I can do this with 2 passes over the file, but I was looking for a way to do this in one pass as files are very big. This is what I intend to do. Imagine a txt file with the following data 
Some garbage
More garbage
data -start <some string> \
     -intermediate <some string> \
     -intermadiate <some string> \
     .
     .
     -end <some string>
Some garbage
More garbage
data -start <some string> \
     -intermediate <some string> \
     -intermadiate <some string> \
     .
     .
     -end <some string>
Some garbage
More garbage
data -start <some string> \
     -intermediate <some string> \
     -intermadiate <some string> \
     .
     .
     -end <some string>
.
.
.

[download]
I want the output file to contain 
data -start <string> -end <string>
data -start <string> -end <string>
data -start <string> -end <string>
.
.
.

[download]
The catch? After removing intermediates, there will be lots of duplicates, which I want to remove. In my current flow, I read in the file, write out an array, and then unique the array 2 pass process seems to be a waste of time. If I can get a one pass algo, it will be great!

In reply to Sorting and subsituting a data file, one pass by tsk1979

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