Script to process a list of items and uncomment lines with that item in a second file

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Old 01-14-2020
Originally Posted by LMHmedchem
I went with this method inserted into a script. It worked well (and very quickly) the first time I tried it, but there was no output the second time.
Difficult to believe with unmodified input files. Pls report back your findings.

It seems to me that sed must be doing more or less the same thing under the hood. Every list item must be checked against every item in the file to be modified, at least until a match is found.
A regex using "alternation" as brought into play in rbatte1's post #4 will be the most efficient approach, as it will scan each input line once with all alternations "in parallel".The "command substitution" to produce the alternation (pasteing from file2) will be done once, and upfront.

I wasn't able to rationalize if it was more efficient to have one or the other file be the inner loop.
Don't use file operations in the inner loop if at all avoidable. They're costly and have to be repeated for every single line read / operated upon in the outer loop.

If the list was the inner loop, you could delete each array element when a match was found and thus shorten the search as the process continues but deleting and shifting around array elements also takes resources.
Yes if you
- are sure no more occurrences of the element will come
- have access to the algorithm. True for your own shell script (slooow by itself), false for binary commands like sed.

Does anyone know what sed is doing to achieve the result so quickly? Is it mainly that is is using compiled code?

I guess it's optimized for (complex!) regex matching. And yes, compiled code.
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