NumPy strings.islower()

The numpy.strings.islower() function checks whether all cased characters in each string element of an input array are lowercase. At least one cased character must be present for it to return True.

Syntax

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numpy.strings.islower(x, /, out=None, *, where=True, casting='same_kind', order='K', dtype=None, subok=True)

Parameters

Parameter	Type	Description
`x`	array_like (StringDType, bytes_, or str_ dtype)	Input array containing string elements.
`out`	ndarray, None, or tuple of ndarray and None, optional	Optional output array where the result is stored. If None, a new array is created.
`where`	array_like, optional	Boolean mask specifying which elements to check. Elements where `where=False` retain their original value.
`casting`	str, optional	Defines the casting behavior when computing the function.
`order`	str, optional	Memory layout order of the output array.
`dtype`	data-type, optional	Defines the data type of the output array.
`subok`	bool, optional	Determines if subclasses of ndarray are preserved in the output.

Return Value

Returns a boolean array indicating whether all cased characters in each element are lowercase and at least one cased character is present. If the input is a scalar, a scalar boolean value is returned.

Examples

1. Checking Lowercase Strings in an Array

We check if each string in an array consists only of lowercase characters.

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import numpy as np

# Define an array of strings
strings = np.array(["hello", "World", "python", "123"])

# Check which strings are completely lowercase
result = np.strings.islower(strings)

# Print the results
print("Input strings:", strings)
print("Are they lowercase?", result)

Output:

Input strings: ['hello' 'World' 'python' '123']
Are they lowercase? [ True False  True False]

2. Using the `out` Parameter

Using an output array to store results instead of creating a new array.

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import numpy as np

# Define an array of strings
strings = np.array(["lowercase", "MIXEDcase", "UPPER", "test123"])

# Create an output array with the same shape
output_array = np.empty_like(strings, dtype=bool)

# Check lowercase strings and store the result in output_array
np.strings.islower(strings, out=output_array)

# Print the results
print("Computed results:", output_array)

Output:

Computed results: [ True False False False]

3. Using the `where` Parameter

Using a condition to check lowercase strings only for selected elements.

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import numpy as np

# Define an array of strings
strings = np.array(["apple", "Banana", "grape", "123"])

# Define a mask (check only where mask is True)
mask = np.array([True, False, True, False])

# Check lowercase strings where mask is True
result = np.strings.islower(strings, where=mask)

# Print the results
print("Computed results with mask:", result)

Output:

Computed results with mask: [ True False  True False]

The lowercase check is performed only for elements where mask=True. The other values remain unchanged.

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NumPy strings.islower() – Checks if Elements Contains only Lowercase