Loops

#python #beginners

Why loops matter

A loop is a control flow structure that executes a block of code repeatedly as long as a condition holds. They matter for three reasons: they reduce redundancy (no manually retyping the same statement), they save time and space (keeping code short and maintainable), and they handle dynamic data (processing a list or user input of any size without rewriting the code for each possible length).

for loops

Use a for loop when you know exactly what you're iterating over - a list, a range of numbers, the characters in a string:

items = ['sugar', 'salt', 'flour', 'soap', 'oil']
for i in items:
    print(i)

# output
# sugar
# salt
# flour
# soap
# oil

for loops pair naturally with range() when you need to repeat something a fixed number of times rather than iterate over existing items:

for i in range(5):
    print("Attempt number", i)

while loops

Use a while loop when the number of repetitions isn't known in advance and depends on a condition staying true:

count = 0
while count < 5:
    print(count)
    count += 1

The loop keeps running as long as count < 5 evaluates to True. Forgetting to update count inside the loop is the classic mistake here - it creates an infinite loop, since the condition never becomes false.

break and continue

break exits the loop immediately, skipping any remaining iterations entirely.
continue skips just the current iteration and moves on to the next one, without exiting the loop.

for num in range(10):
    if num == 5:
        break          # stop the loop entirely once we hit 5
    print(num)
# prints 0, 1, 2, 3, 4

for num in range(10):
    if num % 2 == 0:
        continue        # skip even numbers, keep looping
    print(num)
# prints 1, 3, 5, 7, 9

enumerate()

enumerate() gives you both the index and the value while looping over something - useful whenever you need to know where an item is, not just what it is:

items = ['sugar', 'salt', 'flour']
for index, item in enumerate(items):
    print(index, "->", item)

# output
# 0 -> sugar
# 1 -> salt
# 2 -> flour

Without enumerate(), getting the index would mean manually tracking a separate counter variable and incrementing it yourself each pass through the loop.

Practical examples

Combining several of these — looping through a list, using enumerate() for numbering, and continue to skip an item that doesn't meet a condition:

prices = [180, 320, 85, 60, 0]

for index, price in enumerate(prices):
    if price == 0:
        continue   # skip invalid entries
    print(f"Item {index + 1}: KES {price}")

What I understood from this

break and continue felt interchangeable at first glance, but they solve different problems - break is "I'm done here entirely," continue is "not this one, but keep going." Mixing them up either cuts a loop short when it should have kept processing the rest of the list, or keeps a loop running past a point where it should have stopped. enumerate() was the other thing that changed how I write loops day to day - before I understood it, I was manually creating and incrementing a counter variable alongside every for loop that needed one, which is exactly the kind of repetitive bookkeeping loops are supposed to eliminate in the first place.

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