# Reveresing the order of columns in an array

4 views (last 30 days)
Bartosz Bagrowski on 17 May 2022
Answered: Torsten on 17 May 2022
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]

Les Beckham on 17 May 2022
Edited: Les Beckham on 17 May 2022
% 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

Bartosz Bagrowski on 17 May 2022
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 on 17 May 2022
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

Torsten on 17 May 2022
A = [1 2; 3 4; 5 6; 7 8; 9 10; 11 12];
perm = [1 6 5 4 3 2];
A = A(perm,:)

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