How to randomly swap 2 elements in an array?

Suppose that I have the below array:

C= [1 6 5 10 1 2 11 8 2 7 3 7 9 9]

How can I swap randomly only the elements inside the bold numbers

Assume after swap 11 and 3 : C= [1 6 5 10 1 2 3 8 2 7 11 7 9 9]

or swap 10 and " " between 9 9: C= [1 6 5 1 2 11 8 2 7 3 7 9 10 9]

Answer 1

[Rik]

I'm going to assume at least one of the swap elements must be an number, and that the swapped numbers are allowed to have the same value (so you can some one 9 with another 9).

C=  [1     6     5    10     1     2    11     8     2     7     3     7     9     9];
bold=[1 5 6 9 10 12 13 14];

C_temp=cell(1,2*numel(C));
C_temp(2:2:end)=num2cell(C);
valid_idx=1:numel(C_temp);
valid_idx(bold*2)=[];

swap_idx=[1 1];%invalid output to enter the loop
while ~any(mod(swap_idx,2)==0) || ... %at least 1 needs to be a number
        swap_idx(1)==swap_idx(2) %the indices must be different
    swap_idx=valid_idx(randperm(numel(valid_idx),2));
end

%perform swap
C_temp(swap_idx)=C_temp(swap_idx(end:-1:1));

D=cell2mat(C_temp);
clc,disp(C),disp(D)

[Hang Vu]

Thank you! but can you make it more flexible? I got C from ramdomly displayed.

So these bold numbers are not in the fit position.

And your code display this when I run:

 1     6     5    10     1     2    11     8     2     7     3     7     9     9

6     1     5    10     1     2    11     8     2     7     3     7     9     9

6 is out of 1 1

[Rik]

You didn't describe how you get the bold numbers, so you'll have to figure that out from your input. I might be able to help you there, but I can't read your mind.

Your output should not be possible, that is what the while loop is for. What do the two conditions return if you try them on their own?

[Hang Vu]

it showed this when I run:

C =

     1     6     5    10     1     2    11     8     2     7     3     7     9     9


swap_idx =

    23    15


swap_idx =

    21     3


swap_idx =

    11    25


swap_idx =

     1     4

     6     1     5    10     1     2    11     8     2     7     3     7     9     9

this is how I got C:

I have D = [170, 80, -30, 0, -50, -30, 20, -60, 100, -60 -20];

A contains all + elements and I repelem them( those bold number)

B contains all - elements

Then I randomly put B into A to create C.

Now I want to swap elements between bold numbers

[Rik]

I don't really understand what you mean in your description. At any rate, you should be able to write some code that returns a vector with which elements are bold.

About the result you had the first point: before the first number element there is an empty cell where swapping is allowed. If you don't want that, you can put in an extra condition in the while statement:

while ~any(mod(swap_idx,2)==0) || ... %at least 1 needs to be a number
        swap_idx(1)==swap_idx(2) || ... %the indices must be different
        any(swap_idx==1) %the first element (1 before the first number) isn't allowed
    swap_idx=valid_idx(randperm(numel(valid_idx),2));
end

[Hang Vu]

yes, thank you so much for yor time!

[Hang Vu]

I am sorry for bothering you, but I wonder why sometimes the result is still the same after swapping:

C =

     1     6     5    10     1     2    11     8     2     7     3     7     9     9


swap_idx =

    23     9


swap_idx =

     4     3

     1     6     5    10     1     2    11     8     2     7     3     7     9     9

[Rik]

Look at the indices being swapped. What do you think is happening?

[Hang Vu]

It was invalid then it swaps again, but most of the time it returns the same array.

[Rik]

Look at the indices. What is being swapped if they are 4 and 3?

[Hang Vu]

I got it, it's nothing at 3 and number 6 at 4. Thank you! I will try to fix it!

[Rik]

This should prevent the two swapped values to be adjacent (which swaps a value with a gap, meaning it seems the array stays the same).

while ~any(mod(swap_idx,2)==0) || ... %at least 1 needs to be a number
        any(swap_idx==1) || ... %the first element (1 before the first number) isn't allowed
        abs(diff(swap_idx))<=1 %the indices must be different and more than 1 appart
    swap_idx=valid_idx(randperm(numel(valid_idx),2));
end

[Hang Vu]

Thank you for the help!