Specialized sorting of array while preserving order

how can i sort an array (of even number of elements) so the elements on the left side are smaller than the elements on the right side but without changing the original order of the elements in the array?

for example:

a = [19 21 12 -15 18 1 22 11];

r = [12 -15 1 11 19 21 18 22];

12 is prior to -15 in the original array and so it is in the result.

another example:

a = [8 4 2 6 3 1 9 10];

r = [ 4 2 3 1 8 6 9 10];

[Thomas]

Why is 12 first. The results seems to be random.

but 4 is larger than 2..

Answer 1

[Matt Fig]

% Your original array
a = [19 21 12 -15 18 1 22 11];

% Now the method:
L = length(a)/2;
[~,J] = sort(a);  % For older MATLAB vers,[J,J] = sort(a);
[~,J] = sort(J);
r = [a(J<=L) a(J>L)]

[Zaza]

thanks Matt. it works perfect. can i get an explanation of lines 3-4?

[Matt Fig]

Hello Zaza, I am afraid the best explanation I can give you would be to have you just look at the help for SORT and look at the outputs. To get a better understanding, look at all the outputs (after reading the help for SORT!):

L = length(a)/2;
[I,J] = sort(a)  % Look at these: compare to a
[K,M] = sort(J)  % I(M) is a
r = [a(M<=L) a(M>L)]

[Zaza]

i read the help of sort but i still don't feel comfortable with that...

can you give me please a simple step-by-step ex.? it would be very helpful for me...

[Matt Fig]

Zaza, it seems you already have a step-by-step example! Perhaps this will help. Here is another (preferable) way of solving the problem:

L = length(a)/2;
[~,J] = sort(a);
J(J) = 1:L*2;
r = [a(J<=L) a(J>L)]

Now have a look at Loren's blog entry on this method:

Reversal of a sort