Quicksort implemented in shell script doesn't work - bash

Question

I am learning shell scripting and trying to implement quick sort using it.
But it doesn't work, actually it acting weird.
The script:
#!/bin/bash
declare -a data=()
declare -r size=10
declare -i steps=0
for i in $(seq 0 $size); do
data[$i]=$(expr $RANDOM % $size)
done
function partition() {
pivot=${data[$1]}
left=$(expr $1 + 1)
right=$2
while true; do
while [[ $left -le $right && ${data[$left]} -le $pivot ]]; do
left=$(expr $left + 1)
steps=$(expr $steps + 1)
done
while [[ $right -ge $left && ${data[$right]} -ge $pivot ]]; do
right=$(expr $right - 1)
steps=$(expr $steps + 1)
done
if [[ $left -gt $right ]]; then
break
fi
temp=${data[$left]}
data[$left]=${data[$right]}
data[$right]=$temp
done
temp=${data[$1]}
data[$1]=${data[$right]}
data[$right]=$temp
echo $right
}
function quickSort() {
if [[ $1 -lt $2 ]]; then
local partitionPoint=$(partition $1 $2)
quickSort $1 $(expr $partitionPoint - 1)
quickSort $(expr $partitionPoint + 1) $2
fi
}
# involve the algorithm
quickSort 0 $(expr $size - 1)
echo "Steps: $steps"
echo ${data[#]}
I tried to log some variable but it's just weird I can't figure out what's going on.
When I comment out all the code in the two functions and 'manually' update elements of data variable, it did changed.
I tried to log some variables and they all changing.
But the final output remains untouched.
Or maybe it eventually reversed all the flipping but I don't know.
I can't figure it out.
At last I compare my python implementation line by line. No mistakes. But it just not working.
Am I miss something?
Variable scope or something?
Any advice will be appreciated.

Answer 1

There are several smaller issues in this code, but the biggest issue is here:
partitionPoint=$(partition $1 $2)
This is problematic because $( ... ) runs ... in a subshell -- a separate, fork()ed-off process, consequently with its own variable scope.
If you instead return your result via indirect assignment, making it:
partition "$1" "$2" partitionPoint
and inside the function using:
printf -v "$3" %s "$right"
...to assign the value to the variable so named, things work much better.
#!/bin/bash
PS4=':$LINENO+'; set -x
data=()
size=10
steps=0
for ((i=0; i<size; i++)); do
data[$i]=$((RANDOM % size))
done
partition() {
local pivot left right dest temp
pivot=${data[$1]}
left=$(($1 + 1))
right=$2
dest=$3
while true; do
while (( left <= right )) && (( ${data[$left]} <= pivot )); do
left=$(( left + 1 ))
steps=$(( steps + 1 ))
done
while (( right >= left )) && (( ${data[$right]} >= pivot )); do
right=$(( right - 1 ))
steps=$(( steps + 1 ))
done
(( left > right )) && break
temp=${data[$left]}
data[$left]=${data[$right]}
data[$right]=$temp
done
: '$1='"$1" right="$right" 'data[$1]='"${data[$1]}" 'data[$right]='"${data[$right]}"
temp=${data[$1]}
data[$1]=${data[$right]}
data[$right]=$temp
printf -v "$dest" %s "$right"
}
quickSort() {
local partitionPoint
if (( $1 < $2 )); then
partition "$1" "$2" partitionPoint
quickSort "$1" "$(( partitionPoint - 1 ))"
quickSort "$((partitionPoint + 1))" "$2"
fi
}
# involve the algorithm
quickSort 0 "$(( size - 1 ))"
echo "Steps: $steps"
printf '%s\n' "${data[#]}"