It's been quite a while time since I last wrote code in Ruby (Ruby 2 was new and wow it's 3 already), so I feel like an idiot.
I have a text file containing only the word:
hello
My ruby file contains the following code:
content = File.read("test_file_str.txt","w")
puts content
When I run it, I get:
`read': no implicit conversion of String into Integer (TypeError)
I've never had this happen before, but it has been quite a while since I wrote code, so clearly PEBKAC.
However, when I run this without ,"w" all is seemingly well. What am I doing wrong?
ruby 3.0.3p157 (2021-11-24 revision 3fb7d2cadc) [x64-mingw32]
As per the docs, the second argument for File.read is the length of bytes to be read from the given file which is meant to be an integer.
Opens the file, optionally seeks to the given offset, then returns length bytes (defaulting to the rest of the file). read ensures the file is closed before returning.
So, in your case the error happens because you're passing an argument which must be an integer. It doesn't state this per-se in the docs for File.read, but it does it for File#read:
Reads length bytes from the I/O stream.
length must be a non-negative integer or nil.
If you want to specify the mode, you can use the mode option for that:
File.read("filename", mode: "r") # "r" or any other
# or
File.new("filename", mode: "r").read(1)
Open Files for Reading Don't Accept Write Mode
In general, it doesn't make sense to open a filehandle for reading in write mode. So, you need to refactor your method to something like:
content = File.read("test_file_str.txt")
or perhaps:
content = File.new("test_file_str.txt", "r+").read
depending on exactly what you're trying to do.
See Also: File Permissions in IO#new
The documentation for File in Ruby 3.0.3 points you to IO#new for the available mode permissions. You might take a look there if you don't see exactly the options you're looking for.
Related
I'm trying to save and load the states of Matrices (using Matrix) during the execution of my program with the functions dump and load from Marshal. I can serialize the matrix and get a ~275 KB file, but when I try to load it back as a string to deserialize it into an object, Ruby gives me only the beginning of it.
# when I want to save
mat_dump = Marshal.dump(#mat) # serialize object - OK
File.open('mat_save', 'w') {|f| f.write(mat_dump)} # write String to file - OK
# somewhere else in the code
mat_dump = File.read('mat_save') # read String from file - only reads like 5%
#mat = Marshal.load(mat_dump) # deserialize object - "ArgumentError: marshal data too short"
I tried to change the arguments for load but didn't find anything yet that doesn't cause an error.
How can I load the entire file into memory? If I could read the file chunk by chunk, then loop to store it in the String and then deserialize, it would work too. The file has basically one big line so I can't even say I'll read it line by line, the problem stays the same.
I saw some questions about the topic:
"Ruby serialize array and deserialize back"
"What's a reasonable way to read an entire text file as a single string?"
"How to read whole file in Ruby?"
but none of them seem to have the answers I'm looking for.
Marshal is a binary format, so you need to read and write in binary mode. The easiest way is to use IO.binread/write.
...
IO.binwrite('mat_save', mat_dump)
...
mat_dump = IO.binread('mat_save')
#mat = Marshal.load(mat_dump)
Remember that Marshaling is Ruby version dependent. It's only compatible under specific circumstances with other Ruby versions. So keep that in mind:
In normal use, marshaling can only load data written with the same major version number and an equal or lower minor version number.
Probably a simple question, but I need to delete the contents of a file after a specific line number? So I wan't to keep the first e.g 5 lines and delete the rest of the contents of a file. I have been searching for a while and can't find a way to do this, I am an iOS developer so Ruby is not a language I am very familiar with.
That is called truncate. The truncate method needs the byte position after which everything gets cut off - and the File.pos method delivers just that:
File.open("test.csv", "r+") do |f|
f.each_line.take(5)
f.truncate( f.pos )
end
The "r+" mode from File.open is read and write, without truncating existing files to zero size, like "w+" would.
The block form of File.open ensures that the file is closed when the block ends.
I'm not aware of any methods to delete from a file so my first thought was to read the file and then write back to it. Something like this:
path = '/path/to/thefile'
start_line = 0
end_line = 4
File.write(path, File.readlines(path)[start_line..end_line].join)
File#readlines reads the file and returns an array of strings, where each element is one line of the file. You can then use the subscript operator with a range for the lines you want
This isn't going to be very memory efficient for large files, so you may want to optimise if that's something you'll be doing.
