Trying to compare datetimes in array to a separate predetermined datetime via function/for loop/if statement

Hi all,

I am trying to write a function that removes all rows of data after associated with a certain datetime. The idea was for a loop to go through each row of my datetime column and compare that datetime to 2024-07-06. If greater (after this date) the row gets deleted via my if statement. This is the code I wrote:

 function [output] = remove_after_07(file_name)
   new  = file_name ;
   for index = 1:length(file_name.date_only)
      if index > 2024-07-06
         index = []
      end 
   end
   output                   = new ;
end

This is the error I get:

 ‘Comparison is not defined between datetime and double arrays.’

How can I make it compare the actual dates listed in the column instead of the array?

Thank you!

Answer 1

[dpb]

What does file_name.date_only return an array of datenums or a date string? How to compare depends upon what you're trying to compare to.

In the expression

if index > 2024-07-06

the 2024-07-06 looks like an arithmetic expression and so the result of the line is equivalent to

if index > 2011

after subtracting a total of 13 from 2024.

The above code snippet doesn't look as though it could actually return that error, however, because in

for index = 1:length(file_name.date_only)

index will be a range of integers and so the comparison in the if would be two doubles compared to each other.

If file_name.date_only is actually an array of datenum, then

function [output] = remove_after_07(file_name)
% remove elements from input array after 2024-07-06
  CULL_DATE=datetime('2024-07-06','InputFormat','yyyy-MM-dd');  % put data in variable so can change
  ixBefore=(file_name.date_only<=CULL_DATE);                    % logical vector of those to KEEP
  output=file_name(ixBefore,:);                                 % return those
end

[Kristine]

class(file_name.date_only) returnes 'datetime'.

I have changed 2024-07-06 to a variable I converted to datetime format. I didn't realise that wouldn't just be a default datetime format.

If i try running yoru code:

function [output] = remove_after_07(file_name)
    CULL_DATE=datetime('2024-07-06','InputFormat','yyyy-MM-dd');
    ixBefore=(file_name.date_only<=CULL_DATE);
    output=file_name(ixBefore); 
end

ABC = remove_after_07(splitphytoHLtideshigh);

I get an error:

Subscripting into a table using one subscript (as in t(i)) is not supported. Specify a row subscript and a variable subscript, as in t(rows,vars). To select variables, use t(:,i) or for one variable t.(i). To select rows, use t(i,:).

I don't know what subscripting means.

I've attached a shortened version of the files I'm working with if that helps. It only has a few random data sets from 3 days.

[Walter Roberson]

Use

output=file_name(ixBefore,:); 

[dpb]

Indeed, thanks, Walter. I forgot the variable filename isn't just an array as is the value. I made the correction plus added the missing closing parentheses that @Kristine caught in the Answer...