copying every nth line and duplicating it on it's following line

I am trying to make test files for the project, and I figured in order to make a bradycardia test file from an example file of a normal ECG. therefore I would need to copy every third line and insert it into the next line.

for example:

a =

[1 2 3 4 5 6 7 8 9 10]

and I want:

b=

[1 2 3 3 4 5 6 6 7 8 9 9 10]

and so on... but since the file is 6000 characters long, obviously i cannot manually copy it. And I need it to be 9000 characters long I've tried looking online on how to do this, and am having no luck. Any suggestions?

Answer 1

[Cedric]

Here is an example where I display the output so you can see each step:

 >> a = [1 2 3 4 5 6 7 8 9 10 11 12]
 a =
     1     2     3     4     5     6     7     8     9    10    11    12

 >> tmp = reshape( a, 3, [] )
 tmp =
     1     4     7    10
     2     5     8    11
     3     6     9    12

 >> tmp = [tmp; tmp(end,:)]
 tmp =
     1     4     7    10
     2     5     8    11
     3     6     9    12
     3     6     9    12

 >> b = tmp(:).'
 b =
    1    2    3    3    4    5    6    6    7    8    9    9   10   11   12   12

The only limitation is that the length of a must be a multiple of 3. If it's not the case, you can complete/pad with e.g. NaNs to meet this criterion, and replace the last line which defines b with

 >> b = tmp(~isnan(tmp)).'

Let me know if you need to find an automatic way to complete with NaNs.

[Chace]

you're an angel. thank you!

[Cedric]

You're welcome!