Extract First Zero Crossing After Peak

Question

I have a signal that approximates a noisy sine wave which I want to broadly break down into segments of negative and positive slope. As peak detection on the original trace is messy and the section of positive slope is complex, I plan to extract these features by working with the differential (the maximum velocity of the principle waveform is largely invariant and can be easily thresholded from the lower amplitude waveforms). I can easily detect the MIN peaks in the differential and now want to walk forward in the dataset to the first transition across the zero line and extract this timepoint to a new dataset. Does anyone have a script that I can modify for this purpose? Essentially, I want to trigger with an existing timepoint, iterate to the first timepoint where ti times ti+1 is negative and then write ti+1 to a new waveform.  I also want to walk backwards to the the first transition across the zero line and extract this timepoint to a new dataset but this is secondary.
Thanks!
Matt
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Answer 1

Sorry for not responding earlier about my progess.
I was able to extract the timepoints using your advice but now want to go back to the original data and extract the y-values. For some reason, my for loop below completes but only writes no-values? Extracting the x and y values in parallel won't work because I generated by x-value subset using the differential of the original dataset.
The general question is as follows:
Given a common timepoint, find the y-value associated with that timepoint in another channel.
FOR i=1 TO InitContrLength
   ChDx(i, InitContrY) = ChDx(ChDx(i,InitContr)/Interval, Filtered)
   Next
InitContr is an array of discontinuous time values which represent a subset of the original time series. My for loop is designed to iterate through the InitContr, converting them to an integer representing the appropriate row in my original dataset (Filtered) by dividing the time value by the step interval, and then writing the value from Filtered (which is an array of y-values) that corresponds to that row to InitContrY.
Definitions in case the above is confusing
InitContr = Channel containing a subset of time values
Filtered = Channel containing all y-values from the original waveform
Interval = step width of the original waveform
InitContrY = New Channel that I want to write y-values to that correspond to InitContr x-Values
Any ideas or links to a debugging primer would be greatly appreciated. If I could iterate through my for loop and visualize the variables in real time, I think I could figure this out myself.
Matt