SLogo : Basic Syntax and Builtin Commands

The basic commands that make up the SLogo language are given below and here is an example of a typical program:

repeat 4 [ lt 90 fd 50 ]

Things to keep in mind about the syntax of a program:

• Tokens are guaranteed to be separated by one or more spaces
• Tokens are not case sensitive and any possible correct command name spellings should be accepted (even though the command names below are given in all capital letters)
• Commands can be formatted over any number of lines, including multiple commands on the same line
• Commands all return a double value as described below
• Parameters to a command can be the value of a variable or the value returned from calling any command
• No parentheses are needed to resolve order of operations, the number of parameters expected by each command is enough to resovle any ambiguities

Basic Syntax

Syntax is given as Regular Expressions, a typical way to match a variety of strings that share a common, well-defined, pattern.

Token	Syntax	Semantics	Examples
Constant	-?[0-9]+\.?[0-9]*	any real valued number NOTE, to avoid potential ambiguity in parsing there should not be a space between the negative sign and the value	50 -1.3
Variable	:[a-zA-Z]+	any word preceded by a colon, : NOTE, if a variable name is used that has not previously been given a value, return a default value (i.e., 0)	:distance :side
Command	[a-zA-Z_]+(\?)?	any word can be a command-name NOTE, all commands return a numeric value NOTE, built-in commands are given below and user-defined commands cannot reuse those names	forward fd
List	[ ]	these brackets enclose a list of zero or more commands or variables NOTE, to make parsing easier, these will always be separated from other tokens by spaces	[ fd 50 rt 90 ]
Comment	^#.*	Ignore any lines that are empty, i.e., only zero or more spaces, and lines that begin with comment character, "#".	# a comment #fd 50

Turtle Commands

Name(s)	Description
FORWARD pixels FD pixels	moves turtle forward in its current heading by pixels distance returns distance turtle moved (the value of pixels)
BACK pixels BK pixels	moves turtle backward in its current heading by pixels distance returns distance turtle moved (the value of pixels)
LEFT degrees LT degrees	turns turtle counterclockwise by degrees angle returns amount turtle turned (the value of degrees)
RIGHT degrees RT degrees	turns turtle clockwise by degrees angle returns amount turtle turned (the value of degrees)
SETHEADING degrees SETH degrees	turns turtle to an absolute heading returns number of degrees turned
TOWARDS x y	turns turtle to face the point (x, y), where (0, 0) is the center of the screen returns the number of degrees turtle turned
SETXY x y GOTO x y	moves turtle to an absolute screen position, where (0, 0) is the center of the screen returns distance turtle moved
PENDOWN PD	puts pen down such that when the turtle moves, it leaves a trail returns 1
PENUP PU	puts pen up such that when the turtle moves, it does not leave a trail returns 0
SHOWTURTLE ST	makes turtle visible returns 1
HIDETURTLE HT	makes turtle invisible returns 0
HOME	moves turtle to the center of the screen (0 0) returns distance turtle moved
CLEARSCREEN CS	erases turtle's trails and sends it to the home position returns distance turtle moved

Turtle Queries

Name(s)	Description
XCOR	returns the turtle's X coordinate from the center of the screen
YCOR	returns the turtle's Y coordinate from the center of the screen
HEADING	returns the turtle's heading in degrees
PENDOWN? PENDOWNP	returns 1 if turtle's pen is down, 0 if it is up
SHOWING? SHOWINGP	returns 1 if turtle is showing, 0 if it is hiding

