Applying a function along a numpy array

Question

I\'ve the following numpy ndarray.

[ -0.54761371  17.04850603   4.86054302]

I want to apply this function to all elements of the array

Answer 1

Use np.exp and that will work on numpy arrays in a vectorized fashion:

>>> def sigmoid(x):
...     return 1 / (1 + np.exp(-x))
...
>>> sigmoid(scores)
array([  6.33581776e-01,   3.94391811e-08,   7.68673281e-03])
>>>

You will likely not get any faster than this. Consider:

>>> def sigmoid(x):
...     return 1 / (1 + np.exp(-x))
...

And:

>>> def sigmoidv(x):
...   return 1 / (1 + math.exp(-x))
...
>>> vsigmoid = np.vectorize(sigmoidv)

Now, to compare the timings. With a small (size 100) array:

>>> t = timeit.timeit("vsigmoid(arr)", "from __main__ import vsigmoid, np; arr = np.random.randn(100)", number=100)
>>> t
0.006894525984534994
>>> t = timeit.timeit("sigmoid(arr)", "from __main__ import sigmoid, np; arr = np.random.randn(100)", number=100)
>>> t
0.0007238480029627681

So, still an order-of-magnitude difference with small arrays. This performance differences stays relatively constant, with a 10,000 size array:

>>> t = timeit.timeit("vsigmoid(arr)", "from __main__ import vsigmoid, np; arr = np.random.randn(10000)", number=100)
>>> t
0.3823414359940216
>>> t = timeit.timeit("sigmoid(arr)", "from __main__ import sigmoid, np; arr = np.random.randn(10000)", number=100)
>>> t
0.011259705002885312

And finally with a size 100,000 array:

>>> t = timeit.timeit("vsigmoid(arr)", "from __main__ import vsigmoid, np; arr = np.random.randn(100000)", number=100)
>>> t
3.7680041620042175
>>> t = timeit.timeit("sigmoid(arr)", "from __main__ import sigmoid, np; arr = np.random.randn(100000)", number=100)
>>> t
0.09544878199812956