Why can't I convert from datenum to datetime?

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Louise Wilson
Louise Wilson 2020-4-27
评论： Louise Wilson 2020-4-28
I am trying to convert a vector of datenums to datetime but for some reason having issues. The spreadsheet I am working with is 7gb so I will just include the top rows.
%Data (d) example:
7.3782e+05 79.064 80.016 75.023
7.3782e+05 79.137 80.478 75.399
7.3782e+05 78.144 79.663 75.5
d_times=d(:,1);
d_times=datetime(d_times, 'ConvertFrom', 'datenum'); %convert t column datenums to date and time
When I do this I get the error:
Error using datetime (line 658)
Input data must be a numeric array, a string array, a cell array containing character vectors, or a char matrix.
...but I can see that the first value is a datetime? Where am I going wrong?

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采纳的回答

Stephen23
Stephen23 2020-4-27
You imported that data into a table (e.g. using readtable) and so you are using the wrong indexing:
https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-table.html
You would need to use curly braces to get the numeric data out of the table:
d_times = d{:,1};
% ^ ^ you need these
Even better would be to use the variable name (which you have not told us):
d_times = d.nameofthefirstcolumn;
  1 个评论
Louise Wilson
Louise Wilson 2020-4-28
Thanks Stephen. I didn't include variable names in my table-the first row of the first column was 0. I managed to get around this problem by importing the data using csvread instead. Following this, everything works fine. Since I only needed to do this to check the datetimes were correct following subsetting the data, I will leave it I think. But, this is really good to know! I didn't realise the indexing was wrong. Thank you.

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更多回答（1 个）

per isakson
per isakson 2020-4-27
编辑：per isakson 2020-4-27
The datetime statement is ok.
>> d_times=datetime( now, 'ConvertFrom', 'datenum');
>> d_times
d_times =
datetime
27-Apr-2020 05:27:05
>>
This script
%%
d = [
7.3782e+05 79.064 80.016 75.023
7.3782e+05 79.137 80.478 75.399
7.3782e+05 78.144 79.663 75.5
];
d1 = d(:,1);
%%
d_times = datetime( d1, 'ConvertFrom', 'datenum')
returns
d_times =
3×1 datetime array
30-Jan-2020 00:00:00
30-Jan-2020 00:00:00
30-Jan-2020 00:00:00
>>
I guess there is some kind of problem with your value of d(:,1)
Proposal: Replace
d_times=d(:,1);
by
d1 = d(:,1);
run the script and inspect the value of d1
Try
d1 = d( 1:end-2, 1 );

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