CODEWITHSUNDEEP
pythonlistsum
Martin Spasov
Martin Spasov

Reputation: 313

Generating random numbers with prefix

in the code below is it possible that it prints a number twice?

import random

n = 0
lista = (345, 348, 333, 347, 346, 340, 342, 349, 330, 331, 334, 335, 336, 337, 338, 339, 360,                         366, 368, 320, 324, 327, 328, 329, 380, 388, 389, 391, 392, 393)
while n < 150:
    x = random.randint(234325,876432)
    x = str(x)
    z =  str(random.choice(lista)) + x
    n += 1
    print z

Upvotes: 0

Views: 226

Answers (3)

atlasologist
atlasologist

Reputation: 3964

Yep.

Run it and compare the length of the list to the length of the list without duplicates. Only stop the loop if the numbers are different (there's a duplicate):

import random

listb = []

def random_numbers():
    n = 0
    lista = (345, 348, 333, 347, 346, 340, 342, 349, 330, 331, 334, 335, 336, 337, 338, 339, 360,                         366, 368, 320, 324, 327, 328, 329, 380, 388, 389, 391, 392, 393)
    while n < 150:
        x = random.randint(234325,876432)
        x = str(x)
        z =  str(random.choice(lista)) + x
        n += 1
        listb.append(z)

iterations = 0
while len(listb) == len(set(listb)):
    listb = []
    iterations += 1
    random_numbers()
    print len(listb), len(set(listb))

print iterations

Upvotes: 1

James Sullivan
James Sullivan

Reputation: 126

Is it possible? Absolutely.

The chances aren't great, but there is still a distinct possibility.

To give you an idea of the probability:

x has a range of 642,107 possible values.

lista has a range of 30 possible values.

So you have a total space of 30*642,107 = 19,263,210 possible combinations.

To get the probability, you have to understand the Birthday Paradox to give the chance of a collision.

p(x) = 1 - p'(x) = 1 - \prod\limits_{i = 0}^{149}(1 - \frac{i}{19,263,210}) 
     = 1 - \frac{(19,263,210 permute 149)}{19,263,210^{149}}
     = 0.00057

So you have about a 0.05%-0.06% chance of a collision every time this runs.

On average, after about 2,000 executions you will have a collision at least once.

Upvotes: 2

carlosdc
carlosdc

Reputation: 12132

Yes, very much so. If you don't want that you need to change the algorithm completely, perhaps recording the numbers you've printed.

Upvotes: 1

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