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 Commands

Using commands you tell the turtle or KTurtle to do something. Some commands need input, some give output. In this section we explain all the commands that can be used in KTurtle. Please note that all build in commands we discuss here are highlightedcode editor, this can help you to distinguish them. with blue in the

There are several commands to move the turtle over the screen.

forward (fw)

forward moves the turtle forward by the amount of X pixels. When the pen is down the turtle will leave a trail. forward can be abbreviated to fw

backward (bw)

backward moves the turtle backward by the amount of X pixels. When the pen is down the turtle will leave a trail. backward can be abbreviated to bw.

turnleft (tl)

turnleft commands the turtle to turn an amount of X degrees to the left. turnleft can be abbreviated to tl.

turnright (tr)

turnright the turtle to turn an amount of X degrees to the right. turnright can be abbreviated to tr.

direction (dir)

direction set the turtle's direction to an amount of X degrees counting from zero, and thus is not relative to the turtle's previous direction. direction can be abbreviated to dir.

center

center moves the turtle to the center on the canvas.

go

go commands the turtle to go to a certain place on the canvas. This place is X pixels from the left of the canvas, and Y pixels form the top of the canvas.

gox

gox using this command the turtle will move to X pixels from the left of the canvas whilst staying at the same height.

goy

gox using this command the turtle will move to Y pixels from the top of the canvas whilst staying at the same distance from the left border of the canvas.

Using the commands go, gox, goy and center the turtle will not draw a line, no matter if the pen is up or down.

There are two commands which return the position of the turtle on the screen.

getx

getx returns the number of pixels from the left of the canvas to the current position of the turtle.

gety

gety returns the number of pixels from the top of the canvas to the current position of the turtle.

The turtle has a pen that draws a line when the turtle moves. There are a few commands to control the pen. In this section we explain these commands.

penup (pu)

penup lifts the pen from the canvas. When the pen is “up” no line will be drawn when the turtle moves. See also pendown. penup can be abbreviated to pu.

pendown (pd)

pendown presses the pen down on the canvas. When the pen is press “down” on the canvas a line will be drawn when the turtle moves. See also penup. pendown can be abbreviated to pd.

penwidth (pw)

penwidth sets the width of the pen (the line width) to an amount of X pixels. penwidth can be abbreviated to pw.

pencolor (pc)

pencolor sets the color of the pen. pencolor takes an RGB combination as input. pencolor can be abbreviated to pc.

There are several commands to control the canvas.

canvassize (cs)

With the canvassize command you can set the size of the canvas. It takes X and Y as input, where X is the new canvas width in pixels, and Y is the new height of the canvas in pixels. canvassize can be abbreviated to cs.

canvascolor (cc)

canvascolor set the color of the canvas. canvascolor takes an RGB combination as input. canvascolor can be abbreviated to cc.

There are two commands to clean up the canvas after you have made a mess.

clear (ccl)

With clear you can clean all drawings from the canvas. All other things remain: the position and angle of the turtle, the canvascolor, the visibility of the turtle, and the canvas size.

reset

reset cleans much more thoroughly than the clear command. After a reset command everything is like is was when you had just started KTurtle. The turtle is positioned at the middle of the screen, the canvas color is white, the turtle draws a black line on the canvas and the canvassize is set to 400 x 400 pixels.

First a brief explanation of what sprites are: sprites are small pictures that can be moved around the screen, like we often see in computer games. Our turtle is also a sprite. For more info see the glossary on sprites.

Next you will find a full overview on all commands to work with sprites.

[The current version of KTurtle does not yet support the use of sprites other than the turtle. With future versions you will be able to change the turtle into something of your own design]

spriteshow (ss)

spriteshow makes the turtle visible again after it has been hidden. spriteshow can be abbreviated to ss.

spritehide (sh)

spritehide hides the turtle. This can be used if the turtle does not fit in your drawing. spritehide can be abbreviated to sh.

The answer is: “yes”. The turtle can write: it writes just about everything you command it to.

print

The print command is used to command the turtle to write something on the canvas. print takes numbers and strings as input. You can print various numbers and strings using the “+” symbol. See here a small example:

fontsize

fontsize sets the size of the font that is used by print. fontsize takes one input which should be a number. The size is set in pixels.

