Find second largest value in array

Question

I have an array like this:

array=(1 2 7 6) 

and would like to search for the second largest value, with the output being

secondGreatest=6 

Is there any way to do this in bash?

Answer 1

printf '%s\n' "${array[@]}" | sort -n | tail -2 | head -1 

Print each value of the array on it's own line, sort it, get the last 2 values, remove the last value

secondGreatest=$(printf '%s\n' "${array[@]}" | sort -n | tail -2 | head -1) 

Set that value to the secondGreatest variable.

---

Glenn Jackman had an excellent point about duplicate numbers that I didn't consider. If you only care about unique values you can use the -u flag of sort:

secondGreatest=$(printf '%s\n' "${array[@]}" | sort -nu | tail -2 | head -1) 

Answer 2

A bash-specific loop through the array could do it; you have to keep track of the largest and second-largest. The only other tricky part is to be careful about initializing those values; the largest value is initialized to the first element; the second-largest value is initialized the first time we see a value that's smaller than the largest value. Subsequently for the second-largest value, we only update it if it's strictly less than the current largest value:

#!/bin/bash array=(7 7 6 2 1) if [ "${#array[@]}" -lt 2 ] then echo Incoming array is not large enough >&2 exit 1 fi largest=${array[0]} secondGreatest='unset' for((i=1; i < ${#array[@]}; i++)) do if [[ ${array[i]} > $largest ]] then secondGreatest=$largest largest=${array[i]} elif (( ${array[i]} != $largest )) && { [[ "$secondGreatest" = "unset" ]] || [[ ${array[i]} > $secondGreatest ]]; } then secondGreatest=${array[i]} fi done echo "secondGreatest = $secondGreatest" 

It's still slower than calling out to sort, but it has the added benefit of picking the strictly-smaller second-largest value in the face of multiple high values (such as 7 and 7 above).

Answer 3

It's a good job for dc :

array=(1 2 7 6) echo ${array[*]} | dc -f - -e ' [lasbdsa]sB [dla!>Bsc1z>A]sA lAx [secondGreatest=]nlbp'