How to find something that you're not looking for?!

Hey everyone!

So I haven written a couple matlab codes to predict the future values of some time series. Within those codes i need the following command:

 start=datenum(2011,01,01,0,0,0);
 finish=datenum(2011,31,12,23,0,0);
 a=find(date==start);
 b=find(date==finish);

The vector "date" contains dates and I want to find the exact position of the dates that I have assigned to the variables start and finish. However depending on the data I am using, sometimes there are some gaps within the dates in vector "date". For example vector "date" contains all dates from 01-01-2009 until 12-31-2013 but the dates from 12-28-2011 to 01-05-2012 are missing. Consequently if I ran this code, I would get the index of my start date (assigned to variable a) but the index of my finish date (assigned to variable b) is empty.

Is there any way to determine the closest match to the date I am looking for? For example in this case I would want matlab to tell me the exact position of the date 12-28-2011 within the vector "date" if it cannot tell me the position the the date "finish".

I would be grateful for any ideas how to work around this. Restructuring the vector "date" and filling it with the missing dates is no solution for me at the moment...

Thank you so much!!

Answer 1

[Guillaume]

How about:

[~, idx] = min(abs(date - finish));

[MC3105]

Thanks! So matlab gives me a vector with two elements as answer. The first element seems to be the date that is <= the date I am looking for. What is the second element supposed to be?

[Guillaume]

The above just give you one value, idx which is the index of the closest date to finish (aka the index of the minimum absolute difference between date and finish).

I'm not sure what you did, but it wasn't what I wrote:

mydates = [datenum([2009 1 1]):datenum([2011 12 27]) datenum([2012 1 6]):datenum([2013 12 31])];
start = datenum([2011 1 1]);
finish = datenum([2011 12 31]);
[~, a] = min(abs(mydates - start));
[~, b] = min(abs(mydates - finish));
[a b]

returns

      ans  =  [731  1091]

datestr(mydates([a b]))

returns

    ans = 
          01-Jan-2011
          27-Dec-2011

By the way, don't call your variable date since it shadows the name of a matlab built-in function.

[MC3105]

Great! Thank you so much for your help! I found my error now and your solution does work in the way you describe it!!

Answer 2

[per isakson]

Try this

    ix = find( date >= finish, 1, 'first' );

and see Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero?

[MC3105]

Thank you! This works fine if I want to find something that is >= a specific date.. .although if I try to find something <= it is not working...

In this case I tried this:

 find(date<=finish,1,'first')

It just returns 1. I even tried out putting in 'last'. But this returns a very large number that exceeds the matrix indexes of 'date'. Do you have any idea what the problem could be?

[Guillaume]

My answer will give you the closest date regardless if it's before or after.