guides

Lists, records and tables

Lists

A list holds values in order, written in square brackets with no commas:

sizes = [ 10 25 40 ]add 55 to sizesremove 10 from sizesprint (count sizes)print (item 1 of sizes)

Items are counted from 1. Asking for an item that is not there gives an empty value (0 for numbers) and says so, without stopping the sketch.

Records

A record holds named fields, in curly brackets:

fill goldball = { pos: { 200, 150 }, vel: { 3, 2 }, radius: 20 }ball.pos = ball.pos + ball.velcircle at ball.pos radius ball.radius

A field that is given a value anywhere in the sketch belongs to every record made in the same place, and reads 0 until it is set. A field nothing ever sets is a typo, and rocinante says which fields there are.

Many things: a list of records

Particles, balls, birds: a list of records, and a verb that moves one.

balls = []repeat 20 [ add { pos: anywhere, vel: { (random from -3 to 3), (random from -3 to 3) } } to balls ]draw [  background 30  for each b in balls [    b.pos = b.pos + b.vel    circle at b.pos radius 8 fill gold  ]]

The list verbs

Each of these gives a new list or value and leaves the old one alone. Inside their block, it is the item:

ages = [ 12 17 21 30 8 ]adults = keep ages [ it >= 18 ]children = skip ages [ it >= 18 ]doubled = collect ages [ it * 2 ]total = combine ages from 0 [ result + it ]first_adult = find ages [ it >= 18 ]youngest_first = sort ages by [ it ]mixed = shuffle agesprint adults

In combine, result is the value so far.

Shared, or copied

Numbers, text, vectors and colors are copied when they are given to a new name. Lists and records are shared: b = a makes b another name for the same list, and changing one changes the other.

a = [ 1 2 ]b = aadd 3 to bprint (count a)

This prints 3, and rocinante adds a hint on the add line, because changing a list through a second name is usually a surprise. copy makes a separate one:

a = [ 1 2 ]b = copy aadd 3 to bprint (count a)

Sharing is what lets a verb change a record it is given, which the teaching verbs guide uses.

Tables

A table holds text names, each with a value, kept in the order they were put:

counts = tableput counts "cat" 2put counts "owl" 1put counts "cat" 3print (get counts "cat")for each name in (keys counts) [ print name ]remove "owl" from countsprint counts

A name that is not there reads as an empty value. count counts says how many names there are.

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