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# Creating a Simple RPN Calculator

## Problem

You may be able to convert binary to decimal, octal, or hex in your head but it seems that you can't do simple arithmetic anymore and you can never find a calculator when you need one. What to do?

## Solution

Create a calculator using shell arithmetic and RPN notation:

```	#!/usr/bin/env bash
# cookbook filename: rpncalc
#
# simple RPN command line (integer) calculator
#
# takes the arguments and computes with them
# of the form a b op
# allow the use of x instead of *
#
# error check our argument counts:
if [ \( \$# -lt 3 \) -o \( \$((\$# % 2)) -eq 0 \) ]
then
echo "usage: calc number number op [ number op ] ..."
echo "use x or '*' for multiplication"
exit 1
fi

ANS=\$((\$1 \${3//x/*} \$2))
shift 3
while [ \$# -gt 0 ]
do
ANS=\$((ANS \${2//x/*} \$1))
shift 2
done
echo \$ANS```

## Discussion

### Warning

Any arithmetic done within \$(( )) is integer arithmetic only.

The idea of RPN (or postfix) style of notation puts the operands (the numbers) first, followed by the operator. If we are using RPN, we don't write `5 + 4` but rather ```5 4 +``` as our expression. If you want to multiply the result by 2, then you just put `2`* on the end, so the whole expression would be ```5 4 + 2```*, which is great for computers to parse because you can go left to right and never need parentheses. The result of any operation becomes the first operand for the next expression.

In our simple bash calculator we will allow the use of lowercase `x` as a substitute for the multiplication symbol since * has special meaning ...