a better expression for 'percentage divided by 100'

In similar circumstances, the term I usually see is 'proportion', or sometimes 'ratio'. You need to clarify your sentence though, how is the reader to know that if the array a has five elements and x = 0.5, then f(a,x) returns the third element of a and not the second?

Normalized to range [0,1].

This is the usual approach, and you rarely find values normalized to other ranges.

A percent divided by one hundred is a basis point, or 1 per ten thousand. Like similar units (percent 0/0, per mille 0/00 etc), it can be expressed as 0/000.

So, you could possibly say "where x is a basis point value."

The basis point is also less commonly called permyriad.

EDIT: If you want to represent a percentage value as a fraction value between 0 and 1, you may perhaps want decimal fraction. The number 61/100 (61%) can be written as the decimal fraction 0.61.

You can say: ..., where x is a positive proper fraction of the array length.