There is one command that rolls dice for you, it is called random, and it is very useful for some unexpected results.

random (rnd)

random is a command that takes input and gives output. As input are required two numbers, the first (X) sets the minimum output, the second (Y) sets the maximum. The output is a randomly chosen number that is equal or greater than the minimum and equal or smaller than the maximum. Here a small example:

Using the random command you can add a bit of chaos to your program.

A dialog is a small pop-up window that provides some feedback or asks for some input. KTurtle has two commands for dialogs, namely: message and ask

message

The message command takes a string as input. It shows a pop-up dialog containing the text from the string.

ask

ask takes a string as input. It shows a pop-up dialog containing the text from the string, just like the message. But in addition to it also puts an input field on the dialog. Through this input filed the user can enter a number or a string which can be stored in a container. For example

When a user cancels the input dialog, or does not enter anything at all the container is emptied.

Containers are letters or words that can be used by the programmer to store a number or a text. Containers that contain a number are called variables, containers that can contain text are called string. Containers can be identified by the container character “$” that precedes their usage.

Containers that are not used contain nothing. An example:

This will print nothing and you get an error message.

Let us start with an example:

In the first line the letter x is made into a variable (number container). As you see the value of the variable x is set to 3. On the second line the value is printed.

Note that if we wanted to print an “x” that we should have written

That was easy, now a bit harder example:

 

In the first two lines the variables A and B are set to 2004 and 25. On the third line the variable C is set to A + B, which is 2029. The rest of the example consists of 3 print commands with backward 30 in between. The backward 30 is there to make sure every output is on a new line. In this example you also see that variables can be used in mathematical calculations.

In programming code the regular text is usually started and ended with quotes. As we have already seen:

The regular is delimited with quotes. These pieces of regular text we call strings.

Strings can also be stored in containers just like numbers Strings are a lot like variables. The biggest difference is that they contain text in stead of numbers. For this reason strings cannot be used in mathematical calculations and questions. An example of the use of strings:

On the first line the string x is set to “Hello ”. On the second line the string name is set to the output of the ask command. On the third line the program prints a composition of three strings on the canvas.

This program ask you to enter your name. When you, for instance, enter the name “Paul”, the program prints “Hello Paul, how are you?”. Please note that the plus (+) is the only math symbol that you can use with strings.

 

Yes, KTurtle will do your math. You can add (+), subtract (-), multiply (*), and divide (/). Here is an example in which we use all of them:

Do you know what value a, b, c and d have? Please note the use of the assignment symbol =.

If you just want a simple calculation to be done you can do something like this:

Now an example with parentheses:

The expressions inside parentheses will be calculated first. In this example, 20-5 will be calculated, then multiplied by 2, divided by 30, and then 1 is added (giving 2).

KTurtle has advanced mathematical features. It knows the number pi and trigonometrical functions like sin, cos, tan, arcsin, arccos, arctan and the functions sqrt and exp.

If you need integers in calculations, use this function.

round

round the given number to the nearest integer.

With this code the turtle will print the numbers 11 and 10.

A simple example of a question:

In this example the question is the x > 5 part. If the answer to this question is 'true' the code between the brackets will be executed. Questions are an important part of programming and often used together with execution controllers, like if. All numbers and variables (number containers) can be compared to each other with questions.

Here are all possible questions:

Table 4.1. Types of questions

Question glue-words enable us to glue questions into one big question.

In this example the glue-word and is used to glue 2 questions (a < 5, b == 5) together. If one side of the and would answer “false” the whole question would answer “false”, because with the glue-word and both sides need to be “true” in order to answer “true”. Please do not forget to use the brackets around the questions!

Here is a schematic overview; a more detailed explanation follows below:

Table 4.2. Question glue-words

When two questions are glued together with and, both sides of the and have to be 'true' in order to result in 'true'. An example:

In this example you see a glued question glued onto an other question.

