sequence grouping

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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.
  3 Comments
Chien-Chia Huang
Chien-Chia Huang on 13 Apr 2011
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].

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Accepted Answer

Matt Fig
Matt Fig on 13 Apr 2011
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);
  1 Comment
Chien-Chia Huang
Chien-Chia Huang on 13 Apr 2011
Thanks Matt.
Exactly what I want.

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More Answers (1)

Andrei Bobrov
Andrei Bobrov on 12 Apr 2011
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)
  3 Comments
Chien-Chia Huang
Chien-Chia Huang on 13 Apr 2011
Thanks, Matt. That solves the error.

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