Irreal

Most Irreal readers have probably already seen Mickey’s post on Evaluating Lisp Forms in Regular Expressions, but he mentions one trick that is too good to miss so I’m going to repeat it.

Not too long ago I was trying to find a way to swap several occurrences of two words in a buffer (I think it was for the VimGolf lamb had a little Mary challenge). I did find a way but I wasn’t very happy with it. In his post, Mickey shows a way that is, in retrospect, obvious but which didn’t occur to me. Suppose you have

Jill and Jack went up the hill to fetch a pail of water.
Jill fell down and broke his crown and Jack came tumbling after.

and you want to switch Jill and Jack so that you have the traditional rhyme. Here’s Mickey’s trick: do a replace-regexp (or query-replace-regexp) with a FROM string of

\(Jill\)\|Jack

and a TO string of

\,(if \1 "Jack" "Jill")

Notice that we capture only one subgroup and use it to determine which replacement word to use. If \1 is nil then we didn’t match Jill so we must have matched Jack and we want to use Jill as the replacement. On the other hand, if \1 is not nil then we matched Jill and want to replace it with Jack. Very nice.

Mickey’s got some other nice tricks you can do with the form-based TO string in replace-regexp so be sure to read his post if you haven’t already.

Here’s an interesting VimGolf challenge that nicely demonstrates the power of CUA selection mode. Given the word frequency chart

     align here
the        56271872
of        33950064
and        29944184
to        25956096
in        17420636
I        11764797
that        11073318
was        10078245
his        8799755
he        8397205
it        8058110

align the frequency counts so that it looks like

     align here
the  56271872
of   33950064
and  29944184
to   25956096
in   17420636
I    11764797
that 11073318
was  10078245
his  8799755
he   8397205
it   8058110

My first thought was to use align-regexp but that command doesn’t have a shortcut key sequence and the best regular expression I could find was [1-9]. A pretty nice solution but it certainly won’t win on keystrokes.

After messing around a bit, I came up with a solution of 4 keystrokes using the CUA rectangle functionality.

Notice that the trailing digits of a few of the frequency counts aren’t in the rectangle but they get dragged along anyway. I’ve only been using CUA rectangle mode for a short while but I am continually amazed at how powerful it is. Certainly this problem doesn’t look like a rectangle problem but CUA rectangle mode solves it very handily anyway.

Sadly, those darn Vim people were able to solve this in 3 keystrokes so if you can shave off a stroke or two be sure to leave a comment. I don’t think you can squeeze any more keystrokes out of the rectangle strategy so you’ll probably need something entirely different.

Here’s a simple ViGolf challenge that has a simple solution in Emacs. Given

(a) (abc) (abcd) (123456)

turn it into

___ _____ ______ ________

That is, replace everything in the sexpr with an underscore. This is really easy with Emacs using query-replace-regexp. Here’s a solution in only 8 keystrokes. The best Vim solution is 11, so Emacs definitely wins this one.

On the other hand, my experience has been that every time I think I have the best possible solution, one of my way smarter readers shows me a much better way. If you can beat 8 keystrokes, leave a comment and tell us how a real master does it.

In my CUA Mode Video post, I mentioned that the pointer to the video came from a comment to one of the posts in the Google+ Emacs Community. That post asks for advice on the best way of changing

a[0] = b[0];
a[1] = b[1];
a[2] = b[2];

to

c[0] = d[0];
c[1] = d[1];
c[2] = d[2];

My first thought (and what I’d probably just do if I needed this change) is a query-replace to replace a with c, followed by 【Meta+<】 to return to the top of the buffer and then another query-replace to replace b with d. That’s 13 keystrokes. Not too bad but can we do better?

Here’s a solution in 12 keystrokes using cua-selection-mode.

Using Magnar Sveen’s essential multiple-cursors package I can do it in 9 (8 if I don’t exit multiple-cursors mode) with essentially the same strategy as above but, of course, that’s not using stock Emacs. If you have a clever solution, leave a comment.

Over at the Google+ Emacs Community, Jean-Sébastien Ney (in a comment to this post) gives us a pointer to a very nice video on CUA-mode by Mark Mansour. Actually, the video really deals with the rectangle features of CUA-mode that we discussed recently. If, as I recommended, you put cua-selection-mode in your .emacs or init.el file, you can use all the tricks that Mansour demonstrates without having to invoke CUA-mode as he does.

Mansour shows how rectangle mode can do many of the things that Magnar Sveens wonderful multiple-cursors package does. The video is only 3 and a quarter minutes so it won’t take you long to watch it and it’s well worth the time.