Comparisons, Masks and Boolean Logic

Comparisons, Masks and Boolean Logic

• Masking means to extract, modify, count or otherwise manipulate values in an array based on some criterion.

• Boolean masking, also called boolean indexing, is a feature in Python NumPy that allows for the filtering of values in numpy arrays. There are two main ways to carry out boolean masking:

a) Method one: Returning the result array.

b) Method two: Returning a boolean array.

Comparison operators as ufuncs

• The result of these comparison operators is always an array with a Boolean data type. All six of the standard comparison operations are available. For example, we might wish to count all values greater than a certain value, or perhaps remove all outliers that are above some threshold. In NumPy, Boolean masking is often the most efficient way to accomplish these types of tasks.

x = np.array([1,2,3,4,5])

print(x<3) # less than

print(x>3) # greater than

print(x<=3) # less than or equal

print(x>=3) #greater than or equal

print(x!=3) #not equal

print(x==3) #equal

• Comparison operators and their equivalent :

Boolean array:

• A boolean array is a numpy array with boolean (True/False) values. Such array can be obtained by applying a logical operator to another numpy array:

importnumpyasnp

a = np.reshape(np.arange(16), (4,4)) # create a 4x4 array of integers

print(a)

[[ 0 1 2 3]

[ 4 5 6 7]

[ 8 9 10 11]

[12 13 14 15]]

large values (a>10) # test which elements of a are greated than 10

print(large_values)

[[False FalseFalse False]

[False FalseFalse False]

[False Falsefalse True]

[ TrueTrueTrue True]]

even_values = (a%2==0) # test which elements of a are even

print(even_values)

[[True False True False]

[True False True False]

[True False True False]

[True False True False]]

Logical operations on boolean arrays

• Boolean arrays can be combined using logical operators :

b = ~(a%3 == 0) # test which elements of a are not divisible by 3

print('array a:\n{}\n'.format(a))

print('array b:\n{}'.format(b))

array a:

[[ 0 1 2 3]

[ 4 5 6 7]

[ 8 9 10 11]

[12 13 14 15]]

array b:

[[False TrueTrue False]

[ TrueTrue False True]

[True False True True]

[False TrueTrue False]]