For Loops & Range - Counted Repetition
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Repeating a Specific Number of Times
"For loops allow us to continuously change the value of a loop variable. Best for repeating a certain number of times."
Understanding Range
range() generates a sequence of stepping-stone numbers
1 Argument:
range(stop)range(5)
0, 1, 2, 3, 4
Starts at 0, stops before 5
2 Arguments:
range(start, stop)range(2, 7)
2, 3, 4, 5, 6
Starts at 2, stops before 7
3 Arguments:
range(start, stop, step)range(0, 10, 2)
0, 2, 4, 6, 8
Leaps by 2, stops before 10
Critical Detail:
range() is non-inclusive - it stops BEFORE the end number. If you want numbers 1–10, use range(1, 11).
How For Loops Work
for ii in range(5):
print(ii)
What's happening:
# Iteration 1: ii = 0
0
# Iteration 2: ii = 1
1
# Iteration 3: ii = 2
2
# Iteration 4: ii = 3
3
# Iteration 5: ii = 4
4
Interactive For Loop & Stepping Stones Explorer
Woodland Stepping Stones Generated by
range():Configure range and click Run
COMPARISON For Loop vs. While Loop — Which Should You Choose? + Explore
# For Loop - when you know how many times
for ii in range(5):
print(ii)
# While Loop - when you don't know ahead of time
counter = 0
while counter < 5:
print(counter)
counter += 1
Mental Model: A
for loop is like a ticket dispenser that counts automatically. A while loop is best when the stopping condition depends on changing events.
CHEAT SHEET
Common for & range() Loop Patterns
+ Explore
- Count to N:
for ii in range(n) - Count from A to B:
for ii in range(a, b) - Count by steps:
for ii in range(0, 10, 2)# evens - Countdown:
for ii in range(10, 0, -1)
Woodland Milestone: You've crossed the stepping stones of
for and range()! Next, let's package reusable spells into Functions.
When you know how many steps to run, use a for loop with range(). Remember that range(5) counts 0, 1, 2, 3, 4—stopping before 5.
