I have a similar problem to how to select all columns from a list in a polars dataframe, but slightly different:
import polars as plimport numpy as npimport stringrng = np.random.default_rng(42)nr = 3letters = list(string.ascii_letters)uppercase = list(string.ascii_uppercase)words, groups = [], []for i in range(nr): word = ''.join([rng.choice(letters) for _ in range(rng.integers(3, 20))]) words.append(word) group = rng.choice(uppercase) groups.append(group)df = pl.DataFrame( {"a_0": np.linspace(0, 1, nr),"a_1": np.linspace(1, 2, nr),"a_2": np.linspace(2, 3, nr),"b_0": np.random.rand(nr),"b_1": 2 * np.random.rand(nr),"b_2": 3 * np.random.rand(nr),"words": words,"groups": groups, })print(df) shape: (3, 8)┌─────┬─────┬─────┬──────────┬──────────┬──────────┬─────────────────┬────────┐│ a_0 ┆ a_1 ┆ a_2 ┆ b_0 ┆ b_1 ┆ b_2 ┆ words ┆ groups ││ --- ┆ --- ┆ --- ┆ --- ┆ --- ┆ --- ┆ --- ┆ --- ││ f64 ┆ f64 ┆ f64 ┆ f64 ┆ f64 ┆ f64 ┆ str ┆ str │╞═════╪═════╪═════╪══════════╪══════════╪══════════╪═════════════════╪════════╡│ 0.0 ┆ 1.0 ┆ 2.0 ┆ 0.653892 ┆ 0.234362 ┆ 0.880558 ┆ OIww ┆ W ││ 0.5 ┆ 1.5 ┆ 2.5 ┆ 0.408888 ┆ 0.213767 ┆ 1.833025 ┆ KkeB ┆ Z ││ 1.0 ┆ 2.0 ┆ 3.0 ┆ 0.423949 ┆ 0.646378 ┆ 0.116173 ┆ NLOAgRxAtjWOHuQ ┆ O │└─────┴─────┴─────┴──────────┴──────────┴──────────┴─────────────────┴────────┘I want again to concatenate the columns a_0, a_1,... into a column a, columns b_0, b_1,... into a column b. However, unlike the preceding question, this time a = [a_0; a_1; ...]. I.e., all the elements of a_0 go first, followed by all the elements of a_1, etc. All the columns whose name doesn't end with a _ followed by a digit (in this example, words and groups) must be duplicated enough times to match the length of a. Let nr and nc be the number of rows/columns in df. Then the output dataframe must have m*nr rows (m=3 in this case) and nc-2*(m-1) columns, i.e.
shape: (9, 4)┌─────────────────┬────────┬─────┬──────────┐│ words ┆ groups ┆ a ┆ b ││ --- ┆ --- ┆ --- ┆ --- ││ str ┆ str ┆ f64 ┆ f64 │╞═════════════════╪════════╪═════╪══════════╡│ OIww ┆ W ┆ 0.0 ┆ 0.653892 ││ KkeB ┆ Z ┆ 0.5 ┆ 0.408888 ││ NLOAgRxAtjWOHuQ ┆ O ┆ 1.0 ┆ 0.423949 ││ OIww ┆ W ┆ 1.0 ┆ 0.234362 ││ KkeB ┆ Z ┆ 1.5 ┆ 0.213767 ││ NLOAgRxAtjWOHuQ ┆ O ┆ 2.0 ┆ 0.646378 ││ OIww ┆ W ┆ 2.0 ┆ 0.880558 ││ KkeB ┆ Z ┆ 2.5 ┆ 1.833025 ││ NLOAgRxAtjWOHuQ ┆ O ┆ 3.0 ┆ 0.116173 │└─────────────────┴────────┴─────┴──────────┘How can I do that?