-1
 -1 0 1 2 3 4 5 6 7 8 9   index
    - - - - - - - - -
    4 5 0 3 7 3 9 3 8     entry


What if the number isn't in the array? What do we return?  -1 or length
If we find the number several times, which answer to return?  first or last

In Python, in an array of length 9 called a,
   a[-1] == a[8]
   a[-2] == a[7]
so -1 is not an invalid array index, and so -1
probably shouldn't be used (in Python) to indicate
"not found". It would be better to return length
to indicate "not found".