Archives August 2022

Desert Mountain Lake — An Introduction to Fill

In a recent update to turtleSpaces, we added the ability to create arbitrary shapes, using the beginfill and endfill primitives.

To create a shape, you simply declare beginfill, draw out its borders and then declare endfill. If the lines do not cross each other, then the shape should be created.

For example:

beginfill forward 50 right 90 forward 50 endfill

will create a triangle. But we already have plenty of ways to create triangles, right?

If you add right 60 forward 20 to just before endfill, you’ll understand. You can create whatever shape you want, you just need to make sure your lines don’t cross!

So what can we do with this? Let’s start with a simple example. We could create randomly-generated mountains. Generative art is becoming popular, so let’s see what all the fuss is about.

Okay, so we’re going to draw out the shape of the mountains by starting at the far left, then angling the turtle 30 to 60 degrees to the right.

penup setx -250 beginfill right 30 + random 30

Next we’ll move forward 50 turtle units plus a random number made up of 100 minus the turtle’s current Y position. This will keep us (mostly) from going off the top of the viewable area.

dountil xpos > 220 [
  right 30 + random 30
  forward 50 + random 100 - ypos

Then we’ll turn right between 70 and 110 degrees, for the downslope, and move forward the value of the current y (vertical) position. This will keep us from going below the centerline of the viewable area.

  right 70 + random 40
  forward ypos

Then we’ll return the turtle to the upright position, and then keep creating new mountains until we end up off the right side of the viewable area.

  setheading 0

Then we set the turtle’s y position to 0 and declare the endfill:

sety 0

So that’s not too bad, but we want more! (We always want more.) We’ve made the background blue (using setbackgroundcolor blue) which is going to be our lake, but maybe we want the sky to be a different color. So let’s add a quad before we start drawing the mountains.

This is pretty simple, we’re just going to set the fill color to red and then create the quad at the point we’re going to start drawing the mountains:

setx -250
setfillcolor red
quad 500 150
raise 1/5

Note that we need to raise the turtle a little bit so our sky and the mountains don’t ‘Z-fight’ with each other.

Okay so what about that cool reflection effect? How do we do that? Well, it’s actually pretty simple.

All we’re going to do is log the positions of the turtle as we create the mountains, then ‘replay’ her movements, but with the y position reversed.

To do that, we’re first going to create an empty list, using make.

make "points []
setx -250

Next we’re going to queue our positions into the list. We need to do this in two places in our mountain creation loop, at the peaks and the troughs.

dountil xpos > 220 [
  queue position "points
  rt 30 + random 30
  fd 50 + random 100 - ypos
  queue position "points
  sety 0 queue position "points
  ;we need to make sure to 'log' the final position

Once the mountains are drawn, we simply declare beginfill again, and use foreach and setposition to move the turtle to each point we logged, but we’re going to use the list primitive to change the y value from a positive to a negative.

setfillshade 5
foreach "i :points [
  setposition (list first :i negative second :i third :i)

In the full example we use definecolor to make our brown color a little more blue, for creating the reflection.

The sun is the proverbial icing on the cake. We’re going to make the sun and its reflection using the pie primitive to make half a circle. We’ll generate a number between 80 and 119 and put it into a container, representing the sun’s size.

make "sun 80 + random 40

We’ll position the turtle either at the left-third, center or right-third of the screen:

setxc pick [-71 0 71]

We’ll create the sun:

setfillcolor yellow
pie 180 :sun

Then we’ll redefine the yellow color to make it a bit more blue and subdued.

definecolor yellow [50 50 20]

Finally we’ll turn 180 degrees, create the reflection, then turn back.

right 180
pie 180 :sun
right 180
lower 1/10

We place all of this before we create the quad that draws the red background. So we need to lower the turtle to ensure the sun and the background don’t z-fight.

