python: random.random in function always returns 0?

Question

I have this function:

def get_starting_centroids(centroids,size,num):
    for i in range(0,num):
        temp = []
        for j in range(0,size):
            random.seed(time.clock())
            g = random.random()
            g = g*.85 + .15
            print "random: %i" %g
            temp.append(g)
        centroids.append(temp)
    return centroids

print tells me it always returns 0. If I test random.random() in the python prompt I get a random value. I don't understand what causes this difference

Also, even if there is a reason to re-seed every time, random.seed() (without parameters) uses the current system time by default anyway. — ZAD-Man
– ZAD-Man, Commented Dec 4, 2014 at 18:47
Yes, if you call seed inside the loop, you're not getting random numbers at all. — Lee Daniel Crocker
– Lee Daniel Crocker, Commented Dec 4, 2014 at 18:54

Martijn Pieters · Accepted Answer · 2014-12-04 18:43:21Z

7

You are formatting your random value to an integer:

print "random: %i" %g

This floors the value, which is below 1 always, so you always end up with 0.

Print a floating point value:

print "random: %.5f" % g

and you'll see there are actually values there, just between 0.15 and 1.

You should not seed the random generator every round; if you need to seed, just do it once outside the loop:

random.seed()  # seeds to the current system time by default
for j in range(0,size):
    g = random.random()
    g = g*.85 + .15
    print "random: %.5f" % g
    temp.append(g)

answered Dec 4, 2014 at 18:43

Martijn Pieters

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1 Comment

jason dancks Over a year ago

Thanks everyone for putting up with these kind of questions. Sometimes you just miss the easy stuff. I think you were the first poster so you got it

mcnnowak · Accepted Answer · 2014-12-04 18:45:09Z

1

Your problem is in the formatting.

Use

print "random: %f" %g

You are formatting as decimal, which truncates g, which is a float. g should be added to the temp array correctly, though.

answered Dec 4, 2014 at 18:45

mcnnowak

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