Only perform formula when column A contain data

Hi, i need some helps from you guys regarding excel macro.

Please kindly refer my attached sample.xls.

Sample.xlsx

I actually want to use macro to create formula as in column B, C and D based on column A.
This is the list generated by other program in .csv format.
Everytime it will write a series of '1' and '0' in column A, the length will varies depends on the sampling rate.

Column D is the outcome that i want, which is display how many '1' at each of the end of series '1'
But before that i need to check how many data's in column A before i only apply those formula to column B, C and D when column A contain data.

I believe i need to use loop to keep track column A data range, let say 20 cells got data, then it will set 20 to a variable, say k = 20,
then next time column B will apply formula based on 'k' range.

Can any one enlighten me?
Thanks in advance.

Since it seems the only reason B and C exist is to help get the values in D, I don't think you really need them.

The following code counts the 1's in column A and outputs to column B.

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