Chapter 7: for Loops and range() — Definite Iteration#

7.1 When You Know How Many Times to Repeat#

while loops are ideal when you don't know in advance how many times you'll repeat (e.g., "keep asking until the user types -1"). But often you DO know exactly how many times you want to repeat — "do this exactly 10 times," or "do this once for every character in a string." For these cases, Python gives you a more direct tool: the for loop.

for i in range(5):
    print(i)

Output:

0
1
2
3
4

This is functionally equivalent to the while loop version from Chapter 6, but the for loop handles initialization and update automatically — you only need to specify the test (implicitly, via range()).

7.2 The range() Function#

range() produces a sequence of integers. It has three forms:

Form 1: range(stop)#

Produces integers from 0 up to (not including) stop.

for i in range(5):
    print(i)
# 0, 1, 2, 3, 4   -- five values, stop NOT included

Form 2: range(start, stop)#

Produces integers from start up to (not including) stop.

for i in range(3, 8):
    print(i)
# 3, 4, 5, 6, 7

Form 3: range(start, stop, step)#

Produces integers from start, advancing by step each time, stopping before reaching stop.

for i in range(0, 10, 2):
    print(i)
# 0, 2, 4, 6, 8

for i in range(10, 0, -1):
    print(i)
# 10, 9, 8, 7, 6, 5, 4, 3, 2, 1   -- counting DOWN

for i in range(1, 20, 5):
    print(i)
# 1, 6, 11, 16

Critical rule: stop is always exclusive. This mirrors string slicing's [start:stop] rule from Week 1 — it's not a coincidence. Python is consistent about this design choice across the language.

range() Produces No Values When Misconfigured#

for i in range(5, 5):
    print(i)         # nothing prints — start equals stop
for i in range(10, 5):
    print(i)         # nothing prints — positive step can't reach a smaller stop
for i in range(5, 10, -1):
    print(i)         # nothing prints — negative step can't reach a larger stop

This is not an error — range() just produces an empty sequence, and the loop body simply never executes. This is a common source of silent bugs: the program runs without crashing, but does nothing, and it's easy to miss.

7.3 for vs while — Choosing the Right Tool#

Both loops are equally powerful — anything written with one can be rewritten with the other. The choice is about which one expresses your intent more clearly.

# for loop version
for i in range(5):
    print(i)

# equivalent while loop version
i = 0
while i < 5:
    print(i)
    i += 1

The for version is shorter and removes the chance of forgetting to update i — Python does that automatically. This is why for loops are generally preferred whenever the number of iterations is known.

7.4 The Accumulator Pattern#

One of the most common and important loop patterns is the accumulator: a variable that builds up a result across many iterations.

# Sum the numbers 1 through 10
total = 0                  # initialize the accumulator BEFORE the loop
for i in range(1, 11):
    total = total + i      # update the accumulator INSIDE the loop
print(total)                # 55

Trace:

The accumulator pattern generalizes far beyond sums:

# Product (factorial) accumulator
factorial = 1               # note: starts at 1, not 0, for multiplication!
for i in range(1, 6):
    factorial = factorial * i
print(factorial)             # 120 (5!)

# String-building accumulator
result = ""                 # starts as empty string
for i in range(5):
    result = result + str(i)
print(result)                # "01234"

# Counting accumulator
count = 0
for i in range(1, 101):
    if i % 7 == 0:
        count = count + 1
print(count)                 # how many numbers 1-100 are divisible by 7

Why does the accumulator's starting value matter? For a sum, start at 0 — adding 0 changes nothing, so it's a safe starting point ("the additive identity"). For a product, start at 1 — multiplying by 1 changes nothing ("the multiplicative identity"). If you started a product accumulator at 0, every multiplication would give 0!

