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Published Jun 15, 2024
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In NumPy, the .arcsin() method is used to calculate the inverse sine of the elements in an array. It is commonly employed when solving trigonometry problems where an angle of a right triangle is unknown, but two of the sides are known. The result is always in the range of -pi/2 to pi/2 radians. It returns the inverse sine in radians.


numpy.arcsin(x, out=None, where=True)
  • x: This is the input array. Its elements must be in the range [-1, 1] for valid results. The method returns NaN if values are outside this range.
  • out: This is an optional parameter. It specifies an array to store the result. If not provided or set to None, a new array is created for the result.
  • where: The condition (array of boolean values) that specifies where the method is to be applied.
    • If the condition is True for an element, the .arcsin() method is applied to that element in the input array.
    • If the condition is False for an element, the corresponding element in the output array remains unchanged.
    • If not provided, the .arcsin() method is applied to all elements in the input array.


The below example shows the .arcsin() method in use:

# Importing the 'numpy' library as 'np'
import numpy as np
# Defining an input array
a_input = np.array([1, 0.5, 0, -0.5, -1])
# Computing the inverse sine of all elements in the array
result_all = a_input.copy()
np.arcsin(a_input, out=result_all)
print("result_all:", result_all)
# Computing the inverse sine of specific elements in the array
result_spec = a_input.copy()
np.arcsin(a_input, out=result_spec, where=np.array([True, True, False, True, False]), dtype=float)
print("result_spec:", result_spec)
# Computing the inverse sine of all elements in the array that are greater than 0
result_gt0 = a_input.copy()
np.arcsin(a_input, out=result_gt0, where=list(map(lambda x: x>0, a_input)), dtype=float)
print("result_gt0:", result_gt0)

The output of the above code is shown below:

result_all: [ 1.57079633 0.52359878 0. -0.52359878 -1.57079633]
result_spec: [ 1.57079633 0.52359878 0. -0.52359878 -1. ]
result_gt0: [ 1.57079633 0.52359878 0. -0.5 -1. ]

Codebyte Example:

The below codebyte example demonstrates the usage of the .arcsin() method:


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