Create elapsed time vector for loop

Hi everyone!

I'm new to this so any help would be greatly appreciated!

I would like to analyse some accelerometer data and I'd like to convert the timestamp to elapsed time, with the start of recording being 0. Then, I'd like to create a for loop to do this for the entire column (a few thousand rows), and write the result in a new column.

(Some additional context, if helpful: I have already converted the datetime from 'yyyy-MM-dd HH:mm:ss.SSS' to 'HH:mm:ss.SSS' since the experiments are short and don't take more than a few minutes each. The sampling frequency is 100Hz and so I have samples every 0.001-0.002 seconds. I will then need to find peaks within different elapsed periods).

Could you please help me with the conversion and for loop creation? I have foundhttps://uk.mathworks.com/matlabcentral/answers/386394-plot-datetime-vector-as-elapsed-time-in-seconds-from-start-of-meaurement, https://uk.mathworks.com/matlabcentral/answers/373223-how-to-calculate-time-elapsed-time-in-gmt-format and https://uk.mathworks.com/matlabcentral/answers/410089-create-a-function-to-find-elapsed-time-with-string-of-date-and-times-through-samples but I am not sure how to repurpose them.

Thank you in advance!

Evelyn

Answer 1

[Seth Furman]

1) You can convert timestamp data into elapsed time using datetime subtraction.

>> dt = datetime(2020,1,1:5)

dt = 

  1×5 datetime array

   01-Jan-2020   02-Jan-2020   03-Jan-2020   04-Jan-2020   05-Jan-2020

>> dt - dt(1)

ans = 

  1×5 duration array

   00:00:00   24:00:00   48:00:00   72:00:00   96:00:00

https://www.mathworks.com/help/matlab/matlab_prog/compute-elapsed-time.html

2) MATLAB supports vectorized operations so you can directly subtract a scalar or vector datetime from a vector datetime as in the example above.

https://www.mathworks.com/help/matlab/matlab_prog/vectorization.html

3) See the following link for how to add new variables to a table.

https://www.mathworks.com/help/matlab/matlab_prog/add-and-delete-table-variables.html

[Evelyn Maniaki]

Thank you very much Seth, that was mega helpful! :)