Creating sublists based on the first column

Question

I have Excel file consisting of two columns such as:

1.2.2014 23:35  |-0.007378

1.2.2014 23:40  |-0.004121

1.2.2014 23:45  |-0.000122

1.2.2014 23:50  |0.000117

1.2.2014 23:55  |-0.000542

When I import that in Mathematica, it creates list like:

List={{1.2.2014 23:35, -0.007378}, {1.2.2014 23:40, -0.004121}, ..., {2.2.2014 00:00, -0.004897}, {2.2.2014 00:05, -0.005179}, ...}

And I need to create a list where only the values from the second column will be and these values will create sublists based on the day of observations. So it should look like (based on "List" above):

List2={{-0.007378, -0.004121, ...}, {-0.004897, -0.005179, ...}, ...}.

The question may be very similar to this one: Split dataset based on the first column ...but I cannot figure out how the mentioned method should be applied in my case, where I have dates in the first column.

Thanks for any help. And sorry if it is an easy problem, but I am still begginer with Mathematica.

Answer 1

Starting with:

data = Import["btc.xlsx"] // First;

your time stamps are being imported in the DateObject format:

data[[1, 1]] // InputForm

DateObject[{2011, 12, 31}, TimeObject[{0, 5, 0.}]]

The first argument is a Y/M/D triplet that we can Extract for use in GatherBy or GroupBy:

GatherBy[data, Extract[{1, 1}]][[All, All, 2]]

Or:

GroupBy[data, Extract[{1, 1}] -> Last] // Values

The operator form of Extract is used in each example.

Answer 2

If I understood rightly, a starting point could be:

dateString[date_] := DateString[date, {"Year", "Month", "Day"}];
dString[arg_] := dateString[First[arg]];
dataOut = Gather[dataIn, (dString[#1] == dString[#2]) &];

If you wish, you can provide us a plain text file version of your data (es. Dropbox ...) and I will try to help further.

Answer 3

list= {{"1.2.2014 23:35  ", -0.007378`}, {"1.3.2014 23:40  ", -0.004121`},
       {"1.2.2014 23:45  ", -0.000122`}, {"1.3.2014 23:50  ",  0.000117`}, 
       {"1.2.2014 23:55  ", -0.000542`}};

list2 = {DateList[{StringTrim@#, 
         {"Month", "Day", "Year", "Hour", "Minute"}}][[;; 3]], #2} & @@@ list;

Values@GroupBy[ First -> Last][list2]
(* or Values@GroupBy[list2, First -> Last] *)
(* {{-0.007378,-0.000122,-0.000542},{-0.004121,0.000117}} *)