get number of latin letter in alphabet use printf

#!/bin/bash

#get number of latin letter in alphabet
#use nlett <letter>, e.g nlett a

SNUM=0
SFND=`echo $1|grep '^[a-z]$'`
if [ -z "$SFND" ]; then
    echo "Error: in first parameter must be one small latin letter!"
    exit 1
fi

S_CODE=`printf '%d' \'$SFND`
A_CODE=`printf '%d' \'a`
SNUM=$( expr $S_CODE - $A_CODE + 1 )
echo "$SFND is $SNUM"