Access array element using independent variable in bash script

Usual arrays are looped over using the indices. For my application I need to use an independent variable to extract values from array.

#!/bin/bash
# Author : Ankit
echo "Please enter the number of files (max index number of file): "
read iv
fname="t3p_"
echo "You entered: $iv, $fname" ;
#---------------------------------------------------------------------------------------------
#Reference values
refxa=(2.0 3.0 4.0)
refza=(1.0 6.0 7.0)
#---------------------------------------------------------------------------------------------
rm result.txt
#echo  $iv,$fname
i=1
while [ $i -lt $(($iv+1)) ] ;  do
xxn=5
zzn=19
refxn=$(echo $refxa | awk '{print $i}')
refzn=$(echo $refza | awk '{print $i}')

delta_x=`echo $xxn - $refxn  | bc -l`
delta_z=`echo $zzn - $refzn  | bc -l`

printf "%f\t%f\t%f\t%f\t%f\n" "$i" "$xxn" "$delta_x" "$zzn" "$delta_z" >>  result.txt

i=$[$i+1]
done
echo -e  "------------------------------------------------------------------------------------"
cat result.txt

My question :

1. As seen above I want to subtract value xxn from ith array element($i) which is the index of the loop. How to access the array element using the variable $i. (Using array index is not an option since in my script $i is related to several other parameters and that code is not shown here.)

I tried ${refxa[i]} which did not work.

Expected output (3rd and 5th column are xxn-refxn[i] and zzn-refzn[i]:

You entered: 3, t3p_
------------------------------------------------------------------------------------
1.000000        5.000000        3.000000        19.000000       18.000000
2.000000        5.000000        2.000000        19.000000       13.000000
3.000000        5.000000        1.000000        19.000000       12.000000