unix sort groups by their associated maximum value? - bash

Question

Let's say I have this input file 49142202.txt:
A 5
B 6
C 3
A 4
B 2
C 1
Is it possible to sort the groups in column 1 by the value in column 2? The desired output is as follows:
B 6 <-- B group at the top, because 6 is larger than 5 and 3
B 2 <-- 2 less than 6
A 5 <-- A group in the middle, because 5 is smaller than 6 and larger than 3
A 4 <-- 4 less than 5
C 3 <-- C group at the bottom, because 3 is smaller than 6 and 5
C 1 <-- 1 less than 3
Here is my solution:
join -t$'\t' -1 2 -2 1 \
<(cat 49142202.txt | sort -k2nr,2 | sort --stable -k1,1 -u | sort -k2nr,2 \
| cut -f1 | nl | tr -d " " | sort -k2,2) \
<(cat 49142202.txt | sort -k1,1 -k2nr,2) \
| sort --stable -k2n,2 | cut -f1,3
The first input to join sorted by column 2 is this:
2 A
1 B
3 C
The second input to join sorted by column 1 is this:
A 5
A 4
B 6
B 2
C 3
C 1
The output of join is:
A 2 5
A 2 4
B 1 6
B 1 2
C 3 3
C 3 1
Which is then sorted by the nl line number in column 2 and then the original input columns 1 and 3 are kept with cut.
I know it can be done a lot easier with for example groupby of pandas of Python, but is there a more elegant way of doing it, while sticking to the use of GNU Coreutils such as sort, join, cut, tr and nl? Preferably I want to avoid a memory inefficient awk solution, but please share those as well. Thanks!

Answer 1

As explained in the comment my solution tries to reduce the number of pipes, unnecessary cat commands and more especially the number of pipeline sort operations since sorting is a complex/time consuming operation:
I reached the following solution where f_grp_sort is the input file:
for elem in $(sort -k2nr f_grp_sort | awk '!seen[$1]++{print $1}')
do
grep $elem <(sort -k2nr f_grp_sort)
done
OUTPUT:
B 6
B 2
A 5
A 4
C 3
C 1
Explanations:
sort -k2nr f_grp_sort will generate the following output:
B 6
A 5
A 4
C 3
B 2
C 1
and sort -k2nr f_grp_sort | awk '!seen[$1]++{print $1}' will generate the output:
B
A
C
the awk will just generate in the same order 1 unique element of the first column of the temporary output.
Then the for elem in $(...)do grep $elem <(sort -k2nr f_grp_sort); done
will grep for lines containing B then A, then C what will provide the required output.
Now as enhancement, you can use a temporary file to avoid doing sort -k2nr f_grp_sort operation twice:
$ sort -k2nr f_grp_sort > tmp_sorted_file && for elem in $(awk '!seen[$1]++{print $1}' tmp_sorted_file); do grep $elem tmp_sorted_file; done && rm tmp_sorted_file

Answer 2

So, this won't work for all cases, but if the values in your first column can be turned into bash variables, we can use dynamically named arrays to do this instead of a bunch of joins. It should be pretty fast.
The first while block reads in the contents of the file, getting the first two space separated strings and putting them into col1 and col2. We then create a series of arrays named like ARR_A and ARR_B where A and B are the values from column 1 (but only if $col1 only contains characters that can be used in bash variable names). The array contains the column 2 values associated with these column 1 values.
I use your fancy sort chain to get the order we want column 1 values to print out in, we just loop through them, then for each column 1 array we sort the values and echo out column 1 and column 2.
The dynamc variable bits can be hard to follow, but for the right values in column 1 it will work. Again, if there's any characters that can't be part of a bash variable name in column 1, this solution will not work.
file=./49142202.txt
while read col1 col2 extra
do
if [[ "$col1" =~ ^[a-zA-Z0-9_]+$ ]]
then
eval 'ARR_'${col1}'+=("'${col2}'")'
else
echo "Bad character detected in Column 1: '$col1'"
exit 1
fi
done < "$file"
sort -k2nr,2 "$file" | sort --stable -k1,1 -u | sort -k2nr,2 | while read col1 extra
do
for col2 in $(eval 'printf "%s\n" "${ARR_'${col1}'[#]}"' | sort -r)
do
echo $col1 $col2
done
done
This was my test, a little more complex than your provided example:
$ cat 49142202.txt
A 4
B 6
C 3
A 5
B 2
C 1
C 0
$ ./run
B 6
B 2
A 5
A 4
C 3
C 1
C 0

Answer 3

Thanks a lot #JeffBreadner and #Allan! I came up with yet another solution, which is very similar to my first one, but gives a bit more control, because it allows for easier nesting with for loops:
for x in $(sort -k2nr,2 $file | sort --stable -k1,1 -u | sort -k2nr,2 | cut -f1); do
awk -v x=$x '$1==x' $file | sort -k2nr,2
done
Do you mind, if I don't accept either of your answers, until I have time to evaluate the time and memory performance of your solutions? Otherwise I would probably just go for the awk solution by #Allan.