Logo

An implementation of the educational programming language Logo with the addition of variables, evaluations and basic control functions (repeat, if..then..else, while, break, continue etc.)

Commands

Below is a quick overview of the available commands. For some sample programs, see the examples folder.

Contents

1. Basic Movements
2. Colours and Pen Up/Down
3. Variables
4. Repeat Loops
5. If..Then..Else, Break and Continue
6. While Loops

Basic Movements

The turtle can be moved in four directions without turning:

forward X;
backward X;
left X;
right X;

In each case X can be an integer literal, e.g. forward 100;, or can be a variable, e.g. forward length; (where length has been declared as a number variable), or it can be an evaluation consisting of integer literals and/or variables, e.g. forward 2 * length;.

It is also possible to rotate the turtle clockwise or anti-clockwise:

rightturn X;
leftturn X;

Again, X can be an integer literal, a variable or an evaluation consisting of integers and variables.

It is also possible to manually set the X and Y positions, or the direction of the turtle:

setx X;
sety Y;
setdirection D;

As usual, the parameter can be a integer literal, variable or evaluation. When setting the direction, the value D will wrap around 0 and 360.

If you wish to simply centre the turtle and set the direction to upward (zero) we can use the following command:

centerturtle;

We can toggle the displaying of the turtle with the following commands:

hideturtle;
showturtle;

Colours and PenUp PenDown

By default the turtle pen colour will always be black, but it is possible to set the ARGB components:

setcolora X;
setcolorr X;
setcolorg X;
setcolorb X;

X can be an integer literal, a variable or an evaluation consisting of integers and variables, but the value will be capped to the range 0-255.

By default the turtle will always draw lines as it moves across the canvas. If you wish to toggle whether the turtle pen is touching the canvas or not, you will need the following commands:

penup;
pendown;

Variables

Variables can be declared using the keyword number followed by a variable name.

number length;

Once variables have been created, we can assign values to them with the assignment operator =:

length = 10;

It is also possible to initialise variables:

number width = 15.8;

We can also initialise variables with evaluations:

number area = length * width;

Finally it is possible to create boolean variables:

boolean bool1;
boolean bool2 = false;
boolean bool3 = bool1 && bool2;

Repeat Loops

We can repeat a set of commands by using the repeat command:

repeat N {
  Command1;
  Command2;
  ..
  CommandN;
}

Where N can be an integer literal, variable or evaluation.

For example, to draw a square we might use the following:

repeat 4 {
  forward 100;
  rightturn 90;
}

If Then Else Break and Continue

The interpeter also supports if..then..else commands, plus the ability to break from loops or to continue.

For example, if we wished to draw a semi-circle we could use the following:

number counter = 0;
repeat 36 {
  if(counter <= 18) {
    forward 10;
    rightturn 10;
  } else {
    break;
  }
  counter = counter + 1;
}

While Loops

The interpeter also supports while loops:

number counter = 0;
while (counter < 36) {
  
  forward 10;
  rightturn 10;
  
  counter = counter + 1;
}