'calc' in while loop causes undefined variable error in bash script

Question

The following code is a snippet from a larger bash script I'm writing to parse through a sequence of amino acids and carry out calculations based on specific pairs. For simplicity, I have shortened it and made it self contained - but the problem I'm coming up against is that if I run the example test calculation "calc ....." line - (which is commented out below,) I get a "... is undefined" error - which refers back to the variable fed into the while-read loop. Perhaps this is to do with subshells and/or variable accessibility but I'm stumped as to the solution, especially as I have similar scripts which work.

Any help would be appreciated. - Also, I'm running this in a mac OSX terminal.

#!/bin/bash

#Set input
Seq=HTCHMAREQEMNMHG
SeqMAX=${#Seq}

#set start position for read
Pos=1

#Analyse sequence
echo -e "\nReading: $Seq "

#Read sequence, one character at a time 
while  read -r -n1 Res ; do

    #If Res variable has hit a "blank", then move on
    if [[ $Res = "" ]] ; then

        break

    #Else, if Res variable is the first     
    elif [[ $Pos = "1" ]] ; then

        #Calculate rate

        #UNCOMMENTING THIS LINE CAUSES ERROR
        #calc 9 + 10

            echo "$Left $Right"
            Pos=$((Pos+1))
            Left=$Res

        #Else, if Res variable is the last  
        elif [[ $Pos = $SeqMAX ]] ; then

            Left=$Res
            Right=xC
            ###CALC............
            Pos=$((Pos+1))
            echo -e "$Left $Right"  

        #Finally, if charcter is internal       
        else 

            Right=$Res
            echo "$Left $Right"
            Left=$Res
            Pos=$((Pos+1))

        fi

done < <(echo $Seq)

exit

Answer 1

It appears that calc reads from standard input, so you need to prevent it from consuming data from the stream provided to the while loop intended for the read command:

calc 9 + 10 < /dev/null

Answer 2

You are missing: '

Try:

calc '9 + 10'