Programming :: C-shell Scripting Syntax Error When Defining Two Variables A1 And A2

I am trying to modify a script for research purposes and am having difficulty here as I have little prior experience with C-shell scripting.

The script looks as follows (it includes tcl commands like runFEP that you can ignore)

#!/bin/bash

for ((old=1, new=2; old<=4; old++,new++))
a1=${old}%50
a2=${new}%50
do
cat > input${new}.conf <<EOF
${a1}
code....

My question: I keep getting a syntax error when defining my two variables a1 and a2. I essentially need these variables to be
a1 = value of variable old divided by 50
a2 = value of variable new divided by 50