Grouping and summarizing

Maps let you group data by a key, such as a sale’s category. These examples use sales from the previous page.

Transform Map values

mapv changes a Map’s value and keeps its key. ifv tests the value:

def prices = {"pen" -> 2, "book" -> 5}
def doubled_prices = prices mapv . * 2
def expensive = prices ifv . > 2

doubled_prices // {"book" -> 10, "pen" -> 4}

expensive // {"book" -> 5}

. means the value; .. means the key:

def labeled = prices mapv (name: .., price: .)

labeled
// {
//   "book" -> (name: "book", price: 5)
//   "pen" -> (name: "pen", price: 2)
// }

Reduce a Bag

A reducer summarizes a whole Bag. Repeats count:

count({1, 2'2}) // {3}

sum({1, 2'2}) // {5}

The reference below lists the other reducers.

Group repeated keys

aggv groups equal keys and runs once per group. Inside it, .. is the key and . is the Bag of values:

def counts = sales by .category aggv count(.)

counts // {"a" -> 2, "b" -> 2}

Build a sales summary

Combine filtering, keying, and grouping:

def summary = sales if .amount > 1 by .category
  aggv (sales: count(.), total: sum(.amount))

summary
// {
//   "a" -> (sales: 2, total: 5)
//   "b" -> (sales: 1, total: 4)
// }

1. if keeps sales with an amount above 1.
2. by gives each sale its category as a key.
3. aggv counts the sales and adds their amounts for each category.

Quick reference

Reducer	Summary	Empty input
count	Number of occurrences	{0}
min	Smallest Value	Empty Bag
max	Largest Value	Empty Bag
all	Are all Values true?	{true}
any	Is any Value true?	{false}
sum	Total, including repeats	{0}
avg	Average, including repeats	Empty Bag