Divide largest value by second largest value - awk

Question

I am having a file in the following format. Column one has ~20,000 uniq entry and column 2 has ~120,000 different entry and column 3 has count associated with column 2. For a single entry in column 1 there can be multiple entry in column 2. For each unique entry in column 1, I am trying to get ratio of maximum value to second maximum value of column 3.
F1.txt
S1 S2 C1
A A1 1
A AA 10
A A6 5
A A0 4
B BB 12
B BC 11
B B1 19
B B9 4
Expected Output
S1 S2 C1
B B1 19 1.58333
A AA 10 2
I can do in steps like bellow. But is there a smart way of doing in in one script?
awk 'NR==1; NR > 1 {print $0 | "sort -k3 -n -r "}' F1.txt | awk '!seen[$1]++' >del1.txt
awk 'FNR==NR{a[$2]=1; next}FNR==1{print $0;}!a[$2]' del1.txt F1.txt | awk 'NR==1; NR > 1 {print $0 | "sort -k3 -n -r"}' | awk '!seen[$1]++' >del2.txt
awk 'FNR==NR{a[$1]=$3; next}FNR==1{print $0"\t";"RT"}FNR>1 a[$1]{print $0"\t"$3/a[$1]}' del2.txt del1.txt

Answer 1

#!/usr/bin/awk -f
NR == 1 { print $1, $2, $3; next }
{ data[$1][$3] = $2 }
END {
for (key in data) {
asorti(data[key], s, "#ind_num_desc")
print key, data[key][s[1]], s[1], s[1] / s[2]
}
}
This^^^ assumes an arbitrary permutation of the lines (and requires gawk (which is pretty common) or another implementation with native multi-dimensional “arrays”).
If you can make more assumptions about the input — e.g. that it is always grouped by the first column —, then you can make it more memory-efficient and get rid of multi-dimensional arrays (by not delaying the evaluation until END and instead calculating it in a per-line block each time the first column’s value changes (and then one last time in END).)
To get a different handling of equal numeric values (e.g. to report the “subkey” (column 2) of the first (instead of last) encountered occurrence of a value), you could add if (!($3 in data[$1])) ... or the like.

Answer 2

Whenever you find yourself creating a pipeline containing awk, there is a very good chance that what you are trying to do can be done in a single call to awk much more efficiently.
A non-GNU awk approach that presumes all field1 'A' records are together and all 'B' records are together (as you show in your sample data) could be:
awk '
FNR==1 { print; next } ## 1st line, output heading
$1 != n { ## 1st field changed
if (n) { ## if n set, output result of last block
printf "%s\t%s\n", rec, max / nextmax
}
rec = $0 ## initialize vars for next block
n = $1
max = $3
nextmax = 1
next ## skip to next record
}
{
if ($3 > max) { ## check if 3rd field > max
rec = $0 ## save record
nextmax = max ## update nextmax
max = $3 ## update max
}
else if ($3 > nextmax) { ## if 3rd field > nextmax
nextmax = $3 ## update nextmax
}
} ## output final block results
END { printf "%s\t%s\n", rec, max / nextmax }
' file
Example Use/Output
With your data in the file file, you would have:
$ awk '
> FNR==1 { print; next } ## 1st line, output heading
> $1 != n { ## 1st field changed
> if (n) { ## if n set, output result of last block
> printf "%s\t%s\n", rec, max / nextmax
> }
> rec = $0 ## initialize vars for next block
> n = $1
> max = $3
> nextmax = 1
> next ## skip to next record
> }
> {
> if ($3 > max) { ## check if 3rd field > max
> rec = $0 ## save record
> nextmax = max ## update nextmax
> max = $3 ## update max
> }
> else if ($3 > nextmax) { ## if 3rd field > nextmax
> nextmax = $3 ## update nextmax
> }
> } ## output final block results
> END { printf "%s\t%s\n", rec, max / nextmax }
> ' file
S1 S2 C1
A AA 10 2
B B1 19 1.58333

Answer 3

Using any awk in any shell on every Unix box and using almost no memory (important since your input file would be huge given your description of it):
$ cat tst.awk
BEGIN { FS=OFS="\t" }
NR == 1 { print; next }
$1 != prev {
if ( prev != "" ) {
print prev, val, max, (preMax ? max/preMax : 0)
}
prev = $1
max = ""
}
(max == "") || ($3 > max) {
val = $2
preMax = max
max = $3
}
END { print prev, val, max, (preMax ? max/preMax : 0) }
$ awk -f tst.awk F1.txt
S1 S2 C1
A AA 10 10
B B1 19 1.58333