Loops

Have you ever needed to repeat an action multiple times, like counting numbers or printing a message repeatedly? Instead of duplicating lines of code, loops simplify the process by automating repetition.

Python provides two main types of loops: for loops and while loops. Let’s dive in!

Loops allow you to write a block of code once and execute it multiple times. They are essential for automating repetitive tasks, saving time and effort.

Types of loops

for Loops: Iterating over a sequence

A for loop repeats an action for every item in a sequence, such as a list, a string, or a range of numbers.

Syntax:

for item in sequence:
    # do something with item

Example:

for number in [1, 2, 3, 4, 5]:
    print("This is number:", number)

Explanation: The loop iterates over the list [1, 2, 3, 4, 5], and for each iteration, the variable number takes one value from the list. It prints each value during the process.

Using range() with for Loops

If you need a sequence of numbers but don’t have a predefined list, use the range() function. It generates a sequence of numbers.

Example:

for i in range(5):
    print("Hello!")

Explanation: range(5) generates numbers from 0 to 4. The loop runs 5 times, printing "Hello!" on each iteration.

while Loops: Repeating based on a condition

A while loop continues executing as long as a condition is true. Be cautious, though. If the condition never becomes false, you’ll create an infinite loop.

Syntax:

while condition:
    # do something

Example:

counter = 1
while counter <= 5:
    print("Counting:", counter)
    counter += 1

Explanation: This loop prints numbers from 1 to 5. The variable counter increments by 1 on each iteration. When counter reaches 6, the condition counter <= 5 becomes false, and the loop ends.

Controlling Loops

Breaking out of a Loop with break

Sometimes, you may need to exit a loop before it naturally ends. Use the break statement to stop the loop immediately.

Example:

for number in range(10):
    if number == 5:
        break  # Exit the loop when number equals 5
    print(number)

Explanation: This loop prints numbers from 0 to 4. When number reaches 5, the break statement exits the loop.

Skipping steps with continue

If you want to skip the rest of the loop’s code for the current iteration and move to the next one, use continue.

Example:

for number in range(5):
    if number == 2:
        continue  # Skip when number equals 2
    print(number)

Explanation: This code prints numbers 0, 1, 3, and 4, skipping 2. The continue statement skips the rest of the loop’s body when the condition is met.

Practical examples

Example 1: Calculating the sum of numbers

total = 0
for number in range(1, 6):  # Numbers 1 to 5
    total += number
print("Total:", total)

Output:

Total: 15

Explanation: The loop adds each number from 1 to 5 to the variable total, which accumulates the sum.

Example 2: Password check

password = "secret"
attempt = ""
attempts = 0

while attempt != password and attempts < 3:
    attempt = input("Enter password: ")
    attempts += 1

if attempt == password:
    print("Access granted!")
else:
    print("Access denied!")

Explanation: This loop allows the user three attempts to guess the password. If the correct password is entered within three tries, it prints "Access granted!" Otherwise, it denies access.

Best practices for Loops

• Avoid infinite Loops: Ensure conditions in while loops will eventually become false. For example:

  # Infinite loop (avoid!)
  while True:
      print("This never ends!")
  

• Use descriptive Loop variables: Choose variable names that clearly represent their role.

  # Good
  for student in students:
      print(student)
  
  # Vague
  for s in students:
      print(s)
  

• Use range() efficiently: Customize the range with start, stop, and step values.

  for i in range(2, 10, 2):  # Start at 2, go up to 10, increment by 2
      print(i)
  

Conclusion

Loops are indispensable tools in programming. With for and while loops, you can handle repetitive tasks efficiently, iterating over sequences or running until a condition is met.

By mastering break and continue, you gain finer control over how loops execute, enabling you to solve a wide range of problems with ease.