# Remove elements from numpy array according to their values
###### tags: `GPT Stuff`, `numpy`, `python`

prompt:

i have a numpy array and would like to remove some elements based on their values (provided as a list), how to do this

response:

You can accomplish this by using a boolean mask to filter the array. Here is a step-by-step guide:

1. Create a numpy array.
2. Create a list of values you want to remove.
3. Use the `np.isin` function to create a boolean mask which is `True` for each element of your array that is in the list of values to remove.
4. Use the `~` operator to invert the boolean mask, so that it is `True` for elements you want to keep.
5. Use the boolean mask to index your array, creating a new array with only the elements you want to keep.

Here's an example:

import numpy as np

# Step 1: Create a numpy array
arr = np.array([1, 2, 3, 4, 5, 6])

# Step 2: Create a list of values you want to remove
remove_list = [2, 4]

# Step 3: Use np.isin to create a boolean mask
mask = np.isin(arr, remove_list)

# Step 4: Use the ~ operator to invert the boolean mask
mask = ~mask

# Step 5: Use the boolean mask to index your array
filtered_arr = arr[mask]

print(filtered_arr)  # prints: [1 3 5 6]

In this example, `np.isin(arr, remove_list)` returns an array of the same shape as `arr`, with `True` at the positions where the `arr` element is in `remove_list`, and `False` elsewhere. The `~` operator inverts this mask, so it is `True` where the `arr` element is not in `remove_list`, and `False` where it is. Finally, `arr[mask]` selects only the elements of `arr` where the mask is `True`, effectively removing the elements that are in `remove_list`.