What does 'a' mean in Ruby `open()`, and what does |f| mean? - ruby

Question

What does 'a' and |f| mean below ?
open('myfile.out', 'a') { |f|
f.puts "Hello, world."
}

Answer 1

From the ruby IO doc:
"a" | Write-only, starts at end of file if file exists,
| otherwise creates a new file for writing.
The |f| is a variable that holds the IO object in the block (everything in the {}). So when you f.puts "Hello World" you're calling puts on the IO object which then writes to the file.

Answer 2

The 'a' is just a file open mode, like you'd see in C / C++. It means append, and is relatively uncommon - you're more likely to be familiar with 'r' (read), 'w' (write), etc.
The {|f| ... } bit is the exciting part. It's called a a block - they're everywhere, and they're probably my favourite part of Ruby - I've gone back to C++ recently, and I find myself cursing the language for not supporting them all the time.
Think of code like foo(bar) {|baz| ... } as creating a nameless function, and passing that function as another (hidden) argument to foo (kinda like this is a hidden argument to member functions in C++) - it's just not as hidden, 'cause you specify it right there.
Now, when you pass the block to foo, it will eventually call your block (using the yield statement), and it will supply the argument baz. If my foo behaved like your File.open function, its definition would look something like this:
def foo(filename, &block)
file = File.open(filename)
yield(file)
file.close
end
You can see how it opens the file, passes it to your block with yield, and then closes the file once your block returns. Very convenient - blocks are your friends!
Another good place to start wrapping your head around them is the each function - one of the simplest and most common block functions in Ruby:
[holt#Michaela ~]$ irb
irb(main):001:0> ['Welcome', 'to', 'Ruby!'].each {|word| puts word}
Welcome
to
Ruby!
=> ["Welcome", "to", "Ruby!"]
irb(main):002:0>
This time, your block gets called three times, and each time a different array element gets yielded to your block as word - it's a super-simple way to call a function for every element of an array.
Hope this helps, and welcome to Ruby!

Answer 3

'a' -> Mode in which to open the file ('append' mode)
f is a parameter to the block. A block is a piece of code that can be executed (it is a Proc object underneath).
Here, f will be the file descriptor, I think.

Answer 4

1) You call the open method, passing in the two arguments:
myfile.out <-- This is your file that you want to access
a <-- you are stating that you want to write to a file, starting at the end of the file(aka append)
2) The method open that exists in Kernel, yields an IO stream object aka |f|, in which you can access throughout your block.
3) You are appending "hello world" to myfile.out
4) Once the block ends, the IO stream closes.

Answer 5

The 'a', which stands for append, opens the file in write-only mode and starts writing at the end of the file. If no file exists, a new file is created. Please see the Ruby Docs for more information.
The |f| is a block parameter, which is being passed within the {}. For more information on blocks, please see The Pragmatic Programmer's Guide.

Answer 6

I would highly suggest reading through the help file for the File class for starters.
You can see there the documentation for the open method.
The method signature is File.open(filename, mode)
So, in your example, a, is the mode which in this case is append. Here's a list of valid values for the mode argument:
'r' - Open a file for reading. The file must exist.
'w' - Create an empty file for writing. If a file with the same name already exists its content is erased and the file is treated as a new empty file.
'a' - Append to a file. Writing operations append data at the end of the file. The file is created if it does not exist.
'r+' - Open a file for update both reading and writing. The file must exist.
'w+' - Create an empty file for both reading and writing. If a file with the same name already exists its content is erased and the file is treated as a new empty file.
'a+' - Open a file for reading and appending. All writing operations are performed at the end of the file, protecting the previous content to be overwritten. You can reposition (fseek, rewind) the internal pointer to anywhere in the file for reading, but writing operations will move it back to the end of file. The file is created if it does not exist.
If File.open is used in a block, such as in your example, then f becomes the block variable that points to the newly-opened file, which allows you to both read and write to the file just using f as the reference, while within the block. Using this form of File.open is nice because it handles closing the file automatically when the block ends.

Answer 7

open('myfile.out', 'a') -> Here 'a' means Write only access. Pointer is positioned at end of file.
|f| is the file descriptor, it does puts of "Hello, World."
Instead of |f|, you can write anything, say |abc| or |line|, it doesn't matter.