Here is the full procedure listing:

TO desertmountainlake
  reset hideturtle penup
  setbackgroundcolor blue
  definecolor red [100 0 0]
  definecolor brown [50 31 15]
  ;redefine the colors
  setx pick [-71 0 71]
  make "sun 80 + random 40
  setfillcolor yellow pie 180 :sun
  definecolor 13 [50 50 20]
  right 180 pie 180 :sun right 180
  lower 1/10
  ;draw the sun
  make "points []
  setx -250
  setfc red quad 500 150
  ;create the red sky quad
  raise 2 / 10 setfc orange
  ;start drawing the mountains
  dountil xpos > 220 [
    queue position "points
    right 30 + random 30
    forward 50 + random 100 - ypos
    queue position "points
    rt 70 + random 40
    ;rt is short for right, lt left etc
    fd ypos setheading 0
    ;fd is short for forward
  sety 0 queue position "points
  ;we need to queue the final position
  ;close the fill
  ;the reflection:
  setfc brown setfs 5 beginfill
  foreach "i :points [
    setposition (list first :i negative second :i third :i)

Logo and the Language Microworld

Logo isn’t just all about the turtles! When Logo was first being developed, there were no graphical displays — output was simple text on teletypes, which printed out interactions with the computer on paper. When the developers of Logo went into classrooms to test out their invention on students, all they had were teletypes.

As a result, the first Logo ‘microworld’ was the language microworld, not the geometry microworld which is more often associated with Logo. Logo was designed with many powerful commands used to manipulate strings and lists. Logo is based on Lisp, which stands for List Processing, a programming language in which data and code are the same, and are interchangeable and manipulatable by the running program. In Lisp, as in Logo, programs can modify, create and run themselves.

Some of these primitives (commands) include:

word – join two pieces of text together, eg word “dog “cat returns “dogcat

list – take two words and form a list, eg list “dog “cat returns [dog cat]

first, second, third, last – returns the appropriate character (word0 or item (list)

item, setitem – retrieve or change an item in a list or word based on its numerical index

fput, lput – returns a copy of a list with an additional item inserted in the front or back

butfirst, butlast – returns a list without the first or last item

leading, trailing – returns the front and back of words, respectively

push, pop, queue, dequeue – add or remove items on to the front or back of lists

There are also primitives that sort and search, retrieve words from a built-in English dictionary, convert characters to ASCII and back, and much more. You can browse them all in the Words and Lists reference.

So what can we do with all of these wonderful, powerful tools? Here’s a few examples:


While we can manipulate strings and lists internally we still need to output the result to the user. In Logo, this can be done using the show, print and type primitives, and in turtleSpaces you can additionally typeset (create graphical text) or say (text-to-speech) them.

This example uses the print and the pick commands, the former types text into the console (text) area, following it with a line feed (carriage return) while the latter chooses (picks) a random item from a supplied list. In the example, this is used to generate a randomized simple story:

TO simplestory
  ;this language module contains a number of
  ;different procedures that manipulate language
  ;in fun ways!
  ;For example, this procedure uses the print
  ;and pick primitives to choose randomly from
  ;a series of lists, creating a simple one-line
    ;encompassing the print command
    ;in round brackets allows us to supply it
    ;more parameters than normal. This also
    ;works with commands like say and functions
    ;like word and sentence
    print "The
    pick [
      red green blue
    pick [
      frog cow cat
    pick [
      jumped ran walked
    pick [
      |over to| over underneath
    ;to create a "word with spaces" (a phrase) in a list,
    ;a single list item containing spaces,
    ;enclose it with pipe symbols.
    word pick [
      overpass mountain |shopping mall|
    ] ".
    ;word is used here to create a single word
    ;made up of the final 'pick' and a period.

Note that a ‘word’ in Logo is a single word, usually represented with a single double quote, eg “frog

Multiple words can either be represented in a list, eg [dog cat pig], or as a phrase |fat dog|, or both [|fat dog| |lazy cat| |pink pig|]

In the case of the last example, the second item in that list is |lazy cat|. You can turn |lazy cat| into a list of two items using the parse primitive.

