How to do permutations with conditions

Hello everyone,

I have the following to get permutations

V= ([1:0.5:5]');
Comb = permn(V,2);

https://uk.mathworks.com/matlabcentral/fileexchange/7147-permn

Now, I still want to permutate but I don't want to have permutations that start with 1 but end with it.

So my results now:

1	1
1	1.5
1	2
1	2.5
1	3
1	3.5
1	4
1	4.5
1	5
1.5	1
1.5	1.5
1.5	2
1.5	2.5
1.5	3
.
.
.

I still want the same results but I don't want the results that start with 1 in the first column but I still want 1 to be present in Comb but in my second column. I know I can simply use this:

find(AllCombinations(:,1) ==1 )

Then delete what I don't want, but I was wondering if there's a way to set it as a condition as I permutate, so just one step.

Thank you, if need more clarifications please let me know.

[Sindar]

I doubt there is a way to do this from the start, but there is a slightly better way to delete:

V= ([1:0.5:5]');
Comb = permn(V,2);
Comb( Comb(:,1)==1,:) = [];

Answer 1

[Sindar]

Actually, this should work

V= (1:0.5:5);
Comb = combvec(V(2:end),V)';
% if you want sorted by first column:
Comb = sortrows(Comb,1);

[John Doe]

Thank you Sindar!

I couldn't use combvec because I don't have the Neural Network Toolbox so I used the following exchange file:

https://uk.mathworks.com/matlabcentral/fileexchange/10064-allcomb-varargin

I just have one more question, what if I want to create several columns instead of 2. With permn I can define my column size. Is it still possible here?

[Sindar]

With these methods, you need to repeat the arguments as many times as you need. Luckily, this stuff scales so fast that you probably won't reach the point where this is too bad:

% 5 elements
Comb = allcomb(V(2:end),V,V,V,V)';

If you need to programmatically change the number of elements, you can probably do it with a cell array and argument expansion:

N = 5;
V_s = {};
for ind=1:(N-1)
    V_s{ind} = V;
end
Comb = allcomb(V(2:end),V_s{:})';

[John Doe]

Thank you so much!! This is great!