Issue obtaining minimum value of list/array

Question

I'm having a weird issue, at least I can't explain the reason for this behaviour.

I generate a list of random values with the function rand_data as defined below. When I attempt to extract the minimum value using the min() function, it returns the whole list, which leads me to believe it is not a list but an array.

But if I attempt to use the .min() attribute it returns the error:

AttributeError: 'list' object has no attribute 'min'

What is happening here? Is x1 a list or an array?

Minimal working example:

import numpy as np

def rand_data():
    return np.random.uniform(low=10., high=20., size=(10,))

# Generate data.
x1 = [rand_data() for i in range(1)]

print min(x1)
print x1.min()

You have a list object, containing the results of 1 rand_data() call. — Martijn Pieters
– Martijn Pieters, Commented Jul 13, 2014 at 0:28
You're generating a list with one element in it and that element is an array — o-90
– o-90, Commented Jul 13, 2014 at 0:28
@GoBrewers14 ohh that's an easy explanation. Would you mind posting it as an answer? — Gabriel
– Gabriel, Commented Jul 13, 2014 at 0:30
Nevermind, I see @MartijnPieters just posted that exact answer. — Gabriel
– Gabriel, Commented Jul 13, 2014 at 0:31

Martijn Pieters · Accepted Answer · 2014-07-13 00:30:13Z

3

You used a list comprehension:

x1 = [rand_data() for i in range(1)]

You now have a Python list object containing one result from rand_data().

Since rand_data() uses numpy.random.uniform() that means you have a list containing a numpy array.

Don't use a list comprehension here, it is clearly not what you wanted:

x1 = rand_data()

answered Jul 13, 2014 at 0:30

Martijn Pieters

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1 Comment

Gabriel Over a year ago

I get it now. I was using that method to generate several lists of random data, hence the [rand_data() for i in range(1)]. Thank you!

Alessandro Suglia · Accepted Answer · 2014-07-13 00:46:57Z

0

import numpy as np

def rand_data(num_elements):
# numpy returns always an array with a defined number
# of elements specified by the size parameter
     return np.random.uniform(low=10., high=20., size=num_elements)

#rand_data will automatically generate the random data for you
# simply specifying the number of elements to generate
x1 = rand_data(10)

print('Minimum value for {0} is {1}'.format(x1, min(x1)))

edited Jul 13, 2014 at 0:46

answered Jul 13, 2014 at 0:34

Alessandro Suglia

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