A one-line version:

TO oneline
  ; a simple single line 'story':
  print (sentence pick [|The duck| |Old John| |A tabby cat| |Little Susie|] pick [ran walked surfed jumped danced] pick [|away from| towards] pick [|the city| |the mall| |their house|])

Questions and Answers, Repeats and Repcounts

The next example takes a user-supplied string and turns it into a word pyramid, by repeatedly displaying more and more of the word on successive lines.

It does this first by retrieving the desired word from the user using the question primitive, which prompts the user for input and then ‘stores’ the result in the ‘answer’ primitive, a function that returns the user’s input.

Then, it determines the length of the word using the count primitive, and employs repeat and repcount to generate the pyramid. repcount returns the current iteration of a repeat loop and is a very useful primitive.

reverse is also used in this example, which reverses a word or list.

TO wordpyramid
  ;this procedure creates a text 'pyramid'
  ;out of a string provided by the user:
  question |word| make "word answer
  ;the question primitive prompts the user
  ;for input, which is retrieved using the
  ;answer function
  ;the make primitive places the output from
  ;answer (the input to question) in a container
  ;called "word, which is read using :word
  repeat count :word [
    ;count is a function that returns the number
    ;of characters in the provided word, which is
    ;in this case the contents of the "word container
    ;symbolized using :word (which is the equivalent
    ;of 'thing "word', the primitive 'thing' able
    ;to retrieve and return the contents of containers.)
    repeat (count :word) - repcount [
      ;expressions are resolved right to left. What
      ;this means is that without the round brackets,
      ;Logo would try to subtract repcount (which returns
      ;the current number of repeat loop iterations)
      ;from the contents of the :word container, a string,
      ;which would result in an error. We solve this by
      ;placing round brackets around (count :word) to
      ;ensure that repcount is subtracted from the
      ;result of count :word instead
      type spacebar
      ;'type' prints a character(s) without adding a
      ;carriage return at the end, allowing one to type
      ;more on the same line. spacebar is a function that
      ;returns the space character
    if repcount = 1 [
      ;if the current repeat iteration is 1 (the first):
      print first :word
      ;print the first character of the contents of the :word container
    ] [
      print (word
        ;print a word made up of:
        reverse leading repcount :word
        ;the leading (first) repeat loop iterations characters
        ;in the :word container, but in reverse.
        ;Remember, Logo evaluates expressions from right to left,
        ;so it first slices off the first however many 'leading'
        ;characters from the front of :word and then reverses it.
        butfirst leading repcount :word
        ;the second part of our combined word is the leading
        ;repcount number of characters, excluding (butfirst)
        ;the first character.
  ;don't forget you can hover the mouse over keywords to
  ;get a popup that explains what they do!

Word(s) around the world!

The turtle is starting to get a bit bored, so let’s give her a bit of a workout! The next example uses the typeset and orbit primitives to create a ‘word circle’ out of user-provided input.

The orbit commands in turtleSpaces allow the turtle to orbit around a specific point in turtleSpace. We’re going to use the item primitive and a bit of math to distribute the input into a ring around the home [0 0 0] point:

TO wordcircle
  ;this procedure create a graphical 'word circle'
  ;out of the supplied word:
  clearscreen hideturtle
  dropanchor penup question |word|
  ;dropanchor moves the 'anchor point'
  ;used by orbit primitives to the turtle's
  ;current location
  pullout 8 * count answer
  ;pullout backs the turtle away from the
  ;anchor point without moving it with
  ;the turtle
  ;in this case, we're 'pullling out'
  ;8 turtle-units * the number of characters
  ;in the 'answer' provided to the question
  ;by the user
  repeat count answer [
    ;loop for however many characters are in the answer:
    typeset item repcount answer
    ;typeset (create a graphical character) for the
    ;character 'item' in the place of the current repeat iteration
    ;eg if the word was 'frog', item 1 is 'f', item 4 is 'g'
    ;and on the third loop iteration it would be 'o'.
    slideleft 10
    ;the turtle moves when it typesets the character, but
    ;we're going to 'orbit' to make a circle, so we need to
    ;move the turtle back to where it started
    orbitright 360 / count answer
    ;'orbit' the turtle 360 degrees divided by the number of
    ;letters in the answer
  ;all done creating the circle
  forever [
    cam:rollright 1 wait 1
  ;rotate the camera forever

