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# How do I reverse the order of a vector?

Asked by Umer on 9 Feb 2011

Generate a 4x4 matrix P, whose first column is an array of 0, 2, 4 and 6; second column is an array of 1, 3, 5, and 7; third is the second column in reverse order and fourth column is the first column in reverse order. I want to use Reverse function or something like that..

## Products

Answer by Andreas Goser on 9 Feb 2011

I think you are looking for commands like flipud and fliplr, right?

Answer by Jan Simon on 9 Feb 2011

It would be an inefficient idea to use a function to create a 4x4 matrix with fixed values!

```P = [0, 1, 7, 6; ...
2, 3, 5, 4; ...
4, 5, 3, 2; ...
6, 7, 1, 0];
```

Now this homework is solved. Kind regards to your teacher.

Jan Simon on 9 Feb 2011

I think this is more prone to errors and confuses beginners.

Walter Roberson on 10 Feb 2011

I think of it as a way of differentiating between those who learn and those who copy without understanding.

Jim Dowd on 10 Feb 2011

Funny stuff Jan Simon!

Answer by Matt Fig on 10 Feb 2011

Walter, I am shocked that you didn't include the obvious nested FOR loop.

```cnt = 0;
for ii = 1:2,
for jj = 1:4
A(4-jj+1,4-ii+1) = cnt;
A(jj,ii) = cnt;
cnt = cnt + 2;
end
cnt = 1;
end
```

## 1 Comment

Walter Roberson on 10 Feb 2011

I was working on fitting it to a 100'th order polynomial, but discovered that any order beyond 23 gives absolutely completely wrong answers!

Answer by Walter Roberson on 9 Feb 2011

t = (0:2:6).'; P = [t, t+1, 7-t, 6-t];

## 1 Comment

Andrew Newell on 10 Feb 2011

That told him!

Answer by Matt Fig on 10 Feb 2011

Or, a one liner:

```reshape(permute(reshape([0:7 7:-1:0],2,4,2),[2,1,3]),4,4)
```

Answer by Walter Roberson on 9 Feb 2011
```t = (0:2:6).';
trev = t(length(t):-1:1);
P = [t, t+1, trev+1, trev];
```

Answer by Walter Roberson on 9 Feb 2011
```t = (0:2:6).'
trev = zeros(size(t));
trev(end:-1:1) = t;
P = [t, t+1, trev+1, trev];
```

Answer by Walter Roberson on 9 Feb 2011
```for K = 1 : 4
t(K) = 2*(K - 1);
end
for K = 1:length(t)
trev(K) = t(end+1-K);
end
```

etc.