Sorting pairs into seperate matrices based on whether they are ascending or descending

I have a 10x2 matrix of pairs of numbers

pairs = 10×2

2 8

7 5

6 8

4 7

8 6

2 5

4 2

6 2

1 7

3 1

And I wish to seperate them into two different nx2 matrices depending on whether the pairs are ascending or descending

%Seperates ascending and descending pairs
asc=ones(length(pairs),2);
desc=ones(length(pairs),2);
i=2
for i=1:(length(pairs))
    diff=diff(pairs(i,:));
    if diff>0
        asc=asc(i,:).*pairs(i,:);
    else
        desc=desc(i,:).*pairs(i,:);
    end
end

However everytime i try to run the for loop it returns the "Index exceeds the number of array elements" error.

I tried running it without the loop and it works until i run it again and this error appears. What am i doing wrong?

Answer 1

[Star Strider]

The loop is not necessary anyway.

Just use ‘logical indexing’ —

pairs = [2     8
     7     5
     6     8
     4     7
     8     6
     2     5
     4     2
     6     2
     1     7
     3     1];

Lv = pairs(:,2) > pairs(:,1)

Lv = 10×1 logical array
   1
   0
   1
   1
   0
   1
   0
   0
   1
   0

asc = pairs(Lv,:)

asc = 5×2

2 8 6 8 4 7 2 5 1 7

desc = pairs(~Lv,:)

desc = 5×2

7 5 8 6 4 2 6 2 3 1

.

[Anonymous]

Thanks thats a much more elegant way of doing it

[Star Strider]

As always, my pleasure!

Answer 2

[Dyuman Joshi]

Firstly, do not use built-in functions as variables (or script) names, like you have done here -

diff=diff(pairs(i,:));

Secondly, the default operating dimension of diff() is to find difference of successive elements along the rows.

To work through the difference of columns, you need to specify the dimension -

pairs = [2     8
     7     5
     6     8
     4     7
     8     6
     2     5
     4     2
     6     2
     1     7
     3     1];

%Find the (1st) difference along columns
idx = diff(pairs, 1, 2)>0

idx = 10×1 logical array
   1
   0
   1
   1
   0
   1
   0
   0
   1
   0

%Use logical indexing to get the ascending and descending pairs, respectively
asc = pairs(idx, :)

asc = 5×2

2 8 6 8 4 7 2 5 1 7

desc = pairs(~idx, :)

desc = 5×2

7 5 8 6 4 2 6 2 3 1