5. Loops & Iteration

Repeating work without repeating yourself

A loop runs a block of code again and again. This is how programs process every item in a list, keep asking until input is valid, or count from 1 to a million without you typing a million lines. Running the body once is called an iteration.

The while loop

A while loop repeats as long as a condition stays true. It checks the condition, runs the body, then checks again. Use it when you do not know in advance how many times you will repeat — for example, "keep going until the user types 'quit'."

count = 1
while count <= 3:
    print("Hello", count)
    count = count + 1      # move toward the exit!
print("Done")

Line by line: count starts at 1, the condition 1 <= 3 is true so the body prints and increments to 2, then 3, then 4. At 4 <= 3 the condition is false and the loop ends. The variable that drives the loop is the loop counter.

Infinite loops

If the condition never becomes false, the loop runs forever — an infinite loop. The usual cause is forgetting to change the counter. Always make sure something inside the body moves you toward the exit condition.

# BUG: count never changes, so this never stops
count = 1
while count <= 3:
    print("stuck")   # prints forever

The for loop

A for loop is built for a known number of repetitions or for walking through every item in a collection. It bundles the setup, condition and update into one tidy line, which makes "off-by-one" mistakes rarer.

for i in range(1, 4):     # i takes 1, then 2, then 3
    print("Hello", i)

for letter in "cat":      # iterate over each character
    print(letter)         # c, a, t

for vs while

Aspect	for loop	while loop
Best when	You know the count, or iterate a collection	You repeat until a condition changes
Counter	Managed automatically	You manage it by hand
Infinite-loop risk	Lower	Higher (easy to forget the update)
Typical use	"for each student, print the name"	"while not logged in, keep asking"

break and continue

Two statements fine-tune a loop. break exits the loop immediately. continue skips the rest of the current iteration and jumps to the next one. Use them sparingly — a clear condition is usually better than a loop peppered with jumps.

for n in range(1, 100):
    if n == 5:
        break        # stop entirely at 5
    if n % 2 == 0:
        continue     # skip even numbers
    print(n)         # prints 1, 3

Key idea: loops repeat work; for suits a known number of steps, while suits "until something changes," and every loop must be able to end.