How to get a block at an offset in the IO.foreach loop in ruby? - ruby

Question

I'm using the IO.foreach loop to find a string using regular expressions. I want to append the next block (next line) to the file_names list. How can I do that?
file_names = [""]
IO.foreach("a.txt") { |block|
if block =~ /^file_names*/
dir = # get the next block
file_names.append(dir)
end
}
Actually my input looks like this:
file_names[174]:
name: "vector"
dir_index: 1
mod_time: 0x00000000
length: 0x00000000
file_names[175]:
name: "stl_bvector.h"
dir_index: 2
mod_time: 0x00000000
length: 0x00000000
I have a list of file_names, and I want to capture each of the name, dir_index, mod_time and length properties and put them into the files_names array index according to the file_names index in the text.

Answer 1

You can use #each_cons to get the value of the next 4 rows from the text file:
files = IO.foreach("text.txt").each_cons(5).with_object([]) do |block, o|
if block[0] =~ /file_names.*/
o << block[1..4].map{|e| e.split(':')[1]}
end
end
puts files
#=> "vector"
# 1
# 0x00000000
# 0x00000000
# "stl_bvector.h"
# 2
# 0x00000000
# 0x00000000
Keep in mind that the files array contains subarrays of 4 elements. If the : symbol occurs later in the lines, you could replace the third line of my code with this:
o << block[1..4].map{ |e| e.partition(':').last.strip}
I also added #strip in case you want to remove the whitespaces around the values. With this line changed, the actual array will look something like this:
p files
#=>[["\"vector\"", "1", "0x00000000", "0x00000000"], ["\"stl_bvector.h\"", "2", "0x00000000", "0x00000000"]]
(the values don't contain the \ escape character, that's just the way #p shows it).
Another option, if you know the pattern 1 filename, 4 values will be persistent through the entire text file and the textfile always starts with a filename, you can replace #each_cons with #each_slice and remove the regex completely, this will also speed up the entire process:
IO.foreach("text.txt").each_slice(5).with_object([]) do |block, o|
o << block[1..4].map{ |e| e.partition(':').last.strip }
end

Answer 2

It's actually pretty easy to carve up a series of lines based on a pattern using slice_before:
File.readlines("data.txt").slice_before(/\Afile_names/)
Now you have an array of arrays that looks like:
[
[
"file_names[174]:\n",
" name: \"vector\"\n",
" dir_index: 1\n",
" mod_time: 0x00000000\n",
" length: 0x00000000\n"
],
[
"file_names[175]:\n",
" name: \"stl_bvector.h\"\n",
" dir_index: 2\n",
" mod_time: 0x00000000\n",
" length: 0x00000000"
]
]
Each of these groups could be transformed further, like for example into a Ruby Hash using those keys.