Rectangle functions in emacs

Question

I've read in several places that the rectangle functions in emacs are very useful. I've read a bit about them, and I can't quite figure why. I mean, when you want to kill a paragraph, you mark the first row/column and then the last one, and that's actually a rectangle, right? But you can still use the normal kill...

So what kind of transformations would you do with them?

Answer 1

If you have data in columns in a text file with M-x delete-rectangle or M-x kill-rectangle you can delete a single column of data. Similarly, M-x yank-rectangle will paste in a column of text.

For example, take the following text:

1. alligator    alphorn
2. baboon       bugle
3. crocodile    cornet
4. dog          didgeridoo
5. elephant     euphonium 
6. fish         flugelhorn   
7. gopher       guitar

Select from the a of alligator to the g of guitar. The beginning and end of the selection mark out two opposite corners of the rectangle. Enter M-x kill-rectangle and you immediately have:

1. alphorn
2. bugle
3. cornet
4. didgeridoo
5. euphonium 
6. flugelhorn
7. guitar

Next put the mark at the end of the top line, add a few spaces if required and enter M-x yank-rectangle and ta-da! You have re-ordered the columns:

1. alphorn      alligator    
2. bugle        baboon       
3. cornet       crocodile    
4. didgeridoo   dog          
5. euphonium    elephant     
6. flugelhorn   fish         
7. guitar       gopher       

Answer 2

I like to use rectangle for 2 main purposes, inserting the same text on every line, or killing a column of text (similar to Dave Webb's answer).

There are 2 useful shortcuts for these, C-x r k will kill a rectangle, and C-x r t to insert (there are other rectangle commands with a C-x r prefix, but these are the ones I use).

So let's say you want to take some code and format it so that you can post it in a Stack Overflow post... you need to prefix with 4 spaces. So, go to the beginning of the first line, C-SPC to mark, then go to the beginning of the last line and C-x r t <SPC> <SPC> <SPC> <SPC> <RET>, and there you have it! Then you can just copy and paste it into Stack Overflow. I have run into more complex situations where this is useful, where you actually have text you want to insert on every line at a particular place.

So the other situation like Dave Webb's situation, if you want to kill a rectangle, use C-x r k though, because it's just a lot quicker ;-)

Also, according to my reference card that I printed out when I first started, you can do the following:

• C-x r r: copy to a register
• C-x r y: yank a rectangle
• C-x r o: open a rectangle, shifting text right (whatever that means...)
• C-x r c: blank out a rectangle (I assume that means replace it with spaces, but you'd have to try it out to see)
• C-x r t: prefix with text (as described above)
• C-x r k: killing (as described above)

Answer 3

In emacs24+ there's also function for numbering lines:

(rectangle-number-lines START END START-AT &optional FORMAT)

Insert numbers in front of the region-rectangle.

START-AT, if non-nil, should be a number from which to begin
counting.  FORMAT, if non-nil, should be a format string to pass
to `format' along with the line count.  When called interactively
with a prefix argument, prompt for START-AT and FORMAT.

It is binded to C-x r N by default.