Reveresing the order of columns in an array

Hi guys,

I would like to write a code to reverse the columns order as in a following example:

q=[1 2 3 4, 5 6 7 8, 9 10 11 12, 13 14 15 16, 17 18 19 20];

for example, the program randomly picks:

i1=2

i2=4

so we want to rearrange the order of colums from the second till the forth one and as an output get

qnew=[1 4 3 2, 5 8 7 6, 9 12 11 10, 13 16 15 14, 17 20 19 18]

Answer 1

[Les Beckham]

% Note: use semicolons instead of commas for row breaks so you get 4 columns
q = [1 2 3 4; 5 6 7 8; 9 10 11 12; 13 14 15 16; 17 18 19 20]

q = 5×4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

i1=2;
i2=4;
qnew = q;
qnew(:,i1) = q(:,i2);
qnew(:,i2) = q(:,i1);
disp(qnew)

     1     4     3     2
     5     8     7     6
     9    12    11    10
    13    16    15    14
    17    20    19    18

Answer 2

[Bartosz Bagrowski]

I meant changing the order of the columns in such a way:

q=[1 2; 3 4; 5 6; 7 8; 9 10; 11 12]

i1=2

i2=6

qnew=[1 2; 11 12; 9 10; 7 8; 5 6; 3 4]

so we reverse the order of columns from the second one till the sixth one

[Les Beckham]

Well, that is a different question. This q only has two columns. It appears that now you want to swap the rows.

q=[1 2; 3 4; 5 6; 7 8; 9 10; 11 12]

q = 6×2

1 2 3 4 5 6 7 8 9 10 11 12

i1=2;
i2=6;
qnew = q;
qnew(i1,:) = q(i2,:);
qnew(i2,:) = q(i1,:);
disp(qnew)

     1     2
    11    12
     5     6
     7     8
     9    10
     3     4

Answer 3

[Torsten]

A = [1 2; 3 4; 5 6; 7 8; 9 10; 11 12];
perm = [1 6 5 4 3 2];
A = A(perm,:)