Deriving a list of time differences from a column in a dataset and appending the list as a new column

Question

I know it's a beginner question, but I've struggled with it for some time now and can't find a solution.

I have a dataset consisting of timestamps, where I've managed to compute the time differences with the help of a for loop:

For[i = 1; t = 0, i < 3, i++,  
  t = 
    UnitConvert[
      DateDifference[
        DateObject[sessions5311[i, "datetimeaccess"]], 
        DateObject[sessions5311[i + 1, "datetimeaccess"]]], 
      "Minutes" ]; 
  Print[t]]

sessions5311 = sessions5311[All, Append[#, "Differences" -> t] &]

When I try to append it, only the last value of t is being inserted in the "Differences" column, but I would want to have each of the current values of the t variable during the computation there.

Your help is much appreciated :).

Answer 1

Since you did not give us your dataset to work with, I contrived this one for exposition purposes.

ds = 
  Dataset[
    {Association["a" -> 1, "b" -> "x", 
      "date" -> DateObject[{2019, 4, 14, 20, 14, 47.767113}, "Instant", Gregorian", -4.]], 
     Association["a" -> 2, "b" -> "y", 
       "date" -> DateObject[{2019, 4, 15, 20, 14, 47.767451}, "Instant", "Gregorian", -4.]], 
     Association["a" -> 3, "b" -> "z", 
       "date" -> DateObject[{2019, 4, 16, 20, 14, 47.767678}, "Instant", "Gregorian", -4.]], 
     Association["a" -> 4, "b" -> "x", 
       "date" -> DateObject[{2019, 4, 17, 20, 14, 47.7679}, "Instant", "Gregorian", -4.]], 
     Association["a" -> 5, "b" -> "y", 
       "date" -> DateObject[{2019, 4, 18, 20, 14, 47.768123}, "Instant", "Gregorian", -4.]], 
     Association["a" -> 6, "b" -> "z", 
       "date" -> DateObject[{2019, 4, 19, 20, 14, 47.768346}, "Instant", "Gregorian", -4.]]}]; 

With this dataset, I can add a column of date differences given in minutes starting with the following code.

rows = Length @ ds;
dates = ds[All , "date"] // Normal
diffs = 
  Association["differce" -> #] & /@
    Prepend[
      Table[DateDifference[dates[[i]], dates[[i + 1]], "Minutes"], {i, rows - 1}],
      Quantity[0., "Minutes"]]

Evaluation gives a list of differences expressed as associations.

{<|"difference" -> Quantity[0., "Minutes"]|>, 
 <|"difference" -> Quantity[1440., "Minutes"]|>, 
 <|"difference" -> Quantity[1440., "Minutes"]|>, 
 <|"difference" -> Quantity[1440., "Minutes"]|>, 
 <|"difference" -> Quantity[1440., "Minutes"]|>, 
 <|"difference" -> Quantity[1440., "Minutes"]|>}

The list can be appended to ds as new column having the key "difference" with the following one-liner.

ds = Join @@@ Transpose @ {ds // Normal, diffs} // Dataset