Blocks & the Iteration Vocabulary

A block is an object. Every loop is a message that takes a block. Learn six selectors and you can process any data.

Blocks — deferred computation

Form	Meaning	Run it
[ 2 + 2 ]	a zero-arg block (not yet run)	[ 2 + 2 ] value → 4
[ :x \| x * x ]	one parameter	[ :x \| x*x ] value: 6 → 36
[ :a :b \| a + b ]	two parameters	value: 3 value: 4 → 7

Making collections

#(1 2 3 4 5)          "literal Array"
{ 1+1. 2 factorial }  "dynamic Array (evaluated)"
1 to: 100             "an Interval, iterable"
OrderedCollection new "growable; addFirst:/add:/removeFirst"

The six you use daily

Selector	Does	Example → result
do:	side-effect for each; returns the receiver	#(1 2 3) do: [:e \| ... ]
collect:	transform each (map)	#(1 2 3) collect: [:x \| x*x] → #(1 4 9)
select:	keep those that pass (filter)	#(1 2 3 4) select: [:x \| x even] → #(2 4)
reject:	drop those that pass	#(1 2 3 4) reject: [:x \| x even] → #(1 3)
detect:	first match (error if none)	#(1 2 3) detect: [:x \| x>1] → 2
detect:ifNone:	first match or fallback	... detect: [:x \| x>9] ifNone: [0] → 0
inject:into:	fold to one value (reduce)	(1 to: 100) inject: 0 into: [:a :e \| a+e] → 5050

Read the shapes collect: same size, transformed. select:/reject: same-or-smaller, same elements. detect: one element. inject:into: one accumulated value; the block is [:accumulator :each | ...] and the seed comes after inject:.

Dictionary — key → value

Message	Does
d at: #a put: 3	store 3 under key #a
d at: #a	read (error if key missing)
d at: #a ifAbsent: [ 0 ]	read, or block's value if missing
d at: #a ifAbsentPut: [ X ]	get-or-create: store & return X if missing
d keys d values	all keys / all values (iterable)
d associationsDo: [ :as \| as key … as value ]	iterate key+value pairs
d includesKey: #a	test membership

Handy extras

coll size          coll isEmpty          coll includes: 3
coll sum           coll max              coll average
coll sorted        coll sort: [:a :b | a > b]
coll do: [ :e | ... ] separatedBy: [ ... ]

Companion to Lesson 0002. Ask your teacher to expand any row into live practice in your image.