How to write in the middle of a file

Ruby
1

Greetings all. For some reason the answer to this stumps me.
Say I have an existing file, 1K long, and I want to write something
right in the middle of it, without changing the file size.
I would have thought that
a = File.open ‘file’, ‘a+’
a.seek 500
a.write ‘abc’
a.close

would work but doesn’t seem to. Thoughts?
-R

2

On Jun 25, 2008, at 8:59 PM, Roger P. wrote:

Greetings all. For some reason the answer to this stumps me.
Say I have an existing file, 1K long, and I want to write something
right in the middle of it, without changing the file size.

meaning you want to OVERWRITE something in the middle, yes?

I would have thought that
a = File.open ‘file’, ‘a+’
a.seek 500
a.write ‘abc’
a.close

would work but doesn’t seem to. Thoughts?
-R

Try a mode of ‘r+’. The ‘a’ part says you want all the writes to go
the the end-of-file (whatever that happens to be at the time).

File.open ‘file’, ‘w’ do |f| f.write ‘x’*1024 end

a = File.open ‘file’, ‘r+’
a.seek 500
a.write ‘abc’
a.close

all = File.read(‘file’)

irb> puts all.size
1024
=> nil
irb> puts all.index(‘abc’)
500
=> nil

You can puts(all) yourself if you want :wink:

-Rob

Rob B. http://agileconsultingllc.com
[email protected]

3

Try a mode of ‘r+’. The ‘a’ part says you want all the writes to go
the the end-of-file (whatever that happens to be at the time).

Works like a charm. You rock!
-R