How to filter rows of a numpy array

Question

I am looking to apply a function to each row of a numpy array. If this function evaluates to True I will keep the row, otherwise I will discard it. For example, my function might be:

def f(row):
    if sum(row)>10: return True
    else: return False

I was wondering if there was something similar to:

np.apply_over_axes()

which applies a function to each row of a numpy array and returns the result. I was hoping for something like:

np.filter_over_axes()

which would apply a function to each row of a numpy array and only return rows for which the function returned True. Is there anything like this? Or should I just use a for loop?

Roger Fan · Accepted Answer · 2014-10-02 05:37:11Z

41

Ideally, you would be able to implement a vectorized version of your function and use that to do boolean indexing. For the vast majority of problems this is the right solution. Numpy provides quite a few functions that can act over various axes as well as all the basic operations and comparisons, so most useful conditions should be vectorizable.

import numpy as np

x = np.random.randn(20, 3)
x_new = x[np.sum(x, axis=1) > .5]

If you are absolutely sure that you can't do the above, I would suggest using a list comprehension (or np.apply_along_axis) to create an array of bools to index with.

def myfunc(row):
    return sum(row) > .5

bool_arr = np.array([myfunc(row) for row in x])
x_new = x[bool_arr]

This will get the job done in a relatively clean way, but will be significantly slower than a vectorized version. An example:

x = np.random.randn(5000, 200)

%timeit x[np.sum(x, axis=1) > .5]
# 100 loops, best of 3: 5.71 ms per loop

%timeit x[np.array([myfunc(row) for row in x])]
# 1 loops, best of 3: 217 ms per loop

edited Oct 2, 2014 at 5:37

answered Oct 2, 2014 at 5:19

Roger Fan

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1 Comment

killajoule Over a year ago

Thanks Roger, the function I wanted to use was a bit more complex than just taking the sum, so I might end up using the list comprehension solution.

cottontail · Accepted Answer · 2023-04-03 00:11:44Z

As @Roger Fan mentioned, applying a function row-wise should really be done in a vectorized fashion on the entire array. The canonical way to filter is to construct a boolean mask and apply it on the array. That said, if it happens that the function is so complex that vectorization is not possible, it's better/faster to convert the array into a Python list (especially if it uses Python functions such as sum()) and apply the function on it.

msk = arr.sum(axis=1)>10                # best way to create a boolean mask

msk = [f(row) for row in arr.tolist()]  # second best way
#                            ^^^^^^^^   <---- convert to list

filtered_arr = arr[msk]                 # filtered via boolean indexing

A working example and a performance test

As you can see from the timeit test below, looping over a list (arr.tolist()) is much faster than looping over a numpy array (arr), partly because Python's sum() and not np.sum() is called in the function f(). That said, the vectorized method is much faster than both.

def f(row):
    if sum(row)>10: return True
    else: return False
    
arr = np.random.rand(10000, 200)

%timeit arr[[f(row) for row in arr]]
# 260 ms ± 14 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

%timeit arr[[f(row) for row in arr.tolist()]]
# 114 ms ± 4.22 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

%timeit arr[arr.sum(axis=1)>10]
# 10.8 ms ± 2.03 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

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