Extracting the Hour from A imported CSV file in Matlab - matlab

Question

Probably a silly question but in matlab I have imported CSV data. One of the columns I have imported into my workspace is a timestamp and it is of the format "4568x1 cell" (stored as strings). It is of the format
timestamp(1) = 'Reading time"
timestamp(2) = '2014-12-19 00:00:43 UTC'
timestamp(3) = '2014-12-19 00:01:43 UTC'
,etc How do I just get the hour so I get 00,01,etc and store it in a new array?

Answer 1

I'd just use datevec here, after some trimming:
timestamp(1) = []; % remove the first line
timestamp = cell2mat(timestamp); % now all are same length can do this
timestamp = timestamp(:,1:end-4); % remove UTC part
timevec = datevec(timestamp); % probably don't need format specifier
time_date should now be a 4567 x 6 matrix (since we removed one row), where the columns are year, month, day, hour, minute, and second. Therefore for the hours we just need this:
hours = time_date(:,4);

Answer 2

You need to use datenum to convert the date string to a numerical format, e.g.
timestamp{2} = '2014-12-19 00:00:43 UTC'; % use curly braces as cell array
timestamp{3} = '2014-12-19 00:01:43 UTC';
date_num(1) = datenum(timestamp{2}(1:end-4),'yyyy-mm-dd HH:MM:SS'); % remove the ' UTC' characters at the end of the string
date_num(2) = datenum(timestamp{3}(1:end-4),'yyyy-mm-dd HH:MM:SS'); % remove the ' UTC' characters at the end of the string
% etc... (use a for loop to go through all elements of the cell array)
hh = [0 diff(date_num)*24*60]; % elapsed time in hours, taking the first time stamp as reference

Answer 3

Since the hour consists of digits surrounded by : symbols, you could easily apply a regular expression with lookaround, and then convert the detected strings into numbers:
%// Data
timestamp{1} = 'Reading time';
timestamp{2} = '2014-12-19 00:00:43 UTC';
timestamp{3} = '2014-12-19 00:01:43 UTC';
%// Get hours as a cell array of cells containing strings:
hours_cells = regexp(timestamp(2:end), '(?<=:)(\d+)(?=:)', 'match'); %// no title
%// Convert to numbers:
hours = cellfun(#(x) str2num(x{1}), hours_cells);
If the character structure is fixed (each number includes left zero padding to equalize width), you can identify the positions of the hour (characters 15 and 16 in this case) and do it more simply:
hours = cellfun(#(x) str2num(x([15 16])), timestamp(2:end));
or, possibly faster,
hours = cellfun(#(x) (x([15 16])-48)*[10;1], timestamp(2:end));
%// 48 is ASCII for '0'