Plotting random numbers in Python

Question

I'm trying to generate and plot random numbers using:

from numpy import random
import matplotlib.pyplot as plt

z = 15 + 2*random.randn(200) #200 elements, normal dist with mean = 15, sd = 2

plt.plot(z)
plt.show(z)

The graph is plotted, but Python (2.7.5) freezes and I get the error

Traceback (most recent call last):
  File "G:\Stage 2 expt\e298\q1.py", line 25, in <module>
    plt.show(z)
  File "C:\Python27\lib\site-packages\matplotlib\pyplot.py", line 145, in show
    _show(*args, **kw)
  File "C:\Python27\lib\site-packages\matplotlib\backend_bases.py", line 90, in     __call__
    if block:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

It's completely fine when I do a for loop like so:

from numpy import random
from pylab import plot,show

yvec = [] # set up an empty vector 
for i in range(200): # want 200 numbers
    yy = 25 + 3*random.randn() # normal dist with mean = 15, sd = 2
    yvec.append(yy) # enter yy into vector

plot(yvec)
show(yvec)

Could someone please clarify?

The difference is in the list type: The explicit for loop uses python's builtin list, while random.randn will return a numpy array. — miku
– miku, Commented Jan 29, 2014 at 2:11

Gustav Larsson · Accepted Answer · 2014-01-29 02:13:56Z

5

The function pylab.show does not take a list or array, it takes an optional boolean (and certainly not your data array). The numpy array in the first example can't be implicitly converted to a boolean, thus throwing an error. The second one can however be converted to a boolean, and it will evaluate to True if non-empty.

To fix it, just call show without any arguments.

answered Jan 29, 2014 at 2:13

Gustav Larsson

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