Start loop at specific line of text file in groovy - groovy

Question

I am using groovy and I am trying to have a text file be altered at specific line, without looping through all of the previous lines. Is there a way to state the line of a text file that you want to wish to alter?
For instance
Text file is:
1
2
3
4
5
6
I would like to say
Line(3) = p
and have it change the text file to:
1
2
p
4
5
6
I DO NOT want to have to do a loop to iterate through the lines to change the value, aka I do not want to use a .eachline {line ->...} method.
Thank you in advance, I really appreciate it!

Answer 1

I dont think you can skip lines and traverse like this. You could do the skip by using the Random Access File in java, but instead of lines you should be specifying the number of bytes.

Answer 2

Try using readLines() on file text. It will store all your lines in a list. To change content at line n, change content at n-1 index on list and then join on list items.
Something like this will do
//We can call this the DefaultFileHandler
lineNumberToModify = 3
textToInsert = "p"
line( lineNumberToModify, textToInsert )
def line(num , text){
list = file.readLines()
list[num - 1] = text
file.setText(list.join("\n"))
}
EDIT: For extremely large files, it is better that you have a custom implementation. May be something on the lines of what Tim Yates had suggested in the comment on your question.
The above readLines() can easily process upto 100000 lines of text within less than a sec. So you can do something like this:
if(file size < 10 MB)
use DefaultFileHandler()
else
use CustomFileHandler()
//CustomFileHandler
- Split the large file into buckets of acceptable size.
- Ex: Bucket 1(1-100000 lines), Bucket 2(100000-200000 lines), etc.
- if (lineNumberToModify falls in bucket range)
insert into line in the bucket
There is no hard and fast rule to define how you implement your CustomFileHandler as it completely depends on the use case scenario. If you need to do the above operation multiple times on the same file, you can choose to do the complete bucket split first, store them in memory and use the buckets for the following operations. Or if it is a one time operation, you can avoid manipulating all the buckets first but deal with only what you need and process the others later on on-demand basis.
And even within the buckets you can define your own intelligence to speed up your job. Say if you want to insert into 99999 line of a bucket with 1-100000 lines, you can exploit groovy's methods and closures to their fullest,
file.readLines().reverse()[1] = "some text"