Delete n lines skip 1 line script

Question

I want to create a basic Vim script that simply deletes n lines, skips 1 line and repeats until the end of the document. I don't care about specific matches, I just want to delete the line. For example; So I want to repeat the command 3ddj for the entire document.

Text I want to keep.   
Text I want to delete.  
Text I want to delete.  
Text I want to delete.  
Text I want to keep.  
Text I want to delete.  
Text I want to delete.  
Text I want to delete.  
Text I want to keep.  
Text I want to delete.  
Text I want to delete.  
Text I want to delete.   

So it looks like...

Text I want to keep.  
Text I want to keep.  
Text I want to keep.  

Thank you.

Answer 1

Very simple approach:

1. Move to the first line you want to delete.
2. Record a macro: qa3ddjq
3. Repeat it with a high number: 1000@a

Step three will repeat the macro a thousand times or until an error is encountered. Hitting end of file (hence no lines to delete) produces an error and repetition of the macro is canceled.

See :help recording.

Answer 2

The easiest solution to me would be:

:%norm j3dd

That is:

• %: for every line
• norm: run the following keys as if in normal mode
• j3dd: go down one line then delete 3 lines

---

So from the first line, go down to the second one and delete the next 3 lines. The second Text I want to keep. is now on the second line. Go down one line, delete 3. Rinse and repeat.

---

Note: if you don't want to process the entire file you can replace the %norm with e.g 5,$norm to process from the 5th line to the end of the file.

Answer 3

You have some great solutions already available. Here is another one:

:g/^/if line('.')%4!=1|:s/^/DELETE ME/|endif
:g/^DELETE ME/d

First, we perform an action on every line (:g matching against the ^ (start of line)) and for every line number perform the result of linenumber % 4. If the result is unequal to 1, we add DELETE ME at the beginning of the line, marking the lines to be deleted by the next command.

The next command is a regular (:g) command, that matches all lines that start with DELETE ME and does so.

Note, You cannot immediately delete the line using the first :g command, since this would change the resulting modulo calculation and as such such, we have to first mark the lines and in a second step delete it.

Answer 4

I know this is the vi channel, but to me this is a sed problem.

sed -ne 'p;n;n;n' <file >newfile

So you can wrangle it into a vi solution:

:0
!Gsed -ne 'p;n;n;n'

Answer 5

An alternative (I suppose we start on the line 1, and delete the lines 2-4, 6-8 and so on, as per an example text):

while line(".") < line("$")
    silent +1delete _ 3
endwhile

If prefer doing this interactively, you can make use of the "command-line register". That is, press

:+1del 3<CR> and then 1000@:

Answer 6

an alternative: you want to keep any lines: 1, 4, etc. You could use:

awk '!((FNR-1)%3)'  file > newfile
      # awk: when a condition is given without action:
      #        prints the lines for which the condition returns 0.
      # FNR designates the line number in the current file.
      # a%3 : takes the modulus of a and 3. ( 0%3=0, 1%3=1, 2%3=2, 3%3=0, 4%3=1, etc.) 
      # This will be 0 for 3, 6, etc. 
      # But we want it to be 0 (true) for 1, 4, etc : so we substract 1 to NR

# to see that it keeps those lines:
seq 1 20 | awk '!((NR-1)%3)'
1
4
7
10
13
16
19

another alternative: if you know the lines you want match some unique pattern :

# for exemple each line you want contains "some pattern":
fgrep "some pattern" file > newfile
# (or regular grep if you want "some pattern" to be interpreted as a regexp)