Storing and manipulating columns of data

Ruby
1

Hi,

Being used to deal with Octave / Matlab, I’m a bit confused by Ruby
structures (arrays, hashes, etc). I have this data file “data.dat”
with 3 columns:

6.199000 1.001000e+00 4.720000e-05
8.265000 9.990000e-01 1.210000e-04

I would like to think of the columns as arrays “a”, “b”, “c”, execute
a loop along their indices, pick the values i want, and do something
with them. What would be a sensible data structure for this?
I can read this datafile line by line using IO:readlines(“data.dat”),
but I cannot find a way to refer to one column in the array of
strings generated.

Best regards,

baptiste

2

baptiste Auguié wrote:

2.480000 1.001000e+00 1.520000e-05
loop along their indices, pick the values i want, and do something with
them. What would be a sensible data structure for this?
I can read this datafile line by line using IO:readlines(“data.dat”),
but I cannot find a way to refer to one column in the array of strings
generated.
In native Ruby, the only way to do that would be:

arr = [[0,1],[2,3],[4,5]]
arr.map{|a| a[0]}

=> [0,2,4]

If you can install rb-gsl (and it’s probably a good idea if you’re
expecting to do matlab/octavey things), then there’s the
GSL::Matrix#column method which (as I understand it, not being a heavy
GSL user) actually gives a reference into the original matrix, rather
than copying to a new one as Array#map does.

3

baptiste Auguié wrote:

but I cannot find a way to refer to one column in the array of strings
generated.

You can use String#split to split each line into an array of 3 numbers.
If you want to use the array values as numbers, you’ll need to use
String#to_f as well to convert the strings to numeric values.

ri String#split
ri String#to_f

4

On Jun 30, 2007, at 6:37 AM, baptiste Auguié wrote:

3.099000 1.003000e+00 3.570000e-05
I can read this datafile line by line using IO:readlines
(“data.dat”), but I cannot find a way to refer to one column in the
array of strings generated.

Maybe something like this is what you’re looking for:

data = DATA.readlines rows = data.map do |row| row.split(/\s+/).map { |elt| elt.to_f } end a, b, c = rows.transpose p a, b, c __END__ 1.240000 9.990000e-01 1.290000e-06 1.550000 1.000000e+00 2.920000e-06 2.066000 1.002000e+00 8.360000e-06 2.480000 1.001000e+00 1.520000e-05 [1.24, 1.55, 2.066, 2.48] [0.999, 1.0, 1.002, 1.001] [1.29e-06, 2.92e-06, 8.36e-06, 1.52e-05]

Regards, Morton

5

Dear Baptiste,

I’d also recommend rb-gsl, but if you have trouble
installing from source, you can also use the Matrix
class from Ruby:

http://www.ruby-doc.org/stdlib/libdoc/matrix/rdoc/classes/Matrix.html

require “matrix”

a=Matrix.columns([[25, 93], [-1, 66]])
=> 25 -1
93 66

a.transpose
=> 25 93
-1 66

Best regards,

Axel

6

Thanks everyone for your suggestions, it’s good to know different
options. This community is great!

Thanks,

baptiste