4.5. Pseudorandomness

You will have noticed that every time you generate random numbers, the values change! However, recreating the same set of random numbers is often important to ensure your results are reproducible. Lucky for us, the random numbers generated by the computer aren’t truly random. They’re what we call pseudorandom. If you kept generating random numbers for long enough, the sequence would repeat!

To force our random numbers to start at the same point in the sequence every time, we can use a seed. We use

random.seed(seed)

where seed is an integer. Here’s an example. You’ll see that the program will always generate the same results.

import random

random.seed(0)
print(random.random())
print(random.random())
print(random.random())

If you don’t use a seed you will get different results every time.

You may have seen seeds mentioned in computer games when you want to reproduce the same conditions e.g. generating the same starting map.

Question 1

You run the following program

import random

random.seed(0)
print(random.randrange(6))

The program outputs

3

What happens when you run the program again?

1. You are guaranteed to get the number 3 again.

2. It’s impossible to say as you’ll get either a 0, 1, 2, 3, 4 or 5.

Solution

A

Since a seed is set we will get the same value each time we run this program

Question 2

You run the following program

import random

random.seed(0)
print(random.randrange(6))
print(random.randrange(6))
print(random.randrange(6))

Which of the following could be possible outputs from this program given your knowledge from question 1? Select all that apply.

1. 3
   1
   4
   

2. 3
   3
   3
   

3. 4
   5
   6
   

4. 3
   9
   4
   

5. 3
   0
   0
   

Solution

A, B and E

From question 1, we know that

import random

random.seed(0)
print(random.randrange(6))

will output 3.

We don’t know what the next two numbers will be, but random.randrange(6) will give us integers between 0 (included) and 6 (not included). Hence we could get a 3 followed by numbers either 0, 1, 2, 3, 4, 5.

You’ll see that we actually get the numbers 3, 3 and 0.

import random

random.seed(0)
print(random.randrange(6))
print(random.randrange(6))
print(random.randrange(6))

3
3
0

Question 3

You run the following program

import random

random.seed(0)
print(random.randrange(6))
print(random.randrange(6))
print(random.randrange(6))

random.seed(0)
print(random.randrange(6))
print(random.randrange(6))
print(random.randrange(6))

Which of the following could be possible outputs from this program given your knowledge from question 1? Select all that apply.

1. 3
   3
   0
   3
   5
   4
   

2. 5
   5
   1
   5
   5
   1
   

3. 3
   3
   0
   3
   3
   0
   

4. 3
   3
   0
   4
   1
   3
   

Solution

C

When we reset the random number generator we’ll get the same sequence of random numbers. In this case the seed is rest on lines 3 and lines 8. This means we are looking for three numbers that will repeat. From question 1 we know the first number in the sequence will be a 3, so the third option is the only one that will be possible.

import random

random.seed(0)
print(random.randrange(6))
print(random.randrange(6))
print(random.randrange(6))

random.seed(0)
print(random.randrange(6))
print(random.randrange(6))
print(random.randrange(6))

3
3
0
3
3
0