#!/bin/bash

declare -A array
array[x1,y1]=100
array[x1,y2]=200
array[x2,y1]=300
array[x2,y2]=400
#alternative 1, extract all the main keys with sort
for key in $(printf '%s\n' "${!array[@]}" | sed 's/,.*//' | sort -u); do
#alternative 2, keeping track of the main keys
keys=(x1 x2)
for key in "${keys[@]}"; do

    #the loop contents are the same
    echo "$key : y1 = ${array[$key,y1]}"
    echo "$key : y2 = ${array[$key,y2]}"
done