, 30)

for coord in coordinates:
  print(coord)

Output:

10
20
30

5. The range() Function

The range() function is often used with for loops to generate a sequence of numbers.

• range(stop): Generates numbers from 0 up to (but not including) stop.
• range(start, stop): Generates numbers from start up to (but not including) stop.
• range(start, stop, step): Generates numbers from start up to (but not including) stop, incrementing by step.

# Print numbers from 0 to 4
for i in range(5):
  print(i)

# Print numbers from 2 to 7
for i in range(2, 8):
  print(i)

# Print even numbers from 0 to 10
for i in range(0, 11, 2):
  print(i)

Output (first loop):

0
1
2
3
4

Output (second loop):

2
3
4
5
6
7

Output (third loop):

0
2
4
6
8
10

6. break and continue Statements

• break: Immediately terminates the loop and transfers control to the statement following the loop.

numbers = [1, 2, 3, 4, 5]

for num in numbers:
  if num == 3:
    break  # Exit the loop when num is 3
  print(num)

Output:

1
2

• continue: Skips the rest of the current iteration and proceeds to the next iteration of the loop.

numbers = [1, 2, 3, 4, 5]

for num in numbers:
  if num == 3:
    continue  # Skip printing 3
  print(num)

Output:

1
2
4
5

7. The else Clause with for Loops

A for loop can have an optional else clause. The else block is executed only if the loop completes normally (i.e., it doesn't encounter a break statement).

numbers = [1, 2, 3, 4, 5]

for num in numbers:
  if num == 6:
    print("Found 6!")
    break
else:
  print("6 not found in the list.")

Output:

6 not found in the list.

If the list contained a 6, the break statement would have been executed, and the else block would not have been executed.

8. Nested For Loops

You can nest for loops inside each other to iterate over multiple sequences.

colors = ["red", "green", "blue"]
sizes = ["small", "medium", "large"]

for color in colors:
  for size in sizes:
    print(f"Color: {color}, Size: {size}")

Output:

Color: red, Size: small
Color: red, Size: medium
Color: red, Size: large
Color: green, Size: small
Color: green, Size: medium
Color: green, Size: large
Color: blue, Size: small
Color: blue, Size: medium
Color: blue, Size: large

9. Looping Through Dictionaries

You can loop through dictionaries in several ways:

• Looping through keys:

my_dict = {"name": "Alice", "age": 30, "city": "New York"}

for key in my_dict:
  print(key)

• Looping through values:

for value in my_dict.values():
  print(value)

• Looping through key-value pairs:

for key, value in my_dict.items():
  print(f"Key: {key}, Value: {value}")

Key Takeaways:

• For loops are essential for iterating over sequences.
• Indentation is crucial for defining the loop's code block.
• range() is a powerful function for generating sequences of numbers.
• break and continue provide control over loop execution.
• The else clause can be used to execute code after a loop completes normally.
• Nested loops allow you to iterate over multiple sequences.
• Dictionaries can be looped through using keys, values, or key-value pairs.