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Asked by Mihai on 30 Jan 2013

Hi,

I have an array with x amount of values. How can I find the index value of the element that is closest or equal to a certain value?

I tried it in the following manner, but it doesn't work when the value of the element in Temp is equal to the RefTemp value.

Temp = [-15.3, 0.2, 15.2, 30, 45.3]; RefTemp = 30; %Value to compare the Temp array values to

for ii = 1:length(Temp) TempCalc(ii) = abs(Temp(ii) - RefTemp); end

find(min(TempCalc));

Thank you very much for your help!

Answer by Shashank on 30 Jan 2013

Accepted answer

Do you have the Stats toolbox? if you do then do a nearest neighbor search as follows:

location = knnsearch(Temp',30);

If you don't have stats toolbox then use delauny to do nn search:

>> tri = delaunayn(Temp'); >> dsearchn(Temp',tri,30)

ans =

4

Mihai on 30 Jan 2013

Interesting! I don't have the stats toolbox, and I've never seen either of those 2 functions before. It seems simple enough. I briefly tried playing around with the delaunayn function, and it seems it wouldn't work if 2 elements in the array were equal. This isn't a problem for me since the elements in my Temp array will be unique.

Shashank on 30 Jan 2013

Nearest neighbor computation comes up in engineering and science more often then you can imagine, from pdf estimation to clustering. There are advanced data structures such as kdtrees which speed up neighbor search for higher dimension data. Also knnsearch has options on what to do during a tie.

Answer by Cedric Wannaz on 30 Jan 2013

Edited by Cedric Wannaz on 30 Jan 2013

You have several options. The first question is: do you really need the index, or could you use a vector of logicals, e.g. for indexing something else. Look at the following; we want to extract all volumes associated with temperatures that are closest to a ref value:

>> temp = [-15.3, 0.2, 15.2, 30, 45.3]; >> volume = [4, 7, 28, 35, 20] ; >> ref = 27.2 ; >> dif = abs(temp-ref) dif = 42.5000 27.0000 12.0000 2.8000 18.1000 >> min(dif) ans = 2.8000 >> match = dif == min(dif) match = 0 0 0 1 0 % Vector of logicals indicate % where dif equals its min. >> idx = find(dif == min(dif)) idx = 4 % Index of element that % the min matches

Now you can extract the corresponding volume with either the vector of logicals (which avoids using find()) or the index.

>> volume(match) ans = 35 >> volume(idx) ans = 35

This is one "vector" way to achieve what you want; without all the extra steps, this reduces to:

>> dif = abs(temp-ref) ; >> volume(dif == min(dif)) ans = 35

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