matlabvectoroctavelexicographic-ordering

matlab/octave compare vectors using dictionary order?

Is there a succinct way in Octave to compare two vectors using dictionary order (lexicographic) order?

i.e. I'd like to compare two vectors a , b by the first element, return result if they differ; otherwise compare the second element, and so on..

For example, if a = [0 1 5] , b = [0 2 1], I'd like to have

a <? b

for some operator/function <? to return true.

If I simply do a < b, this will return a vector of logical values.

ans =

  0  1  0
Solution

  • The following will work for both MATLAB and Octave...

    You can create a comparison function using find like so:

    lexlt = @(a, b) find([a < b true], 1) < find([a > b true], 1);

    It will return true if the first vector argument is lexographically less than the second, and false otherwise (if it's greater than or equal to it). For example:

    >> a = [0 1 5];
>> b = [0 2 1];
>> lexlt(a, a)

ans =

  logical

   0

>> lexlt(a, b)

ans =

  logical

   1

>> lexlt(b, a)

ans =

  logical

   0

    And here's the corresponding function for a "greater than" comparison (i.e. the first argument is lexographically greater than the second):

    lexgt = @(a, b) find([a > b true], 1) < find([a < b true], 1);