Math Operations

Name(s)	Description
SUM expr1 expr2 + expr1 expr2	returns sum of the values of expr1 and expr2
DIFFERENCE expr1 expr2 - expr1 expr2	returns difference of the values of expr1 and expr2
PRODUCT expr1 expr2 * expr1 expr2	returns product of the values of expr1 and expr2
QUOTIENT expr1 expr2 / expr1 expr2	returns quotient of the values of expr1 and expr2
REMAINDER expr1 expr2 % expr1 expr2	returns remainder on dividing the values of expr1 by expr2
MINUS expr ~ expr	returns negative of the values of expr
RANDOM max RAND max	returns random non-negative number strictly less than max
RANDOMRANGE min max RANDR min max	returns random number within the given range min to max, inclusive
SINE degrees SIN degrees	returns sine of degrees
COSINE degrees COS degrees	return cosine of degrees
TANGENT degrees TAN degrees	returns tangent of degrees
ARCTANGENT degrees ATAN degrees	returns arctangent of degrees
SQUAREROOT expr SQRT expr	returns square root of expr
LOG expr	returns natural log of expr
POWER base exponent POW base exponent	returns base raised to the power of the exponent
PI	returns value of the number Pi (Math.PI)

Boolean Operations

Note, since all return values are real valued (double), which are hard to compare exactly due to rounding issues in Java calculations, two values should be assumed as equal if they are within a tolerance of 0.001 of each other.

Name(s)	Description
LESS? expr1 expr2 < expr1 expr2	returns 1 if the value of expr1 is strictly less than the value of expr2, otherwise 0
LESSEQUAL? expr1 expr2 <= expr1 expr2	returns 1 if the value of expr1 is less than or equal to the value of expr2, otherwise 0
GREATER? expr1 expr2 > expr1 expr2	returns 1 if the value of expr1 is strictly greater than the value of expr2, otherwise 0
GREATEREQUAL? expr1 expr2 >= expr1 expr2	returns 1 if the value of expr1 is greater than or equal to the value of expr2, otherwise 0
EQUAL? expr1 expr2 == expr1 expr2	returns 1 if the value of expr1 and the value of expr2 are equal, otherwise 0
NOTEQUAL? expr1 expr2 != expr1 expr2	returns 1 if the value of expr1 and the value of expr2 are not equal, otherwise 0
AND test1 test2	returns 1 if test1 and test2 are non-zero, otherwise 0
OR test1 test2	returns 1 if test1 or test2 are non-zero, otherwise 0
NOT test	returns 1 if test is 0 and 0 if test is non-zero

Variables, Control Structures, and User-Defined Commands

Name(s)	Description
MAKE variable expr SET variable expr	assigns the value of expr to variable, creating the variable if necessary returns expr
REPEAT expr [ command(s) ]	runs command(s) given in the list the value of expr number of times returns the value of the final command executed (or 0 if no commands are executed) NOTE, the value of the current iteration, starting at 1, is automatically assigned to the variable :repcount so that it can be accessed by the command(s)
DOTIMES [ variable limit ] [ command(s) ]	runs command(s) for each value specified in the range, i.e., from (1 - limit) inclusive returns the value of the final command executed (or 0 if no commands are executed) NOTE, variable is assigned to each succeeding value so that it can be accessed by the command(s)
FOR [ variable start end increment ] [ command(s) ]	runs command(s) for each value specified in the range, i.e., from (start - end), going by increment returns the value of the final command executed (or 0 if no commands are executed) NOTE, variable is assigned to each succeeding value so that it can be accessed by the command(s)
IF expr [ command(s) ]	if expr is not 0, runs the command(s) given in the list returns the value of the final command executed (or 0 if no commands are executed)
IFELSE expr [ trueCommand(s) ] [ falseCommand(s) ]	if expr is not 0, runs the trueCommand(s) given in the first list, otherwise runs the falseCommand(s) given in the second list returns the value of the final command executed (or 0 if no commands are executed)
TO commandName [ variable(s) ] [ command(s) ]	assigns command(s) given in the second list to commandName using variable(s) given in first list as parameters when commandName is used later in a program, any given values are assigned to variables that can be accessed when the command list is run and the value of the final command executed is returned (or 0 if no commands are executed) returns 1 if command is successfully defined, otherwise 0