If one of the two questions that are glued together with or is 'true' the result will be 'true'. An example:

In this example you see a glued question glued onto an other question.

not is a special question glue-word because it only works for one question at the time. not changes 'true' into 'false' and 'false' into 'true'. An example:

In this example the glued question is 'true' yet the not changes it to 'false'. So in the end "not hello ;-)" is printed on the canvas.

The execution controllers enable you — as their name implies — to control execution.

Execution controlling commands are highlighted with dark green in a bold font type. The brackets are mostly used together with execution controllers and they are highlighted with bolded black.

If you have done some programming in KTurtle you have might noticed that the turtle can be very quick at drawing. This command makes the turtle wait for a given amount of time.

wait

wait makes the turtle wait for X seconds.

This code draws a circle, but the turtle will wait half a second after each step. This gives the impression of a slow-moving turtle.

if

The code that is placed between the brackets will only be executed if the answer to the question is “true”. Please read for more information on questions in the question section.

On the first line x is set to 6. On the second line the question x > 5 is asked. Since the answer to this question is “true” the execution controller if will allow the code between the brackets to be executed

else

else can be used in addition to the execution controller if. The code between the brackets after else is only executed if the answer to the question that is asked is “false”.

The question asks if x is greater than 5. Since x is set to 4 on the first line the answer to the question is “false”. This means the code between the brackets after else gets executed.

while

The execution controller while is a lot like if. The difference is that while keeps repeating (looping) the code between the brackets until the answer to the question is “false”.

On the first line x is set to 1. On the second line the question x < 5 is asked. Since the answer to this question is “true” the execution controller while starts executing the code between the brackets until the answer to the question is “false”. In this case the code between the brackets will be executed 4 times, because every time the fifth line is executed x increases by 1.

repeat

The execution controller repeat is a lot like while. The difference is that repeat keeps repeating (looping) the code between the brackets for the given number.

for

The for loop is a “counting loop”, i.e. it keeps count for you.

Every time the code between the brackets is executed the x is increased by 1, until x reaches the value of 10. The code between the brackets prints the x multiplied by 7. After this program finishes its execution you will see the times table of 7 on the canvas.

The default step size of a loop is 1, you can use an other value with

break

Terminates the current loop immediately and transfers control to the statement immediately following that loop

exit

Finishes the execution of the code.

learn is a very special command, because it is used to create your own commands. The command you create can take input and return output. Let us take a look at how a new command is created:

The new command is called circle. circle takes one input, a number, to set the size of the circle. circle returns no output. The circle command can now be used like a normal command in the rest of the code. See this example:

 

 

In the next example, a command with a return value is created.

 

In this example a new command called multiplyBySelf is created. The input of this command is multiplied by itself and then returned, using the return command. The return command is the way to output a value from a function you have created.

Commands can have more than one input. In the next example, a command that draws a rectangle is created.

Now you can run box 50, 100 and the turtle will draw a rectangle on the canvas.

In this chapter you will find an explanation of most of the “uncommon” words that are used in the handbook.

degrees

Degrees are units to measure angles or turns. A full turn is 360 degrees, a half turn 180 degrees and a quarter turn 90 degrees. The commands turnleft, turnright and direction need an input in degrees.

input and output of commands

Some commands take input, some commands give output, some commands take input and give output and some commands neither take input nor give output.

Some examples of commands that only take input are:

The forward command takes 50 as input. forward needs this input to know how many pixels it should go forward. pencolor takes a color as input and print takes a string (a piece of text) as input. Please note that the input can also be a container. The next example illustrates this:

Now some examples of commands that give output:

The ask command takes a string as input, and outputs the number or string that is entered. As you can see, the output of ask is stored in the container x. The random command also gives output. In this case it outputs a number between 1 and 100. The output of the random is again stored in a container, named r. Note that the containers x and r are not used in the example code above.

There are also commands that neither need input nor give output. Here are some examples:

intuitive highlighting

This is a feature of KTurtle that makes coding even easier. With intuitive highlighting the code that you write gets a color that indicates what type of code it is. In the next list you will find the different types of code and the color they get in the code editor.

Table 5.1. Different types of code and their highlight color