CODEWITHSUNDEEP
pythonarraysnumpyerror-handlingdivide-by-zero
hlin117
hlin117

Reputation: 22260

How to return 0 with divide by zero

I'm trying to perform an element wise divide in python, but if a zero is encountered, I need the quotient to just be zero.

For example:

array1 = np.array([0, 1, 2])
array2 = np.array([0, 1, 1])

array1 / array2 # should be np.array([0, 1, 2])

I could always just use a for-loop through my data, but to really utilize numpy's optimizations, I need the divide function to return 0 upon divide by zero errors instead of ignoring the error.

Unless I'm missing something, it doesn't seem numpy.seterr() can return values upon errors. Does anyone have any other suggestions on how I could get the best out of numpy while setting my own divide by zero error handling?

Upvotes: 184

Views: 200400

Answers (9)

Marcnu
Marcnu

Reputation: 21

A solution with try/except to manage any kind of those numpy RuntimeWarnings:

with np.errstate(all='raise'):
    try:
        array3 = array1/array2
    except:
        array3 = array1 # Give whatever default value you like

Upvotes: 0

denis
denis

Reputation: 21947

Building on @Franck Dernoncourt's answer, fixing -1 / 0 and my bug on scalars:

def div0( a, b, fill=np.nan ):
    """ a / b, divide by 0 -> `fill`
        div0( [-1, 0, 1], 0, fill=np.nan) -> [nan nan nan]
        div0( 1, 0, fill=np.inf ) -> inf
    """
    with np.errstate(divide='ignore', invalid='ignore'):
        c = np.true_divide( a, b )
    if np.isscalar( c ):
        return c if np.isfinite( c ) \
            else fill
    else:
        c[ ~ np.isfinite( c )] = fill
        return c

Upvotes: 58

Ulf Aslak
Ulf Aslak

Reputation: 8608

DEPRECATED (PYTHON 2 SOLUTION):

One-liner (throws warning)

np.nan_to_num(array1 / array2)

Upvotes: 20

T. Gwen
T. Gwen

Reputation: 104

An other solution worth mentioning :

>>> a = np.array([1,2,3], dtype='float')
>>> b = np.array([0,1,3], dtype='float')
>>> b_inv = np.array([1/i if i!=0 else 0 for i in b])
>>> a*b_inv
array([0., 2., 1.])

Upvotes: -1

DStauffman
DStauffman

Reputation: 4319

In numpy v1.7+, you can take advantage of the "where" option for ufuncs. You can do things in one line and you don't have to deal with the errstate context manager.

>>> a = np.array([-1, 0, 1, 2, 3], dtype=float)
>>> b = np.array([ 0, 0, 0, 2, 2], dtype=float)

# If you don't pass `out` the indices where (b == 0) will be uninitialized!
>>> c = np.divide(a, b, out=np.zeros_like(a), where=b!=0)
>>> print(c)
[ 0.   0.   0.   1.   1.5]

In this case, it does the divide calculation anywhere 'where' b does not equal zero. When b does equal zero, then it remains unchanged from whatever value you originally gave it in the 'out' argument.

Upvotes: 356

Franck Dernoncourt
Franck Dernoncourt

Reputation: 83167

Building on the other answers, and improving on:

Code:

import numpy as np

a = np.array([0,0,1,1,2], dtype='float')
b = np.array([0,1,0,1,3], dtype='float')

with np.errstate(divide='ignore', invalid='ignore'):
    c = np.true_divide(a,b)
    c[c == np.inf] = 0
    c = np.nan_to_num(c)

print('c: {0}'.format(c))

Output:

c: [ 0.          0.          0.          1.          0.66666667]

Upvotes: 54

Pi Marillion
Pi Marillion

Reputation: 4674

Try doing it in two steps. Division first, then replace.

with numpy.errstate(divide='ignore'):
    result = numerator / denominator
    result[denominator == 0] = 0

The numpy.errstate line is optional, and just prevents numpy from telling you about the "error" of dividing by zero, since you're already intending to do so, and handling that case.

Upvotes: 17

hlin117
hlin117

Reputation: 22260

One answer I found searching a related question was to manipulate the output based upon whether the denominator was zero or not.

Suppose arrayA and arrayB have been initialized, but arrayB has some zeros. We could do the following if we want to compute arrayC = arrayA / arrayB safely.

In this case, whenever I have a divide by zero in one of the cells, I set the cell to be equal to myOwnValue, which in this case would be zero

myOwnValue = 0
arrayC = np.zeros(arrayA.shape())
indNonZeros = np.where(arrayB != 0)
indZeros = np.where(arrayB = 0)

# division in two steps: first with nonzero cells, and then zero cells
arrayC[indNonZeros] = arrayA[indNonZeros] / arrayB[indNonZeros]
arrayC[indZeros] = myOwnValue # Look at footnote

Footnote: In retrospect, this line is unnecessary anyways, since arrayC[i] is instantiated to zero. But if were the case that myOwnValue != 0, this operation would do something.

Upvotes: 0

Lee
Lee

Reputation: 31040

You can also replace based on inf, only if the array dtypes are floats, as per this answer:

>>> a = np.array([1,2,3], dtype='float')
>>> b = np.array([0,1,3], dtype='float')
>>> c = a / b
>>> c
array([ inf,   2.,   1.])
>>> c[c == np.inf] = 0
>>> c
array([ 0.,  2.,  1.])

Upvotes: 3

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