4

I have the following code:

import random

rand1 = random.Random()
rand2 = random.Random()

rand1.seed(0)
rand2.seed(0)

rand1.jumpahead(1)
rand2.jumpahead(2)

x = [rand1.random() for _ in range(0,5)]
y = [rand2.random() for _ in range(0,5)]

According to the documentation of jumpahead() function I expected x and y to be (pseudo)independent sequences. But the output that I get is:

x: [0.038378463064751012, 0.79353887395667977, 0.13619161852307016, 0.82978789012683285, 0.44296031215986331]

y: [0.98374801970498793, 0.79353887395667977, 0.13619161852307016, 0.82978789012683285, 0.44296031215986331]

If you notice, the 2nd-5th numbers are same. This happens each time I run the code.

Am I missing something here?

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  • 2
    I don't know much about Python, but the fact that you're using a hardcoded value as the seed might have something to do with it. Commented Feb 25, 2011 at 1:24
  • 1
    Upvoted because it's an interesting problem. I would shy away from declaring something a bug, however, if you need to include the statement "Am I missing something here?" Commented Feb 25, 2011 at 1:31

2 Answers 2

Reset to default
5 
rand1.seed(0)
rand2.seed(0)

You initialize them with the same values so you get the same (non-)randomness. Use some value like the current unix timestamp to seed it and you will get better values. But note that if you initialize two RNGs at the same time with the current time though, you will get the same "random" values from them of course.

Update: Just noticed the jumpahead() stuff: Have a look at How should I use random.jumpahead in Python - it seems to answer your question.

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2 Comments

To elaborate: Most programs that need pseudorandom numbers seed the generator with the execution time or some other value determined at runtime.
How does this explain that the first value is different and the following values are equal in both sequences?
2

I think there is a bug, python's documentation does not make this as clear as it should.

The difference between your two parameters to jumpahead is 1, this means you are only guaranteed to get 1 unique value (which is what happens). if you want more values, you need larger parameters.

EDIT: Further Explanation

Originally, as the name suggests, jumpahead merely jumped ahead in the sequence. Its clear to see in that case where jumping 1 or 2 places ahead in the sequence would not produce independent results. As it turns out, jumping ahead in most random number generators is inefficient. For that reason, python only approximates jumping ahead. Because its only approximate, python can implement a more effecient algorithm. However, the method is "pretending" to jump ahead, passing two similiar integers will not result in a very different sequence.

To get different sequences you need the integers passed in to be far apart. In particular, if you want to read a million random integers, you need to seperate your jumpaheads by a million.

As a final note, if you have two random number generators, you only need to jumpahead on one of them. You can (and should) leave the other in its original state.

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3 Comments

I used jumpahead of 5 and 6 and the outputs were - [0.49573648501739465, 0.31398691064796336, 0.084671360334822476, 0.82978789012683285, 0.44296031215986331, 0.77763679686002374, 0.81291322861811088, 0.96007877998132773, 0.44789551387102489, 0.79695926695921226] [0.77619832841070591, 0.12122767752895014, 0.67710017312526105, 0.82978789012683285, 0.44296031215986331, 0.77763679686002374, 0.81291322861811088, 0.96007877998132773, 0.44789551387102489, 0.79695926695921226]
According to your hypothesis, I should have got atleast 5 unique values.
Oh i see .. you meant it depends upon the difference in the jumps

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