Well, suppose you want to get the value interpolated at X = 4.7. First you want the integer part and float part (4 and 0.7), which are floor(X) and X - floor(X) respectively.
The expression lerp(a, b, x) does linear interpolation - so we want to interpolate between the two nearest array entries, floor(X) and floor(X) + 1.
Which is something like:
lerp(Array(Floor(X)), Array(Floor(X) + 1), X - floor(X))