# 205: Candy Button Paper

Candy Button Paper |

Title text: Nonrewriteable tape? |

## [edit] Explanation

This comic refers to Candy Buttons, a type of candy sold by Necco in the U.S. since 1980. Because of the resemblance of the strips of paper to the tape of a Turing Machine, a small minority of children pretended to be a Turing Machine by creating rules and executing them upon the tape of candy exactly like a real Turing Machine would do.

The title text refers to the fact that, although it would be hypothetically possible to create a Turing Machine that can only delete symbols, the information density of the tape would be greatly reduced, and the original Turing Machine could read and write from the tape it operated on.

## [edit] Transcript

- When it came to eating strips of candy buttons, there were two main strategies. Some kids carefully removed each bead, checking closely for paper residue before eating.
- Others tore the candy off haphazardly, swallowing large scraps of paper as they ate.
- Then there were the lonely few of us who moved back and forth on the strip, eating rows of beads here and there, pretending we were Turing machines.

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# Discussion

It is possible to run a Turing machine on a candy belt:

Marvin Minsky (1967), Computation: Finite and Infinite Machines, Prentice-Hall, Inc. Englewood Cliffs, N.J. In particular see p. 262ff (italics in original):
"We can now demonstrate the remarkable fact, first shown by Wang [1957], that for any Turing machine T there is an equivalent Turing machine TN that *never changes a once-written symbol*! In fact, we will construct a two-symbol machine TN that can only change blank squares on its tape to 1's but can not change a 1 back to a blank." Minsky then offers a proof of this. -- Kopa Leo 69.163.36.90 16:01, 6 July 2013 (UTC)

- In my opinion, intuitively, when writing is demanded, a turing machine just have to copy those symbols to a new location, minding the symbol that needs to be written. It can have a start-of-data mark so this would be transparent to other operations 173.245.48.96 05:45, 27 July 2014 (UTC)