Spiral Stories

Similarly, this next example creates a spiral of words, which grow larger over time as the spiral is built. Each word is spoken by the computer’s text-to-speech facility. Rather than prompting for input, the ‘story’ is defined in a container (variable) using the make primitive.

While in the previous example, item was used to pick out each letter in the user’s input, here it is used to pick out each list item (word) in the “message (story) container.

The foreach primitive is used to typeset each letter in each word independently, to create a more flowing effect. This example demonstrates opportunities for creating text-based animations and artworks:

TO wordspiral
  ;creates a spiral made out of words from a given message:
  cam:pullin 100
  ;pulls the camera closer to the turtle
  cam:tandem myrtle
  ;causes the camera to match the turtle's movements
  ;don't draw lines
  make "message [Once upon a time there was a dog named Jim who used to deliver newspapers with his human Susan. One day they came upon an old house occupied by an ornery orange cat.]
  ;sets the message into the :message container.
  ;The message is a list of words,
  ;as opposed to a single word
  repeat count :message [
    ;repeat the following the same number of times
    ;as there are words in the message:
    cam:pullout 10
    repeat 19 [
      right 2 cam:rollright 2 wait 1
    ;turn in an animated way
    settypesize 10 + (repcount / 2)
    ;make the words larger as we progress
    if repcount = 32 [
      setfillcolor orange
      ;if this is the 32nd word, make the
      ;'fillcolor' orange
    ] [
      ;otherwise pick one from the given list:
      setfc pick [6 7 5 10 15]
      ;many primitives have shorter 'shortcuts'
    ;the fillcolor is the color used to create
    ;shapes such as typeset characters
    say item repcount :message
    ;speak the current word
    foreach "i item repcount :message [
      ;for each letter in the current word:
      typeset :i wait 1
      ;typeset the letter and wait 1/60th of a second
    if repcount != count :message [
      ;if this isn't the last word:
      repeat 12 - count item repcount :message [
        ;repeat 12 subtracted by the number of letters in the word:
        typeset spacebar wait 1
        ;type a 'space' and wait 1/60th of a second
  ;if we're here, we're finished the spiral!
  wait 120
  setxy -100 -150
  ;move the turtle to the center
  repeat 480 [
    cam:pullout 1 cam:rollright 1 wait 1
  ;spin the camera and pull it back

Codes and Cyphers

Whew! Myrtle’s had her workout, so she can take a well-deserved rest now. Let’s look at some fancier text manipulation, starting with a simple cypher.

Cyphers traditionally manipulate each letter in words in a specific, usually secret way, so that one can pass messages without those who intercept them having the ability to read them.

In the case of our simple example, we’re going to advance the ASCII value of each character by one, changing A into B and so-on.

This procedure is also our first encounter (in this article anyhow) with the “functionalisation” of a procedure in Logo.

In Logo, a procedure can either be just a procedure, in the sense that it is executed simply by declaring its name, and nothing more, or it can be a function, which requires input values be provided when it is declared, such as forward 20. It can also be a returner, which returns a result but requires no input, or a functional returner, which takes input and returns a result.

clearscreen – procedure
forward 20 – function
pencolor – returner
sin 20 – functional returner

Our cypher is a functional returner: it requires an input while returning an output. As a result, it needs to be executed by feeding it back into something else, such as print:

print cypher “albatross

This cypher example only encrypts a single word, or phrase if input is provided using the pipe symbols:

print cypher |I wish I was a secret agent!|

Now, as to the code itself, it first uses a boolean (listp) to check if the input is in fact a word / phrase and not a list, and complains if it is not. Booleans (predicates, which is why they typically end with p) return true or false, based on the input they are provided or the state of the turtleSpaces environment. The ‘if’ primitive takes a boolean result and if it is true then executes the supplied list.

