Remove elements by index from a string within numpy array

Question

I have the following example array with strings:

 ['100000000' '101010100' '110101010' '111111110']

I would like to be able to remove some elements by index in each of the strings in the array simultaneously. For example if I will remove elements with index 6 and 8, I should receive the following outcome:

 ['1000000' '1010110' '1101000' '1111110']

All my attempts failed so far maybe because the string is immutable, but am not sure whether I have to convert and if so - to what and how.

You have to use string slicing and assign the results back to the array elements. — Barmar
– Barmar, Commented Feb 2, 2023 at 0:02

hlidka · Accepted Answer · 2023-02-02 00:15:40Z

3

import numpy as np

a = np.array(['100000000', '101010100', '110101010', '111111110'])

list(map(lambda s: "".join([c for i, c in enumerate(str(s)) if i not in {5, 7}]), a))

Returns:

['1000000', '1010110', '1101000', '1111110']

answered Feb 2, 2023 at 0:15

hlidka

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4 Comments

pho Over a year ago

Iterating over a list is faster than iterating over a numpy array: Just mapping a.tolist() instead of a reduces runtime (~30% for this array) to 5.36 µs ± 52.3 ns per loop from 7.52 µs ± 170 ns

Petar Andreev Over a year ago

It provides the same outcome as in the examples, but I am trying to point elements to be removed by index. In this case why we are putting 5 and 7 instead of 7 and 8?

hlidka Over a year ago

Python lists start with index 0, so if you want to remove 6th and 8th, that's indexes 5 and 7. If this doesn't solve your whole problem, try to describe what you imagine the final output should be. This example gives the output you indicated.

Daniel Hao Over a year ago

It can also take this syntax - list(map(lambda s: "".join(c for i, c in enumerate(str(s), 1) if i not in {6, 8}), a)) Note: skip []

Roy Smart · Accepted Answer · 2023-02-02 00:55:43Z

2

Another way to do this is to convert a into a 2D array of single characters, mask out the values you don't want, and then convert back into a 1D array of strings.

import numpy as np

a = np.array(['100000000', '101010100', '110101010', '111111110'])

b = a.view('U1').reshape(*a.shape, -1)

mask = np.ones(b.shape[-1], dtype=bool)
mask[[5, 7],] = False

b = b[:, mask].reshape(-1).view(f'U{b.shape[-1] - (~mask).sum()}')

answered Feb 2, 2023 at 0:55

Roy Smart

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