Logo Interpreter

Types

[ item ... ]

List

"word

Text string (terminated by whitespace)

:variable

Input definition/variable reference

( expression )

Parenthesis can be used to group expressions

( procedure input ... )

Call procedure with an arbitrary number of inputs

Procedures and Flow Control

to procname inputs ... statements ... end

Define a new named procedure with optional inputs

output expr

End the running procedure and output the specified value

stop

Terminate the program

if expr [ statements ... ]

Execute statements if the expression is non-zero

ifelse expr [ statements ... ] [ statements ... ]

Execute first set of statements if the expression is non-zero, otherwise execute the second set

repeat expr [ statements ... ]

Repeat statements expr times

repcount

Return the current iteration number of the current repeat

Variables

make "varname expr

Update a variable or define a new global variable. Quoting the variable name is optional, but this may be unsupported in the future since it precludes making the name itself an expression

String Operations

word expr expr

(word expr ...)

Concatenate two (or more) words into one word, separated with spaces

List Operations

first [ items ... ]

last [ items ... ]

Return first/last item from the list

butfirst [ items ... ]

butlast [ items ... ]

Return list, except for the first/last item

reverse [ items ... ]

Returns a list with the items in reverse order

pick [ items ... ]

Return one item from a list, at random

iseq expr expr

Returns a list with integers from the first to the last, inclusive

Arithmetic Operations

sum expr expr

(sum expr ...)

expr + expr

difference expr expr

expr - expr

product expr expr

(product expr ...)

expr * expr

quotient expr expr

(quotient expr)

expr / expr

modulus expr expr

expr % expr

power expr expr

expr ^ expr

minus expr

- expr

Add, subtract, multiply, divide, modulo (remainder-after-division), negate, and raise-to-the-power-of respectively. Inputs are numbers or numeric expressions, output is a number. A single input to quotient returns the reciprocal.

sqrt expr

arctan expr

(arctan expr expr)

sin expr

cos expr

tan expr

Square root, and the usual trig functions. Inputs are numbers or numeric expressions, output is a number. Angles are in degrees.

random expr

Return a random number from 0 through one less than expr

Logical Operations

and expr expr

(and expr ...)

or expr expr

(or expr ...)

xor expr expr

(xor expr ...)

not expr

Logical "and", "or", "exclusive-or", and "not", respectively. Inputs are numbers or numeric expressions, output is 1 (true) or 0 (false).

Predicates

equalp expr expr

equal? expr expr

expr = expr

notequalp expr expr

notequal? expr expr

expr <> expr

greaterp expr expr

greater? expr expr

expr > expr

greaterequalp expr expr

greaterequal? expr expr

expr >= expr

lessp expr expr

less? expr expr

expr <= expr

lessequalp expr expr

lessequal? expr expr

expr <= expr

Equals, not-equals, greater than, greater than or equal to, less than, less than or equal to, respectively. Inputs are numbers or numeric expressions, output is 1 (true) or 0 (false).

Turtle Graphics

forward expr

fd expr

Move turtle forward expr pixels

backexpr

bk expr

Move backward forward expr pixels

right expr

rt expr

Rotate expr degrees clockwise

left expr

lt expr

Rotate expr degrees counterclockwise

print expr

Write a word (Firefox 3 only)

penup

pu

Turtle stops leaving a trail

pendown

pd

Turtle resumes leaving a trail

clean

Clear the drawing area

home

Return turtle to center, pointing upwards

clearscreen

cs

Same as clean and home together

setpos [ expr expr ]

setxy expr expr

setx expr

sety expr

Move turtle to the specified location without drawing

setheading expr

seth expr

Rotate the turtle to the specified heading

pos

xcor

ycor

Returns the current turtle position as [ x y ], x or y respectively

towards [ expr expr ]

Returns the heading towards the specified [ x y ] coordinates

heading

Returns the current turtle heading

setcolor csscolor

pencolor csscolor

(setcolor expr expr expr)

(pencolor expr expr expr)

Set pen/text color (CSS color names or #rrggbb), must be a string e.g. "red or in the 3-argument version, r/g/b values in 0...255

font cssfont

Set text font (per the CSS 'font' property), must be a string e.g. word "20pt "Verdana

setwidth expr

Set pen width to expr pixels

hideturtle

ht

Hide the turtle

showturtle

st

Show the turtle

Higher Order Functions

map "procname [ expr ... ]

Return a list composed of the results of calling procname for each item in the list

filter "procname [ expr ... ]

Return a list composed of the input list where procname called on the item returns true

reduce "procname [ expr ... ]

(reduce "procname [ expr ... ] initial)

Call procname repeatedly with two arguments - the current value and the next list item. If initial is not specified, the first list element is used instead.