Calculation between values in numpy array

Question

New to python and need some help. I have a numpy array tuple with a shape of (1, 8760) with numbers within each of the 8760 locations. I've been trying to calculate if the values is between -2 and 2 then my new variable will be 0 else just keep the same value in the new variable. Here is what I tried and many others but I probably don't understand the array concept fully.

for x in flow:
    if 2 > x < -2:
        lflow = 0
    else:
        lflow = flow

I get this error:

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

From what I read those functions gives me a true or false but I want to calculate of value instead of telling me if it matches or not. Please help.

Thanks

Just to be clear, you want the new value to be 0 if x is in the range (-2, 2)? Because 2 > x < -2 means x is less than -2 and x is less than 2. — Reti43
– Reti43, Commented Jan 4, 2016 at 19:01
Maybe I made a typo. If x is between -2 and 2 then I want the new value to be zero or else just keep the value within the array. Within the 8760 are set values outputted from a software so I want to evaluate those numbers and set as zero if necessary. Thanks. — New2Python
– New2Python, Commented Jan 4, 2016 at 20:15

Niemerds · Accepted Answer · 2016-01-04 20:59:43Z

1

If your shape is (1,8760) an array of 8760 elements is assigned to x in your iteration, because the loop iterates the first axis containing one element of size 8760. Furthermore I'd suggest to use "where" function instead of a loop:

# create a random array with 100 values in the range [-5,5]
a = numpy.random.random(100)*10 - 5
# return an array with all elements within that range set to 0
print numpy.where((a < -2) | (a > 2), a, 0)

edited Jan 4, 2016 at 20:59

answered Jan 4, 2016 at 19:06

Niemerds

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3 Comments

New2Python Over a year ago

Thanks for your comment. I'm not sure if random is what I'm looking for as I want to evaluate each of the 8760 and set as zero if its between -2 and 2. I will try out the second line of code you replied with. Thanks.

Reti43 Over a year ago

@New2Python The random() function is just to conveniently create an array of numbers for the shake of demonstration. The second line is what you want. If the condition (a < -2) | (a > 2) is true (translates to a less than -2 or a greater than 2), you get the original value of a, else, you get a 0. However, that means you get a 0 for the range [-2, 2], inclusive. If you want 0 when a < 2 and a > -2, then you need to change the above condition to (a <= -2) | (a >= 2).

Niemerds Over a year ago

@Reti43: yes of course your are right concerning the "&". Thanks!

fivetentaylor · Accepted Answer · 2016-01-04 23:07:51Z

0

Numpy uses boolean masks to select or assign values to arrays in bulk. For example, given the array

A = np.array([-3,-1,-2,0,1,5,2])

This mask represents all the values in A that are less than -2 or greater than 2

mask = (A < -2) | (A > 2)

Then use it to assign those values to 0

A[mask] = 0

This is much faster than using a regular loop in python, since numpy will perform this operation in c or fortran code

answered Jan 4, 2016 at 23:07

fivetentaylor

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