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# NumPy | arccos method

schedule Aug 10, 2023
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Numpy's `arccos(~)` method computes the inverse cosine of each of the input values in radians.

# Parameters

1. `a` | `array-like`

The input array.

2. `out` | `Numpy array` | `optional`

Instead of creating a new array, you can place the computed result into the array specified by `out`.

3. `where` | `array` of `boolean` | `optional`

Values that are flagged as False will be ignored, that is, their original value will be uninitialized. If you specified the out parameter, the behavior is slightly different - the original value will be kept intact. Since this is a source of confusion for many, check examples below.

# Return value

A Numpy array that contains the inverse cosine of each value in the input array.

# Examples

## Basic Usage

``` np.arccos([-1, 0, 1]) array([3.14159265, 1.57079633, 0. ]) ```

## Specifying an output array

``` a = np.zeros(3)np.arccos([-1, 0, 1], out=a)a array([3.14159265, 1.57079633, 0. ]) ```

Here, we've output the result into array `a`.

``` np.arccos([-1, 0, 1], where=[False, True, False]) array([2.34e-324, 1.57079633, 1.32e-322]) ```

Here, only the second number was used for calculation since it has a corresponding boolean of `True` in the mask. You should notice how the values with `False` yielded strange results - in fact, you should disregard them because they are uninitialized numbers that are of no practical use.

Now, if you specified the `out` parameter, instead of uninitalized values, the original values will be left intact:

``` a = np.zeros(3)np.arccos([-1, 0, 1], out=a, where=[False, True, False])a array([0. , 1.57079633, 0. ]) ```
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