2x2 Matrix Operations

#!/bin/bash
function inputMat(){
    echo "Enter Matrix A: "
    read -a a
    echo "Enter Matrix B: "
    read -a b
}
function displayMat(){
    for(( i=0; i<4; i++ ))
    do
        if(( i==2 )) # new line for next row
            then echo ""
        fi
        echo -n "${c[i]}  "
    done
}
function add(){
    for(( i=0; i<4; i++ ))
    do
        let c[$i]=${a[$i]}+${b[$i]}
    done
}
function sub(){
    for(( i=0; i<4; i++ ))
    do
        let c[$i]=${a[$i]}-${b[$i]}
    done
}
function mul(){
    let c[0]=${a[0]}*${b[0]}+${a[1]}*${b[2]}
    let c[1]=${a[0]}*${b[1]}+${a[1]}*${b[3]}
    let c[2]=${a[2]}*${b[0]}+${a[3]}*${b[2]}
    let c[3]=${a[2]}*${b[1]}+${a[3]}*${b[3]}
}
inputMat
echo -n "1.Add 2. Subtract 3. Multiply: "
read ip
case $ip in
    1):
        add;;
    2):
        sub;;
    3):
        mul;;
esac
displayMat