7.5 Iterating Over a Range with range(len(s))#

Combining range() with len() lets you process a string position by position:

s = "hello"
for i in range(len(s)):
    print(i, s[i])

Output:

0 h
1 e
2 l
3 l
4 o

This pattern — for i in range(len(s)): — is extremely common. It gives you access to both the index and, via s[i], the character at that index. (Chapter 8 introduces an even more direct way to loop over characters without needing the index at all.)

7.6 A Complete Example: Counting Vowels#

word = input("Enter a word: ")
vowel_count = 0
vowels = "aeiouAEIOU"

for i in range(len(word)):
    if word[i] in vowels:
        vowel_count = vowel_count + 1

print(f"'{word}' has {vowel_count} vowels.")

This combines: for loop, range(len(...)), indexing, the in operator, and the accumulator pattern — five ideas working together, exactly the kind of combination you'll be writing constantly from here on.

7.7 Common Mistakes with for Loops#

Mistake 1: Off-by-One with range()#

# Want to print 1 through 10:
for i in range(10):       # WRONG -- prints 0 through 9
    print(i)

for i in range(1, 11):    # CORRECT -- prints 1 through 10
    print(i)

Mistake 2: Trying to Modify the Loop Variable to Change Iteration Count#

for i in range(5):
    print(i)
    i = 10   # This does NOT affect the loop! range() was already computed.
# Still prints 0, 1, 2, 3, 4 -- range() generates its values independently

This surprises many beginners. range(5) produces the sequence 0, 1, 2, 3, 4 once, in advance (conceptually). Reassigning i inside the body only affects that one pass — the loop still moves on to the next value range() was going to give it regardless.

Mistake 3: Using for When You Need while (and Vice Versa)#

# AWKWARD: using a for loop to simulate "until a condition" — possible
# but unnatural, and caps the iterations arbitrarily
for attempt in range(1000):   # "1000" is an arbitrary guess at a max
    guess = input("Guess the number: ")
    if guess == "42":
        break   # (break is covered in Chapter 10)

# BETTER: while loop expresses "keep going until correct" directly
guess = input("Guess the number: ")
while guess != "42":
    guess = input("Guess the number: ")

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Chapter 6–7 Practice Problems#

Set A: Tracing while Loops#

1. Trace this loop by hand. What does it print?

   n = 3
   while n > 0:
       print(n)
       n -= 1
   print("Liftoff!")

1. Trace this loop. How many times does the body execute?

   x = 1
   while x < 50:
       x = x * 2
   print(x)

1. What's wrong with this code? Fix it.

   count = 0
   while count < 5:
       print("Counting:", count)

Set B: Writing while Loops#

1. Write a while loop that prints all multiples of 3 from 3 to 30 (inclusive).

1. Write a sentinel-controlled loop that keeps asking the user for positive numbers and stops when they enter 0 or a negative number, printing the running total each time.

1. Write a while loop that computes how many times you can divide 100 by 2 before the result is less than 1 (don't use //, use /).

Set C: range() and for Loops#

1. What values does range(4, 20, 4) produce?

1. What values does range(20, 4, -4) produce?

1. Write a for loop that prints the squares of the numbers 1 through 10.

1. Write a for loop using the accumulator pattern to compute the sum of all even numbers from 2 to 100.

1. What's the bug in this code, and what does it actually print?

    for i in range(1, 5):
        print(i)

(The intent was to print 1 through 5 inclusive.)

Set D: Challenge#

1. Without running it, determine exactly what this prints, including how many lines:

    total = 0
    for i in range(1, 10, 2):
        total += i
        print(i, total)

1. Write a for loop that builds the string "a-b-c-d-e" by accumulating characters from "abcde" with a "-" separator (but no trailing dash after the final character — think carefully about this edge case).

1. A while loop and a for loop can each be rewritten as the other. Rewrite this for loop as an equivalent while loop:

    for i in range(2, 20, 3):
        print(i)

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Chapter Summary#

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Next: Chapter 8 — Nested Loops and Iterating Over Strings