Python's For Loop Made the Counter Variable Disappear (Here's How)

With a while loop, I had to manually create a counter, check it, and update it every single time. Python's for loop handles that bookkeeping for you.

The basic syntax

for i in range(1, 11):
    print(i)

range(1, 11) generates the numbers 1 through 10, one at a time, each getting assigned to i for that pass through the loop. Same rule as everywhere else in Python: the upper bound is excluded, which is why it's 11 and not 10 to actually reach ten.

It's not just for numbers

This is the part that actually surprised me. A for loop can iterate over pretty much any collection of values, not just a range of numbers.

for letter in "Darshan":
    print(letter)
# prints D, a, r, s, h, a, n, one per line

numbers = [10, 20, 30]
for n in numbers:
    print(n)

coordinates = (1, 2, 3)
for c in coordinates:
    print(c)

Lists and tuples are getting their own dedicated posts later in this series, so I'm not going deep on them here, but it's worth knowing now that for loops work with them the same natural way they work with strings and ranges.

Controlling the step

range() actually takes a third value: a step, controlling how much it increments each time.

for i in range(1, 10, 2):
    print(i)   # 1, 3, 5, 7, 9

for i in range(1, 10, 3):
    print(i)   # 1, 4, 7

You can also go backward with a negative step:

for i in range(10, 0, -1):
    print(i)   # 10, 9, 8, ..., 1

Rewriting the print-10-times task

Just to compare directly against the while loop version from the last post:

for i in range(1, 11):
    print("Darshan")

No manual counter, no manual increment. The range() object handles all of that internally.

Exercise: population decline over the last 10 years

Say a town's current population is 10,000, and it's been growing 10% per year. Working backward, what was the population in each of the last ten years?

current_population = 10000

for i in range(10, 0, -1):
    print(i, current_population)
    current_population = current_population / 1.1

The reverse range (10, 0, -1) walks backward year by year, and dividing by 1.1 each time undoes one year's 10% growth, working from the present back toward the past.

Common mistakes I'd flag here

• Forgetting range()'s upper bound is exclusive, the same trap as while loop conditions, just in a new shape.

• Getting the step direction wrong. range(10, 0, -1) needs a negative step to count downward, a positive step there produces an empty range instead of an error, which can be a confusing silent failure.

• Assuming for loops only work on numbers, and missing that strings, lists, and tuples all iterate naturally too.

Quick recap

• for i in range(start, stop) loops over a sequence of numbers, with stop excluded, and handles the counter automatically.

• for loops work over strings, lists, and tuples directly, not just number ranges.

• range()'s optional third argument sets the step size, and a negative step counts backward.

• Combining a reverse range with a formula was enough to reconstruct a decade of population history in four lines.

What's next

Next up: nested for loops, and why you'd ever want a loop inside another loop.