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C'est la Z

Learning Elisp 4 - Rot13 part 1

It's been a couple of weeks since my last post. Sorry - been repainting the apartment so I was limited to my laptop for a while.

Now that we've gone over a few basics we can explore new elisp features while building "useful" things. This video has us building code to implement ROT13. ROT13 is a particular instance of a Caesar or rotational cipher. In a Caesar Cipher, you take each letter and "rotate" it by a certain number of places. For example, if you are rotating by 2, the string "abcz" would become "cdeb" with each letter moving two over. Note that the "z" has to rotate back to the beginning of the alphabet and become a "b."

I also put "useful," above in quotes. This is because, while I think ROT13 is interesting and will allow us to explore a number of elisp concepts, it's actually already built in to Emacs.

In the video, I go on for a while about ROT13 and some internet history but if you aren't interested in that, I put a chapter link in the video description to skip that part.

Notes on some of the elisp concepts covered:


In elisp, things in parentheses are lists so (+ 2 3) is a list. If you have this in a program, however, Emacs will try to evaluate the function + on the parameters 2 and 3. In this case, that is what we want.

Other times, we want to use a list as pure date, for example the list (1 2 3 4 5). To prevent Emacs from evaluating this list we start it with a single quote: '(1 2 3 4 5)

Strings and Characters

In Elisp, things in double quotes are strings so "abc" is a string with the three characters a, b, and c. Characters can be written individually by preceding them with a question mark - ?a for the character a.

In the video, we'll go into details and also see how Emacs will interpret a string as a list of characters and how we can then go back from a list of characters to a string.

Mapping functions

In lisp type languages, we use a lot of what are known as mapping functions. The function mapcar in elisp, takes a function and applies it to each element of a list. For example, given this code:

  (defun square (x) (* x x))
  (mapcar #'square '(1 2 3 4 5))

That mapcar function will return the list '(1 4 9 16 25), the list you get by applying the square function we wrote to each element of the list. The video goes into more details. Note that we have to precede the function square with #' in the mapcar call


We also cover anonymous functions in the video - it lets you create a "throwaway" function.

For example, (lambda (x) (* x x)) returns a function that squares its parameter. We can use it to square the number 3 like this: ( (lambda (x) (* x x)) 3). In the video we'll show why this is useful when combined with functions like mapcar.

Misc stuff

For some reason, the video is showing for me at low resolution. If this happens to you, click on the options wheel and select a higher res.

Also, I mentioned rec.humor.spc in the video. I found some old Stupid People's Court posts here. I thought some of you would enjoy them.

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