Convert data types from string to list (Coastal Buoy data) for DateListPlot

Question

I downloaded Coastal buoy data from the CDIP webpages and saved it as a text file. The trouble is that the data has spaces between the YYY MM DD hh mm.

{Edit 4/5/2019: Updated link Please see comment below}

With FileNames and Import, I was able to import the data from a text file.

fn = FileNames["*.txt"]; (*I am interested in the first text file in this list*)
h2018 = Import[fn[[1]], "Data"]

What I want to do is convert the first 5 columns to DateList and then plot the 9th column against this new DateList.

I tried doing this to no effect because I am trying to convert the data to a string and then replacing spaces in the new string with / and then using DateList.

StringReplace[ToString[h2018[[3 ;;, 1 ;; 3]]], "," .. -> "/"];
StringReplace[%, " " -> ""];
dl=DateList[%];

How do I convert this string to a DateList and complete my plotting of column 9 in h2018 with the DateList, dl?

I did try to convert this string to a list of numbers using the information in this question but without success.

Flatten@ToExpression@StringSplit[%];

Answer 1

Use the functions in the Date & Time guide to manipulate dates. This step-by-step answer.

Import the table.

raw = Import["http://www.ndbc.l.noaa.gov/data/realtime2/46218.txt", "Table"];

Notice that Mathematica has imported the numbers as numbers and not as strings. Look at row 3 and columns from the start to column 5.

Head /@ raw[[3, ;; 5]]

{Integer, Integer, Integer, Integer, Integer}

First collapse the 5 column headers for the date-times in row 1 and 2. This is not strictly needed but will keep the headers consistent with the data.

raw[[;; 2]] = Flatten@MapAt[StringJoin, 1]@TakeDrop[#, 5] & /@ raw[[;; 2]];

Next feed the values in the first 5 columns into DateObject. The columns are in the exact order that DateObject expects so there is no need to adjust their positions. This is performed on row 3 until the end.

raw[[3 ;;]] = Flatten@MapAt[DateObject, 1]@TakeDrop[#, 5] & /@ raw[[3 ;;]];

Since the first 5 columns have been collapsed into 1 the information is now in column 5 instead of column 9, (9 - 5 + 1). Take from row 3 until the end and columns 1 and 5 to plot with DateListPlot.

DateListPlot[raw[[3 ;;, {1, 5}]]]

Hope this helps.

Answer 2

Here's one way to do it:

data = Import["http://www.ndbc.l.noaa.gov/data/realtime2/46218.txt",
              "Table", HeaderLines -> 2][[All, {1, 2, 3, 4, 5, 9}]];

ts = TimeSeries[Composition[Flatten, MapAt[DateObject, #, 1] &,
                            Reverse, TakeDrop[#, -1] &] /@ data];

DateListPlot[ts]

(If you don't want to convert to a TimeSeries[], you can omit that part.)

---

Since the other answer has already been accepted, let me just illustrate how one might use the functionality of Dataset[] for this task:

data = Import["http://www.ndbc.l.noaa.gov/data/realtime2/46218.txt", "Table"];
data = MapAt[StringReplace[#, "#" -> ""] &, Delete[data, 2], 1];
data = Dataset[AssociationThread[First[data], #] & /@ Rest[data]];

ds = <|"Timestamp" -> DateObject[{#YY, #MM, #DD, #hh, #mm}], 
       "WaveHeight" -> Quantity[#WVHT, "Meters"]|> & /@ data

Show[ds[DateListPlot, All], FrameLabel -> {None, "Wave Height (m)"}]