Question

I’m doing this for a school project (so I can’t use any advanced features) and I’m using Python 2.6.6.

I have a list of numbers from 1 to 1000 and a seed (448 for example).

How can I generate a random sequence with that seed so that the numbers in my list will be in a different index ?

And is it possible, knowing the seed, to return the list in its original order ?

Answer 1

import random
SEED = 448

myList = [ 'list', 'elements', 'go', 'here' ]
random.seed(SEED)
random.shuffle(myList)

print myList

results in

['here', 'go', 'list', 'elements']

Your list is now pseudorandomized.

‘Pseudo’ is important, because all lists having the same seed and number of items will return in the same ‘random’ order. We can use this to un-shuffle your list; if it were truly random, this would be impossible.

Order = list(range(len(myList)))
# Order is a list having the same number of items as myList,
# where each position's value equals its index

random.seed(SEED)
random.shuffle(Order)
# Order is now shuffled in the same order as myList;
# so each position's value equals its original index

originalList = [0]*len(myList)   # empty list, but the right length
for index,originalIndex in enumerate(Order):
    originalList[originalIndex] = myList[index]
    # copy each item back to its original index

print originalList

results in

['list', 'elements', 'go', 'here']

Tada! originalList is now the original ordering of myList.