What happens if a non-integer value is typed? What if the argument is omitted?

Question

Write a program called twice that takes a single integer argument and doubles its value:
$ twice 15
30
$ twice 0
0
$

Answer 1

It's a very simple script:
echo $(( $1 + $1 ))
Non-integer values are treated as the value zero, so the output is 0. If the argument is omitted, however, then it's a syntax error of $(( + )) with the error:
syntax error: operand expected (error token is " ")