Aggregate with expressions

Summarize each penguin species with several expressions in one agg() call.

The data are body measurements of 344 penguins near Palmer Station, Antarctica, from the R palmerpenguins package (penguins), read by URL.

Requires the phpolars extension (Getting started). Runnable script: expressions.php.

use function Phpolars\{col, len, read_csv};$data = 'https://raw.githubusercontent.com'    .'/phpstats/datasets/main/csv';$penguins = read_csv(    "$data/penguins.csv",    nullValues: 'NA',);$bySpecies = $penguins    ->groupBy('species', maintainOrder: true)    ->agg(        len()->alias('penguins'),        col('body_mass_g')->mean()->round()            ->alias('mass_g'),        col('body_mass_g')            ->filter(col('sex')->eq('female'))            ->mean()            ->round()            ->alias('female_g'),        col('island')->nUnique()->alias('islands'),        col('island')            ->sortBy(                'body_mass_g',                descending: true,                nullsLast: true,            )            ->first()            ->alias('heaviest_on'),    );echo $bySpecies;

shape: (3, 6)
┌───────────┬──────────┬────────┬──────────┬─────────┬─────────────┐
│ species   ┆ penguins ┆ mass_g ┆ female_g ┆ islands ┆ heaviest_on │
│ ---       ┆ ---      ┆ ---    ┆ ---      ┆ ---     ┆ ---         │
│ str       ┆ u32      ┆ f64    ┆ f64      ┆ u32     ┆ str         │
╞═══════════╪══════════╪════════╪══════════╪═════════╪═════════════╡
│ Adelie    ┆ 152      ┆ 3701.0 ┆ 3369.0   ┆ 3       ┆ Biscoe      │
│ Gentoo    ┆ 124      ┆ 5076.0 ┆ 4680.0   ┆ 1       ┆ Biscoe      │
│ Chinstrap ┆ 68       ┆ 3733.0 ┆ 3527.0   ┆ 1       ┆ Dream       │
└───────────┴──────────┴────────┴──────────┴─────────┴─────────────┘

• nullValues: 'NA' reads the file’s NA entries as nulls. mean() skips them, so mass_g averages the penguins with a recorded body mass.
• filter() inside an aggregate limits only that aggregate. female_g averages female penguins, and penguins still counts every penguin of the species.
• sortBy('body_mass_g', ...)->first() picks a value by another column’s order. heaviest_on is the island of the species’ heaviest penguin. nullsLast: true keeps penguins without a body mass out of first place.
• Without maintainOrder: true, the order of the groups can change between runs.

Reference: groupBy() · agg() · Expressions