See what I mean when I say lists of data and code are the same in Logo? You can manipulate lists of code just like any other list, and attempt to execute any list (although the results may not be that useful!)

Next we create an empty output container, and then foreach character in the input, we convert it to its ASCII value, add one, and then convert it back into a character, inserting it into the output container. Finally, we use the output primitive to return the result back to the calling primitive, for example print.

TO cypher :input
  ;A simple cypher:
  ;adding a :parameter to the end of a TO declaration
  ;causes the procedure to require the parameter when it
  ;is called.
  ;in this case, we're requiring a parameter called :input
  ;which when provided will place that given value into
  ;a container called :input _that only exists for the
  ;time this procedure is executed_ then vanishes!
  if listp :input [print |You can only provide a word to this procedure!| stop]
  ;check to see if the input is a list. If it is, reject it
  ;and stop processing this procedure
  make "output "
  ;initialize an empty container called :output
  foreach "letter :input [
    ;for each character in the :input container
    ;(represented by :letter) :
    make "output word :output char (ascii :letter) + 1
    ;update the :output container to contain the current contents
    ;of that container plus:
    ;the character representation of
    ;the ASCII value of
    ;the current :letter
    ;plus one.
    ;So, for example, the ASCII value of A is 65. We're
    ;going to add one to that (66) and then add the char(acter)
    ;representation (B) of that ASCII value to the existing :output
  ;We do this for all the letters in the supplied word.
  output :output
  ;we 'return' our finished :output via the output command.
  ;This means that the output is passed on to the next command
  ;rather than just being printed out. It makes this procedure
  ;a _function_. Functions take input and return output.
  ;So, to show our cypher, we must enter a command such as:
  ;print cypher "elephant
  ;because if we just type: cypher "elephant
  ;Logo will not know what to do with the output

Deciphering the Gibberish

Of course we need to decipher our encrypted messages too:

TO cypherlist :input
  ;this function takes a list rather than a word,
  ;and uses cypher to encrypt each word separately:
  local "output
  ;we use :output in 'cypher', so we need our own
  ;'local' copy that is exclusive to this procedure
  ;and will not be modified by 'cypher'
  make "output []
  foreach "i :input [
    queue cypher :i "output
  output :output
  ;can you write a 'decypherlist'?

The Language of Music

Of course, English isn’t the only language Logo can speak — it can speak all sorts of languages! But especially, it can speak the language of music, using the playnotes primitive.

playnotes takes a list of music ‘commands’ and then plays them. These include:

L# – play the next provided note at the given length. L0 = 16th note … L9 = double whole note!

R# – plays a ‘rest’ (no tone) of the given length (similar to L)

C5 – plays a note of the given tone (C) and length (5)

Can we use list primitives to create random music? You bet we can!

This example uses the forever loop to play randomly generated music… forever! Inside the forever loop the code employs the list, word, pick and random primitives to piece together our computer-created composition:

TO randommusic
  ;but hey, all word and list handling isn't necessariy
  ;about text. Check out this random music generator:
  forever [
      playnotes (pick
        ;pass to the playnotes primitive (which plays
        ;musical notes) one of the following two things:
        list (list word "R random 6)
        ;a list (which is the type of data playnotes
        ;requires) made up of a single word containing the
        ;letter R and a random number between 0 and 5,
        ;the combined word (eg R4) signifying a rest, or:
          word "L random 6
          word pick [C# D# F# G# A#] pick [3 4 5 6]
        ;a list made up of two words, the first containing
        ;the letter L and a number between 0 and 5,
        ;and the second containing a note (eg C#) and
        ;an octave (eg 4)
      ;use round brackets to space out your command statements
      ;across multiple lines, to make them easier to read
      ;and explain!

