Filter through list and lump data [duplicate]

Question

I have a very large data set (around 3000) that looks like:

{{1749.08, 580.}, {1749.17, 626.}, {1749.25, 700.}, {1749.33, 
  557.}, {1749.42, 850.}, {1749.5, 835.}, {1749.58, 948.}, {1749.67, 
  663.}....

The first number is the year plus the month/12. So I want to iterate over all the list adding the second part for each year

So example using data above:

For year 1749: 580+626+700+557+850+835+948+663

I know I have to use a for loop but I can't figure out the syntax. Can anyone lend a helping hand?

Answer 1

One way is to gather the years by using Floor and then adding up the values for each year.

data = {{1749.08, 580.}, {1749.17, 626.}, {1749.25, 
    700.}, {1749.33, 557.}, {1749.42, 850.}, {1749.5, 835.}, {1749.58,
     948.}, {1749.67, 663.}, {1750.00, 653.}};
Total[Part[#, All, 2]] & /@ GatherBy[data, Floor[First[#]] &]
(* {5759., 653.} *)

Answer 2

data = {{1749.08, 580.}, {1749.17, 626.}, {1749.25, 700.}, {1749.33, 
   557.}, {1749.42, 850.}, {1749.5, 835.}, {1749.58, 948.}, {1749.67, 
   663.}, {1751.66, 10}, {1751.77, 20}}

Total /@ GatherBy[data, IntegerPart[#[[1]]] &][[All, All, 2]]

(* {5759.,30} *)

Note the data might be a bit ambiguous: if the month is 12, the year gets bumped to the next. Is that really what you want?