Changes

One can do a surprising amount of Computer Science in Turtle Art. This is an example of a universal computer designed by Alan Turing in order to investigate problems in computability. As far as we know (Church's Thesis), any function that can be computed by an effective algorithm can be computed by a Turing Machine. A multitude of other systems has been proposed, and all are equivalent to the Turing Machine and to each other. However, there are questions that we would like to ask that neither a Turing Machine nor any of those other systems can answer.

One can do a surprising amount of Computer Science in Turtle Art.  

Before we get to those undecidable questions, however, how is a Turing Machine constructed, and what is this Turing Machine going to do?

 

 

Before we get to those undecidable questions, however, how is a Turing Machine constructed, and what is our Turing Machine going to do?

==Plan==

==Plan==

This Turing Machine will be the simplest kind of adding machine. It will start with two blocks of cells with a separator in between. The number of cells in each block is our input. The output will be a single block containing as many cells as the sum of the two input numbers. It will do this by replacing the separator with the color of the number blocks, and erasing one number cell at the end of the second block.

This Turing Machine is the simplest kind of adding machine. It starts, as in the first illustration, with two blocks of cells with a separator in between. The number of cells in each block is our input, which the user can change by editing the Setup action. The output will be a single block containing as many cells as the sum of the two input numbers. It will do this by replacing the separator with the color of the number blocks, and erasing one number cell at the end of the second block, giving a configuration as in the second illustration for its result.

==The Structure of a Turing Machine==

==The Structure of a Turing Machine==

A Turing Machine is not designed for efficiency, like microprocessors. It is designed to be easy to reason about, and so is about as simple as Turing could make it.

A Turing Machine is not designed for efficiency, like commercial microprocessors. It is designed to be easy to reason about, and so is about as simple as Turing and a few other mathematicians could make it.

The essential elements of a Turing machine are

The essential elements of a Turing machine are

* a tape consisting of cells containing symbols; here, a line of colored dots

* a tape that can be extended as much as needed, consisting of cells containing symbols; here, a line of colored dots

* a program table, consisting of three columns and some number of rows of instructions; here, a rectangle of colored dots

* a program table, consisting of three columns and some number of rows of instructions; here, a rectangle of colored dots

* a read-write head that follows instructions in the program table; here, the turtle with certain Actions

* a head that reads and writes symbols on tape, and follows instructions in the program table; here, the turtle with certain defined Actions

The machine can be in any of a finite number of states, represented here by color values. The rows of the program table correspond to combinations of the current state with the current symbol read from the current cell on the tape. For a three-state machine with two symbols, such as the adding machine we will see below, the order of the program table is

The machine can be in any of a finite number of states, represented here by color values. One of the states is interpreted as Halt: the Machine stops executing the program. The rows of the program table correspond to combinations of the current state with the symbol read from the current cell on the tape. For a three-state machine with two symbols, such as our adding machine, the order of the program table is

{|

{|

|}

|}

In operation, the starting point is a given tape with a finite sequence of symbols prewritten on it, a given program, and the Turing Machine head over the leftmost symbol on the tape in state 1. At each step, the head reads the symbol in the current cell, then follows the instructions in the row corresponding to that symbol and the current state. There are three parts to the instruction:

In operation, the starting point is a given tape with a finite sequence of symbols prewritten on it, a given program, and the Turing Machine head over the leftmost symbol on the tape in state 1. At each step, the head reads the symbol in the current cell, then follows the instruction in the row corresponding to that symbol and the current state. There are three parts to the instruction:

* A symbol to write in the current cell

* A symbol to write in the current cell

* An instruction to move one cell left or right

* An instruction to move one cell left or right

* The state for the next step

* The state for the next step

The actual program table in numerical symbols is

The actual program table in numerical symbols is

Where  

Where  

* Symbols 0 and 1 are represented as Red and Blue

* Symbols 0 and 1 are represented as Red and Blue on the tape

* Move 0 is Left and 1 is Right

* Move 0 is Left and 1 is Right

* States are really numbers

* States are really numbers, represented in the table as the color with number 20 times the state so that we can tell the differences among them. The Read Pixel block in Turtle Art pushes the RGB values of the pixel on the stack, not the Turtle Art color number, so we have to have special logic to translate back to states.

==Turing's Research==

==Turing's Research==

Among others things, Turing

Among others things, Turing

* designed a Universal Turing Machine, where the program of a regular Turing Machine would appear in a block on the tape, and the head would operate on the tape before and after it as if the two segments were connected together.

* designed a program for a Universal Turing Machine, where the program of a regular Turing Machine would appear in a block on the tape along with an area for working storage, and the head would operate on the tape before and after it as if the two segments were connected together.

* proved that no Turing machine can analyze the program for another Turing machine to decide whether it would ever halt.

* proved that no Turing machine can analyze the programs for all other Turing machines to decide which of them would ever halt. (If there were such a Halting Oracle, we could easily modify it into a program that would halt for programs that did not halt, and would not halt for programs that halt. When applied to itself, it would thus halt if and only if it did not halt, a contradiction.)

 

==Our Turing Machine==

* invented the Turing Test for Artificial Intelligence, asking whether an AI could successfully pretend to be a person at the other end of a Teletype connection.