Note how we can use the () round brackets to spread out a single complete instruction across multiple lines to make it easier to comment and read.

Doyay owknay igpay atinlay?

Pig Latin is a perennial favourite of children across the ages. A series of simple rules are used to create a ‘secret language’ that can only be understood by those who know them.

The rules are:

  1. If the word only has one letter (a) leave it as-is
  2. If the word only has two letters, add ‘yay’ to it
  3. If the world starts with ‘th’, move it to the end and add ‘ay’ to it
  4. If the word starts with a vowel, add ‘nay’ to it
  5. Otherwise, move the first letter to the end and add ‘ay’ to it.

The following example uses if, elsif and else to sort the input words through all of these rules, stitching together a result:

TO piglatin :input
  ;doyay owknay igpay atinlay?
  ;this procedure takes a list as an input
  make "output []
  ;initialize the output container as an empty list
  foreach "word :input [
    ;for each word in the input string:
    if 1 = count :word [
      ;if the word only has one letter (a):
      queue :word "output
      ;put it in the output untouched
    elsif 2 = count :word [
      queue word :word "yay "output
    ;if the word is two letters, add it to the output
    ;+ 'yay'
    elsif "th = leading 2 :word [
      ;otherwise, if the word starts with 'th':
      queue (word trailing (count :word) - 2 :word "thay) "output
      ;take the remaining part of the word minus the 'th' and add 'thay' to it
      ;then 'queue' it (add it to the end of) into the :output list
    elsif not memberp first :word [a e i o u y] [
      ;- 'not' reverses the boolean, making a true false
      ;- 'memberp' returns true if the first value provided to it
      ;is present in the list supplied as the second value
      ;and so, if the first letter in the word is NOT
      ;a vowel:
      queue (word butfirst :word first :word "ay) "output
      ;take everything except the first letter (butfirst)
      ;and then add the first letter to the end, followed
      ;by "ay, Note the brackets around the word function,
      ;they are needed because we are supplying three
      ;inputs: the end of the word, the first letter and "ay.
      ;finally we queue the result into :output
    else [
      ;FINALLY, if all else fails (the word starts with a vowel):
      queue (word :word "nay) "output
      ;we think you can figure this one out for yourself! :)
  show :output
  ;we're going to show the output rather than outputting it,
  ;but you could change this to output :output if you wanted
  ;to use this as a function.

Note the use of the leading and trailing primitives, which return the front and back of input words, respectively.

And now for something (somewhat) completely different…

You know that you can make containers containing values, and that you can use expressions to create those values. Typically these expressions are evaluated before they are put into the container, and so they are fixed at the point in time the make is executed. But what if you could put the expression into the container instead? This is where make! comes in handy:

TO nameexample
  ;this is a brief example of make!
  ;which creates a container whose value can be different
  ;every time you 'look' in it! (hence make!)
  ;in this case, when we define our container using
  ;make!, the expression goes into the container, not the result.
  ;This means that every time we show the container's 'contents'
  ;what we get back is an evaluation of the expression. So if
  ;we provide a dynamic expression, then the result will be
  make! "name sentence pick [Big Little Stout Thin Tall Short] pick [Jim Fred Mark Paul Mike]
  show :name
  show :name
  show :name
  make! "time (se hours minutes seconds)
  show :time

As you can see from the example, containers created with ‘make!’ are more like magic boxes, whose contents change based on when you look inside them. Logo is magical, indeed!

A Sophisticated Story

Earlier, we told a simple story, now let’s tell a more sophisticated one.

In the following example, a series of list containers are created containing various story elements. These elements are then laid out on the graphical display using the typeset primitive. Note the absolute positioning set using the setxy primitive.

