Off-by-one

301.14 Random Number Generation

Random Number Generation — Note 3 of 3

Avoid fencepost errors.

What you need to know

Off-by-one errors with random numbers happen when you use the wrong range boundaries. Remember that randint(a, b) includes both a and b.

What to know

randint(1, 6) includes both 1 and 6
randint(0, 9) gives 10 possible values (0 through 9)
Don't confuse with range() which excludes the end
Test with the minimum and maximum values to verify

Code examples

Off-by-one check

python

import random
# Want numbers 1 to 10 inclusive
num = random.randint(1, 10)  # Correct: includes 10

# Common mistake with range:
# range(1, 10) gives 1-9 (excludes 10)
# randint(1, 10) gives 1-10 (includes 10)

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Specification notes for programming are in Paper 2.