What are you trying to Do?!?
A first-order hold is most useful to model the action of a DAC at the
interface of a discrete-time digital control system with the outside
world. In fact, I think of it as a mathematical construct which takes
impulses as input and outputs rectangular pulses that just happen to be
at the system sampling rate -- as such it is doomed to forever be a
mathematical construct, because impulses do not exist in reality.
The closest thing that I can think of to a first-order hold would be a
sample-and-hold block, whose behavior in system-modeling language would
be a sampler followed by a hold. A sample-and-hold would be an easy
thing to implement in C -- maintain a state variable for it, and only
update the state on the samples that you're supposed to remember.
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