sequence grouping

Hi all,

I am trying to construct a few groups from a given sequence by giving some pivots, one or more. For example, a = 1:15 and the pivots are 6 and 9. The desired groups are [1:3], [4:8], [7:11] and [12:15]. Is loop (brute force) the only solution?

for i = 1:length(piv)
par{i} = piv(i)-length(piv):piv(i)+length(piv);
end
par1 = 1:piv(1)-1;
par2 = piv(2)+length(piv)+1:end;

Thanks.

[Matt Fig]

That last line doesn't make sense. Are you missing a parenthesis around the END statement?

[Oleg Komarov]

I don't get the logic of the grouping...Can you elaborate a little more on the concept?

[Chien-Chia Huang]

Sorry about being vague in description.

When a sequence [1:15] and pivots (6,9) are given, from each of the pivot, we enclose its neighbors (number of pivots from the right and left).

Hence, we have [(6-4):(6+2)] = [4:8] and [(9-2):(9+2)] = [7:11].

One of the rest will be those on the left of [4:8], namely, [1:3] and the other will be those on the right of [7:11], namely, [12:15].

Answer 1

[Matt Fig]

Does this do what you are wanting?

Lp = length(piv);
G = ones(Lp,2*Lp+1);
G(:,1) = piv-Lp;
G = cumsum(G,2);
D{1} = min(a):min(G(:))-1;  % Holds the groups
D(2:Lp+1) = mat2cell(G,ones(1,Lp),2*Lp+1);
D{Lp+2} =  max(G(:))+1:max(a);

.

EDIT

.

Note that this may be much faster, though it does have a loop.

L2 = length(piv);
D2 = cell(L2+2,1);
D2{1} = min(a):min(piv-L2-1);
for ii = 2:L2+1
    D2{ii} = piv(ii-1)-L2:piv(ii-1)+L2;
end

D2{L2+2} = max(piv+L2)+1:max(a);

[Chien-Chia Huang]

Thanks Matt.

Exactly what I want.

Answer 2

[Andrei Bobrov]

variant without loop

a = 1:15;
c = [6 9];

li=length(c);
lii = -li:li;
[non,I]=ismember(c,a);
C = bsxfun(@(x,y)x+y,I.',lii);
cl = cell(length(c)+2,1);
cl([1,end]) = {1:C(1)-1,C(end)+1:numel(a)};
cl([2:end-1]) = mat2cell(C,ones(length(c),1),l)

[Chien-Chia Huang]

Thanks Abobroff.

But an error message occurred at this line => [~,I]=ismember(c,a);

Anything wrong?

[Matt Fig]

I bet the error message has to do with your use of an older version of MATLAB. Replace this line with:

[I,I] = ismember(c,a);

[Chien-Chia Huang]

Thanks, Matt. That solves the error.