Question

In the Python’s standard max function (I also can pass in a key parameter):

s = numpy.array(['one','two','three'])
max(s) # 'two' (lexicographically last)
max(s, key=len) # 'three' (longest string)

With a larger (multi-dimensional) array, I can not longer use max, so I tried to use numpy.amax, however I can’t seem to be able to use amax with strings…

t = np.array([['one','two','three'],['four','five','six']])
t.dtype # dtype('|S5')
numpy.amax(t, axis=0) #Error! Hoping for: [`two`, `six`]

Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/usr/lib/python2.7/dist-packages/numpy/core/fromnumeric.py", line 1833, in amax
        return amax(axis, out)
TypeError: cannot perform reduce with flexible type

Is it possible to use amax (am using it incorrectly!), or is there some other numpy tool to do this?

Answer 1

Instead of storing your strings as variable length data in the numpy array, you could try storing them as Python objects instead. Numpy will treat these as references to the original Python string objects, and you can then treat them like you might expect:

t = np.array([['one','two','three'],['four','five','six']], dtype=object)
np.min(t)
# gives 'five'
np.max(t)
# gives 'two'

Keep in mind that here, the np.min and np.max calls are ordering the strings lexicographically – so “two” does indeed come after “five”. To change the comparison operator to look at the length of each string, you could try creating a new numpy array identical in form, but containing each string’s length instead of its reference. You could then do a numpy.argmin call on that array (which returns the index of the minimum) and look up the value of the string in the original array.

---

Example code:

# Vectorize takes a Python function and converts it into a Numpy
# vector function that operates on arrays
np_len = np.vectorize(lambda x: len(x))

np_len(t)
# gives array([[3, 3, 5], [4, 4, 3]])

idx = np_len(t).argmin(0) # get the index along the 0th axis
# gives array([0, 0, 1])

result = t
for i in idx[1:]:
    result = result[i]
print result
# gives "two", the string with the smallest length