How can I randomly weight an element from an array and then pick it based on those weights?

Question

I want to pseudo-randomly pick one element from an array of 5 elements: I want to control the occurrence probability of each of those 5 elements.

Example: I have an array like this: [A B C D E]

• I want the probability to pick A: 0.10 (10%)
• I want the probability to pick B: 0.10 (10%)
• I want the probability to pick C: 0.20 (20%)
• I want the probability to pick D: 0.20 (20%)
• I want the probability to pick E: 0.40 (40%)

I have seen that I can weight a random selection from an array here: Weighted random selection from array.

How can I weight elements contained in an array like this?

Answer 1

You'd use the bash builtin variable RANDOM, with a little arithmetic

weighted_selection() {
 local ary=("$@")
 case $(( RANDOM % 10 )) in
 0) index=0 ;; # one out of ten
 1) index=1 ;; # one out of ten
 2|3) index=2 ;; # two out of ten
 4|5) index=3 ;; # two out of ten
 *) index=4 ;; # remaining is four out of ten
 esac
 echo ${ary[index]}
}

Let's test it:

a=(A B C D E)
declare -A count
for ((i=1; i<1000; i++)); do
 (( count[$(weighted_selection "${a[@]}")]++ ))
done
declare -p count

outputs

declare -A count='([A]="99" [B]="100" [C]="211" [D]="208" [E]="381" )'

• chepner: RANDOM ranges from 0 to 32767, so this really produces a distribution like 10.006%/10.006%/20.012%/20.012%/39.99%. If that difference matters (and it probably shouldn't), you could do something like x=32768; until (( x < 32760 )); x=$RANDOM, then check $x % 10.
• Soma Hesk: does it have go version for this algo?