#!/bin/sh -u
# parabola loop with display X,Y made positive to fit on terminal screen
#
# NOTE: This script fragment is incomplete - it does not follow all
# the script guidelines set out in "script_style.txt".  It also
# contains many "Instructor-Style" comments that you must not use in
# your own scripts.  See script_style.txt for details.
#
# -IAN! idallen@idallen.ca

PATH=/bin:/usr/bin ; export PATH
LC_COLLATE=C ; export LC_COLLATE
LANG=C ; export LANG
umask 022

# To make this loop work for terminal graphics output, both X and Y
# have to be positive and have to fit inside the terminal window.
# We adjust x to get "dispx" and y to get "dispy".
#
# Integer Arithmetic Only!
#  Shell scripts only deal in integers. The order in which you do
#  arithmetic matters! let "x=1/80*160" is zero, because "1/80" is zero
#  in integer artithmetic. let "x=160*1/80" is the right answer "2".
#  Always do multiplication before division in integer math.
#
# Y adjust for "dispy":
#  We assume that the terminal window has 41 lines (rows). (A better
#  version of this script would use "tput lines" to find the real
#  number.) Our internal value of Y goes from -20 to +20. To make this
#  range from 0 to 41, we add 20 (the value of num).
#
# X adjust for "dispx":
#  We assume that the terminal window has 80 columns. (A better version
#  of this script would use "tput cols" to find the real number.) Our
#  internal value of X (Y squared) goes between 0 and 400. To make this
#  range from 0 to 80, we multiply by 80 then divide by 20 squared (num
#  squared).  Do multiplication before division in integer math!
#
let num=20
let y=-num
while [ $y -le $num ] ; do
    let x=y*y

    # Calculate the display values of X,Y from the program values.
    # The display values must be non-negative and fit on terminal screen.
    #
    let dispy=y+num
    let dispx=x*80/num/num   # integer math: do multiplication before division
    echo "y is $y, x is $x, dispy is $dispy, dispx is $dispx"

    let y=y+1
    sleep 0.1
done