Unexpected Interaction Of Features

   
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An Unexpected Interaction of Features - 2018/06/07

I've been dealing with some data, and using my usual technique of using command-line tools to play with it for a while before writing a program to do the full analysis.

But something was wrong, and it took me a while to work it out.

I was sorting a file:

which
aerodynamically
electroencephalogram
exotically
aerodynamically
a
differentiation
-> a
aerodynamically
aerodynamically
differentiation
electroencephalogram
exotically
which

But my file has as the first field a count:

5 which
15 aerodynamically
20 electroencephalogram
10 exotically
15 aerodynamically
1 a
15 differentiation
-> 10 exotically
15 aerodynamically
15 aerodynamically
15 differentiation
1 a
20 electroencephalogram
5 which

That's not what I wanted, but this was a game I'd played before. The utility sort is working on the data as text, so it's alphabetical. I need to sort using -n to get it to sort numerically:

5 which
15 aerodynamically
20 electroencephalogram
10 exotically
15 aerodynamically
1 a
15 differentiation
-> 1 a
5 which
10 exotically
15 aerodynamically
15 aerodynamically
15 differentiation
20 electroencephalogram

Excellent, but now I realise there are repeated lines, and I need to de-duplicate. So I use sort -u to do that:

5 which
15 aerodynamically
20 electroencephalogram
10 exotically
15 aerodynamically
1 a
15 differentiation
-> 10 exotically
15 aerodynamically
15 differentiation
1 a
20 electroencephalogram
5 which

The duplication is gone, but the screwy ordering is back, because I forgot the "numerical" flag, so sort -nu is what I need:

5 which
15 aerodynamically
20 electroencephalogram
10 exotically
15 aerodynamically
1 a
15 differentiation
-> 1 a
5 which
10 exotically
15 aerodynamically
20 electroencephalogram

Spot the difference.

Yes, the "differentiation" line has gone, and I can only assume that when both the n and u flags are set, it only takes the numbers into account when deciding if there are duplicates. I haven't explored whether, for a given number, it (a) sorts and keeps the first, (b) sorts and keeps the last, (c) keeps the first in the input then sorts, (d) keeps the last in the input then sorts, or (e) something else.

But it's certainly not what I expected.

So now it's back to using "sort -n | uniq" rather than "sort -nu".

For reference: "sort --version" returns "sort (GNU coreutils) 8.21"


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