reshape a char array

Hi,

I have the following MWE and I don't understand what matlab is producing:

I start with a simple 21-length char array:

ex =  'caggtgcagctggtgcagtct'

that I would like to reshape into a 7-by-3 array. Of course, I could use a loop for this, but I thought that I could just reshape it. However, when I do

reshape(ex, [7,3])
ans =

  7×3 char array

    'cag'
    'agc'
    'gca'
    'gtg'
    'tgt'
    'ggc'
    'ctt'
   

This makes no sense to me. I was expecting

'cag'
'gtg'
'cag'
'ctg'
'gtg'
'cag'
'tct'

I also tried reshape(ex, 7,3) and reshape(ex(:), 7, 3) without any luck. What is matlab doing? TIA

[Guillaume]

Matlab is column major. This means that when you reshape something into something else, the something else is filled column by column, not row by row. So, indeed as David shows, to get the output you want you reshape then transpose.

Answer 1

[David Hill]

reshape(ex,[3,7])';

First number is row and second is column. Matrix is numbered down first.

[Laura]

wow, that is really counter-intuitive. I guess I am used to other programming languages (i.e. python) that do not "number down first"...

Thank you!

[Guillaume]

You'll find a number of languages (C-based mostly) are row majors and an equivalent number are column-major (Matlab, Fortran, and other math-focused languages) while a few can't make their mind up (eg. OpenGL).

So, it's only counterintuitive if you've never been exposed to the other side.