Problems with sortrows function

I am trying to sort a two dimensional matrix Coord consisting of (x,y) pairs of coordinates (of size n x 2) in lexicographical order, with respect to x and then y. I used the command

[Coord_Lex,ix]=sortrows(Coord,[1,2]);

This should sort x coordinates in ascending order and within each set of matrix rows with the same x coordinate the y coordinates in ascending order. But in some cases it does not work correctly. For example, a part of the answer that I am getting is

1.2250 0.2067

1.2250 0.1722

1.2250 0.1378

1.2250 0.1033

1.2250 0.0689

1.2250 0.0344

1.2250 -0.2933

1.2250 -0.3278

1.2250 -0.3622

1.2250 -0.3967

1.2250 -0.4311

i.e., the y coordinaes are not sorted correctly. I checked the values of x coordinates in the incorrect part for the round-off error polution and the difference between them always gives 0.

[Eric]

What version of MATLAB are you using? I just tried this in R2023b, and it worked fine for me.

[Steven Lord]

Can you attach a small sample of data with which you can reproduce this behavior to your post, or perhaps display it using format hex so we can see the exact values that are stored in your variables? I suspect there's a difference in the last bit that's preventing sortrows from behaving as you think it should (even though it is behaving as it is designed to do.)

format % Show a few decimal places
p = pi

p = 3.1416

format longg % Show more decimal places
p = pi

p = 
          3.14159265358979

format hex % Show the actual bits stored in memory
p = pi

p = 
   400921fb54442d18

Answer 1

[Milan Mihajlovic]

Dear Steven,

this seems to be the case for some nodes. I created the matrix of differences between the x-coordinates of the nodes that "misbehaved" and in some cases the difference is equal to a round-off of 2.2e-16 which does cause the problem. Thank you for your insight. Best wishes, Milan