Replace values falling within a range in a 2D array in python?

Question

I want to replace all values smaller than -999 with NaN and values falling within -999 to 0 with -0.1 in a 2D array.

I can replace one value using

data[data < -999] = 'nan'

However, when I use

data[data < -999] = 'nan'
data[data < 0] = -0.1

It says

RuntimeWarning: invalid value encountered in less
data[data < 0] = -0.1

How to replace values < -999 with NaN and values within the range of -999 to 0 with -0.1

Just to be clear, what data type is data? What does print(type(data)) return? — DSM
– DSM, Commented Mar 14, 2016 at 19:23

DaveBensonPhillips · Accepted Answer · 2016-03-14 19:32:05Z

1

Your line

data[data < -999] = 'nan'

Isn't doing what you think. data < -999 evaluates to falsey in Python2.7, even though it's a weird thing to do; you're comparing a list to an int. This means the expression maps to

data[0] = 'nan'

Which is why you see only one value change. Note that this syntax is not allowed in Python3

It looks like you're trying to do

for column in data:
    for ix, num in enumerate(column):
        if num < -999:
            column[ix] = 'nan'
        elif num < 0:
            column[ix] = -0.1

A quick example of why yours doesn't work:

>>> data = [[-9999, -9999], [-9999, -9999]]
>>> data[data<-999] = 'nan'
>>> data
['nan', [-9999, -9999]]
>>>

edited Mar 14, 2016 at 19:32

answered Mar 14, 2016 at 18:54

DaveBensonPhillips

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6 Comments

dsbisht Over a year ago

I am using Python 2.7 and has to replace the values in a 2D array. data[data < -999] = 'nan' worked fine for me. I was wondering whether I can use it further.

DaveBensonPhillips Over a year ago

@Mario The only way it will appear to work will be if the only value in data less than -999 is the very first. See my example above for a counterexample to it working

dsbisht Over a year ago

Yes, the first value in the 2D array was -9999, but it replaced all the those values with NaN, here the if else with for loop is not working for me.

DaveBensonPhillips Over a year ago

@Mario apologies, I had a small typo; please try again

DaveBensonPhillips Over a year ago

@Mario it's fixed to work for a two-dimensional array - let me know if you have an issue

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