index of first greater than during two array comparison

Question

I have two arrays threshold and values.

threshold=[10 22 97]  
values=[99 23 77 11 8 10]

I want to output idx such that threshold(idx-1)<values(i)<=threshold(idx). That is for the above example output will be

output=[4 3 3 2 1 1]

The naive code that can produce above output will be

output=ones(1,length(values))*(length(values)+1); 
for i=1:length(values)  
  for j=1:length(threshold)  
    if(values(i)>threshold(j))  
       output(i)=j;
    end  
  end  
end

Is there a simple way of doing it. I want to avoid loops.

Also the inner loop with j, can be replaced with find function. That is, find(threshold>values(i),1,'first'); — Ramakrishnan Kannan
– Ramakrishnan Kannan, Commented Nov 12, 2013 at 8:03
Why do you want to avoid loops ? You want a simple way, yet you already have a naive way. What is the desired difference between a naive way and a simple way ? — High Performance Mark
– High Performance Mark, Commented Nov 12, 2013 at 8:04

Shai · Accepted Answer · 2013-11-12 08:23:42Z

3

You can use histc command, with a slight adjustment of threshold array

>> threshold=[-inf 10 22 97 inf];
>> values=[99 23 77 11 8 10]; 
>> [~, output] = histc( values, threshold+.1 )
output =

 4     3     3     2     1     1

The modification of threshold is due to "less-than"/"less-than-equal" type of comparison for bin boundary decisions.

answered Nov 12, 2013 at 8:23

Shai

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4 Comments

Shai Over a year ago

@RodyOldenhuis yes... I was surprised to see your answer - I honestly thought you got ahead of me and I was about to delete my answer...

Rody Oldenhuis Over a year ago

seems my answer is wrong in the context of non-sorted values...I'll delete mine and drink some coffee :)

Shai Over a year ago

@RodyOldenhuis please do not delete your answer. it is a good example of comparison using bsxfun.

Rody Oldenhuis Over a year ago

@Shai: well, if you insist.

Rody Oldenhuis · Accepted Answer · 2013-11-12 08:19:37Z

2

No loops often means you'll gain speed by increasing peak memory. Try this:

threshold = [10 22 97];
values    = [99 23 77 11 8 10];

%// Do ALL comparisons
A = sum(bsxfun(@gt, values.', threshold));

%// Find the indices and the ones before 
R = max(1, [A; A-1]);

%// The array you want
R(:).'

If you run out of memory, just use the loop, but then with a find replacing the inner loop.

Loops aren't all that bad, you know (if you have MATLAB > R2008). In theory, the solution above shouldn't even be faster than a loop with find, but oh well...profiling is key :)

answered Nov 12, 2013 at 8:19

Rody Oldenhuis

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