This is actually a fairly simple example, with the opportunity for participation from the entire classroom, in choosing the various words and phrases that make up the story:

TO story
  ;Let's tell a random story. This can be a fun yet simple project for students
  ;since they can pick the various items that go into each list of story elements:
  make "protagonist [|an old man| |a young lady| |a small dog| |a white cat| |a red walrus|]
  make "city [Vancouver Seattle |Los Angeles| |New York| Paris Rome]
  make "vehicle [|their car| |their bike| |the bus| |a taxi| |an Uber|]
  make "destination [supermarket mall dentist |computer store| |art gallery|]
  make "badevent [|slipped and fell| |got mugged| |lost their wallet| |got sick|]
  make "goodevent [|found $100| |won the lottery| |met Ryan Reynolds| |ate a burger|]
  ;first we 'make' lists for each element in the story
  ;note that phrases (also called 'long words') have to be surrounded with pipes ||
  clearscreen penup setxy -200 100
  ;setxy positions the turtle at the given x and y co-ordinates
  settypesize 9
  typeset (sentence |There was once| pick :protagonist "from word pick :city ",)
  ;sentence assembles multiple words into a list (including 'long words' or phrases)
  ;we're using word here to add punctuation to a word (or phrase) taken from the :city list.
  setxy -200 80
  typeset (sentence |who took| pick :vehicle |to the| word pick :destination ".)
  setxy -200 60
  typeset (sentence |Sadly, they| word pick :badevent "!)
  setxy -200 40
  typeset (sentence |But luckily, then they| word pick :goodevent "!)
  setxy -200 -50
  typeset |And they lived happily ever after.|

And they lived happily ever after.

The next word, and the next word…

This next example is a simple little cracker, which uses the english primitive to retrieve random words from turtleSpaces built-in English dictionary, and then typesets them into the graphical display, rolling and turning the turtle to create a 3D ‘word cloud’.

If you click and drag on the graphical display area you can rotate around the output.

TO wordcloud
  ;creates a 3D word cloud out of random words:
  clearscreen penup
  repeat 100 [
    right pick [-90 90]
    typeset pick english 6
    ;'english' returns a list of english words of the given length
    slideright 10 rollright 90

If you are unable…

The next example tries to pick a random word out of the dictionary and turn it into an un-able variation, eg unfixable.

A naive version would look something like:

print (word “un pick english 5 “able)

which while straightforward is going to output a lot of rubbish.

Like piglatin, we’re going to define some rules that we’ll use to process the word and see if we can create something that makes sense. But this time we’re going to do this through exclusion — if the random word doesn’t conform to our rules, we will simply pick another one.

We’ll do this using the dountil primitive, which does something until specific conditions are met:

TO unable
  ;tries to create the 'un-able' version of a random word, eg unstoppable
  ;A simple version:
  ;print (word "un pick english 5 "able)
  ;A better version:
  dountil (
    ;we're going to cheat and find a good un-able word:
    ;'dountil' means perform the provided list, THEN
    ;check the conditions, and if they are false, perform
    ;the provided list of instructions AGAIN until the
    ;conditions are all true.
    ;do this until all of the following are true:
    "un != leading 2
    ;the first two letters aren't 'un' -- we don't want 'ununable'
    :pick "er != trailing 2
    ;the last two letters aren't 'er' -- that would just be weird
    :pick not memberp last :pick [a e i o u y s d g]
    ;the last letter isn't a vowel or s, d or g
    ;note that the opening round bracket beside 'dountil'
    ;has allowed us to spread the above out across multiple lines.
    ;Without the brackets, all of that would have needed to be
    ;on the same line, or Logo would have become confused.
  ) [
    make "pick pick english 6
    ;pick an english word with 6 letters and place it into
    ;the :pick container
  print (word "un :pick "able)
  ;print the un-able word

Is this cheating? We’ll leave that for you to decide.

Check out these and other examples in the following turtleSpaces project: