How to drop row in polars-python

Question

How to add new feature like length of data frame & Drop rows value using indexing. I want to a add a new column where I can count the no-of rows available in a data frame, & using indexing drop rows value.

for i in range(len(df)):
    if (df['col1'][i] == df['col2'][i]) and (df['col4'][i] == df['col3'][i]):
        pass
    elif (df['col1'][i] == df['col3'][i]) and (df['col4'][i] == df['col2'][i]): 
        df['col1'][i] = df['col2'][i]
        df['col4'][i] = df['col3'][i]
    else:
       df = df.drop(i)

Please provide enough code so others can better understand or reproduce the problem. — Community, Mar 16 '22 at 08:22

ritchie46 · Accepted Answer · 2022-06-16T09:19:15.710

Polars doesn't allow much mutation and favors pure data handling. Meaning that you create a new DataFrame instead of modifying an existing one.

So it helps to think of the data you want to keep instead of the row you want to remove.

Below I have written an example that keeps all data except for the 2nd row. Note that the slice will be the fastest of the two and will have zero data copy.

df = pl.DataFrame({
    "a": [1, 2, 3],
    "b": [True, False, None]
}).with_row_count("row_nr")

print(df)

# filter on condition
df_a = df.filter(pl.col("row_nr") != 1)

# stack two slices
df_b = df[:1].vstack(df[2:])

# or via explicit slice syntax
df_b = df.slice(0, 1).vstack(df.slice(2, -1))

assert df_a.frame_equal(df_b)

print(df_a)

Outputs:

shape: (3, 3)
┌────────┬─────┬───────┐
│ row_nr ┆ a   ┆ b     │
│ ---    ┆ --- ┆ ---   │
│ u32    ┆ i64 ┆ bool  │
╞════════╪═════╪═══════╡
│ 0      ┆ 1   ┆ true  │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ 1      ┆ 2   ┆ false │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ 2      ┆ 3   ┆ null  │
└────────┴─────┴───────┘
shape: (2, 3)
┌────────┬─────┬──────┐
│ row_nr ┆ a   ┆ b    │
│ ---    ┆ --- ┆ ---  │
│ u32    ┆ i64 ┆ bool │
╞════════╪═════╪══════╡
│ 0      ┆ 1   ┆ true │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌┤
│ 2      ┆ 3   ┆ null │
└────────┴─────┴──────┘

It says `slice` is faster but do you mean *slice*? Since there's no method call for slice here. :) — creanion, Jun 16 '22 at 09:05
Right. I was looking at your answer and trying to understand what part of the code you said was better, so hence the question. — creanion, Jun 16 '22 at 09:13
I shall add an example with an explicit `slice` as well. :+1: — ritchie46, Jun 16 '22 at 09:17

How to drop row in polars-python

1 Answers1

Outputs: