I want to read a signal stream, and conditionally alter the value of individual samples coming through before writing to an array. Right now I am doing a [snapshot~] at the sample rate and multiplying by 1 or 0, depending on whether it is above a threshold value, and banging a [tabwrite] at the sample rate to write to the array. It works but this all seems clumsy; is there a way to let the signal remain a signal but intercept and manipulate it at the sample level?

I tried to make a [fexpr~ $x1# > ] but no such object seems to exist

What kind of manipulation are you attempting?
You can do simple things with conditional expr~ statements.

like if you needed to subtract 0.3 from all samples below 0.7
if ($v1 < 0.7, $v1 - 0.3, $v1)

if you need to pass or not pass a stream based on a threshold, you could use
if ($v1 > $v2, 0, $v1)
where $v2 is the threshold

or some variation of it

have I understood what your trying to do?

As I understood from the [expr~] help files, [expr~] and $v1 (etc.) evaluated the signal each block instead of each sample, or is that wrong?