I am following Wicked cool ruby scripts book.
here,
there are two files, file_output = file_list.txt and oldfile_output = file_list.old. These two files contain list of all files the program went through and going to go through.
Now, the file is renamed as old file if a 'file_list.txt' file exists .
then, I am not able to understand the code.
Apparently every line of the file is read and the line is stored in oldfile hash.
Can some one explain from 4 the line?
And also, why is gets used here? why cant a .each method be used to read through every line?
if File.exists?(file_output)
File.rename(file_output, oldfile_output)
File.open(oldfile_output, 'rb') do |infile|
while (temp = infile.gets)
line = /(.+)\s{5,5}(\w{32,32})/.match(temp)
puts "#{line[1]} ---> #{line[2]}"
oldfile_hash[line[1]] = line[2]
end
end
end
Judging from the redundant use of quantifiers ({5,5} and {32,32}) in the regex (which would be better written as {5}, {32}), it looks like the person who wrote that code is not a professional Ruby programmer. So you can assume that the choice taken in the code is not necessarily the best.
As you pointed out, the code could have used each instead of while with gets. The latter approach is sort of an old-school Ruby way of doing it. There is nothing wrong in using it. Until the end of file is reached, gets will return a string, and when it does reach the end of file, gets will return nil, so the while loop works as the same when you use each; in each iteration, it reads the next line.
It looks like each line is supposed to represent a key-value pair. The regex assumes that the key is not an empty string, and that the key and the value are separated by exactly five spaces, and the the value consists of exactly thirty-two letters. Each key-value pair is printed (perhaps for monitoring the progress), and is stored in oldfile_hash, which is most likely a hash.
So the point of using .gets is to tell when the file is finished being read. Essentially, it's tied to the
while (condition)
....
end
block. So gets serves as a little method that will keep giving ruby the next line of the file until there is no more lines to give.
I am creating a grammar corrector app. You input slang and it returns a formal English correction. All the slang words that are supported are kept inside arrays. I created a method that looks like this for when a slang is entered that is not supported.
def addtodic(lingo)
print"\nCorrection not supported. Please type a synonym to add '#{lingo}' the dictionary: "
syn = gets.chomp
if $hello.include?("#{syn}")
$hello.unshift(lingo)
puts"\nCorrection: Hello.\n"
elsif $howru.include?("#{syn}")
$howru.unshift(lingo)
puts"\nCorrection: Hello. How are you?\n"
end
end
This works, but only until the application is closed. how can I make this persist so that it amends the source code as well? If I cannot, how would I go about creating an external file that holds all of the cases and referencing that in my source code?
You will want to load and store your arrays in a external file.
How to store arrays in a file in ruby? is relevant to what you are trying to do.
Short example
Suppose you have a file that has one slang phrase per line
% cat hello.txt
hi
hey
yo dawg
The following script will read the file into an array, add a term, then write the array to a file again.
# Read the file ($/ is record separator)
$hello = File.read('hello.txt').split $/
# Add a term
$hello.unshift 'hallo'
# Write file back to original location
open('hello.txt', 'w') { |f| f.puts $hello.join $/ }
The file will now contain an extra line with the term you just added.
% cat hello.txt
hallo
hi
hey
yo dawg
This is just one simple way of storing an array to file. Check the link at the beginning of this answer for other ways (which will work better for less trivial examples).
I've got a parser written using ruby's standard StringScanner. It would be nice if I could use it on streaming files. Is there an equivalent to StringScanner that doesn't require me to load the whole string into memory?
You might have to rework your parser a bit, but you can feed lines from a file to a scanner like this:
File.open('filepath.txt', 'r') do |file|
scanner = StringScanner.new(file.readline)
until file.eof?
scanner.scan(/whatever/)
scanner << file.readline
end
end
StringScanner was intended for that, to load a big string and going back and forth with an internal pointer, if you make it a stream, then the references get lost, you can not use unscan, check_until, pre_match, post_match,
well you can, but for that you need to buffer all the previous input.
If you are concerned about the buffer size then just load by chunk of data, and use a simple regexp or a gem called Parser.
The simplest way is to read a fix size of data.
# iterate over fixed length records
open("fixed-record-file") do |f|
while record = f.read(1024)
# parse here the record using regexp or parser
end
end
[Updated]
Even with this loop you can use StringSanner, you just need to update the string with each new chunk of data:
string=(str)
Changes the string being scanned to str and resets the scanner.
Returns str
There is StringIO.
Sorry misread you question. Take a look at this seems to have streaming options