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perf(rust, python): remove O^2 behavior in melt #7003

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Feb 18, 2023
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perf(rust, python): remove O^2 behavior in melt #7003

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perf(rust, python): remove O^2 behavior in melt
ritchie46 Feb 18, 2023
21a7e97
fix
ritchie46 Feb 18, 2023
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6 changes: 5 additions & 1 deletion polars/polars-core/src/frame/explode.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -282,7 +282,11 @@ impl DataFrame {

for value_column_name in &value_vars {
variable_col.extend_trusted_len_values(std::iter::repeat(value_column_name).take(len));
let value_col = self.column(value_column_name)?.cast(&st)?;
// ensure we go via the schema so we are O(1)
// self.column() is linear
// together with this loop that would make it O^2 over value_vars
let (pos, _name, _dtype) = schema.try_get_full(value_column_name)?;
let value_col = self.columns[pos].cast(&st).unwrap();
values.extend_from_slice(value_col.chunks())
}
let values_arr = concatenate_owned_unchecked(&values)?;
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