Thread Subject: Index of points between two vectors

Hi ppl!

I have two vectors:

Nodes1, which is a 256 lines and 2 columns vector of cartesian coordinates of nodes in a FEM mesh.

and

Pts, which is a 48 lines and 2 columns vector containing the coordinates of specific points (that I specify).

I want to find the indices of those points in the Nodes1 vector and I am using this coding that I found in this Newsgroup:

%Nodes under electrodes
N1=[];
for i5=1:size(elec_pts)
N11=find(min(abs(Nodes1(:,1)-Pts(i5,1)) & min(abs(Nodes1(:,2)-Pts(i5,2)))));
N1=[N1;N11];
end

However, I am not getting the indexes of all the points I specify, or nodes if you will.

I obtained the cartesian coordintes using pol2cart function in MATLAB, since it was easiest to use polar coordinates for a circular boundary.

Hope someone can help.

Cheers.

The function you want is ismember. Type "help ismember" (without quotes) at the command line for the correct usage.
Darren

When working with coordinates, you typically need to be careful about round off errors. You can look up the feutil command in OpenFEM (open source) or SDT (commercial).

I just post the code on other thread, but I duplicate here for convenience. This function is similar to ismember, but works with floating points:

% Bruno

function varargout = ismemberf(A, S, varargin)
% function tf = ismemberf(A, S)
% [tf loc ] = ismemberf(A, S)
%
% Purpose: Floating point ISMEMBER (i.e., with round-off tolerance)
%
% ismemberf(A, S, 'row') operate on row
% ismemberf(..., 'tol', tol) fix the tolerance
% tol can be scalar or vector (using with 'row')
%
% If not provided, tol is 1e-10 relative to variations of S values
%
% Example:
% [X Y]=meshgrid(0:0.1:10,0:0.1:10);
% S=[3 1;
% 3 3;
% 5 6;
% 5.2 5.5;
% 3 3];
% A = [X(:) Y(:)];
% [tf loc]=ismemberf(A, S, 'row', 'tol', 0.5);
% imagesc(reshape(loc,size(X)));


% Call native ismember for strings
if ~isnumeric(A) || ~isnumeric(S)
    out = cell(1,max(nargout,1));
    [out{:}] = ismember(A, S, varargin{:});
    varargout = out;
    return
end

% Preprocess the optional inputs (for parsing later on)
vin = varargin;
for k=1:length(vin)
    if ischar(vin{k})
        vin{k} = strtrim(lower(vin{k}));
    else
        vin{k}='';
    end
end

% parsing the varargin to set tol
tol = NaN;
tolloc = strmatch('tol', vin);
if ~isempty(tolloc)
    tolloc = tolloc(end);
    if length(vin)>=tolloc+1
        tol = varargin{tolloc+1};
    end
end

% one dimensional ismember
if isempty(strmatch('row', vin))
    [tf loc] = ismember1(A, S, tol);
    
else % row option
    % Check fo compatible dimension
    if size(A,2) ~= size(S,2)
        error('A and S must have the same number of columns');
    end
    
    % duplicate tol
    if isempty(tol)
        tol = nan(size(A,2),1);
    elseif isscalar(tol)
        tol(1:size(A,2)) = tol;
    end
    
    % Loop over column
    B = true;
    for j=1:size(A,2)
        % Get the binary matrix
        [tfj locj Bj] = ismember1(A(:,j), S(:,j), tol(j));
        B = B & Bj;
    end
    [iA iS] = find(B);
    tf = false(size(A,1),1);
    tf(iA) = true;
    loc = accumarray(iA,iS,size(tf),@(i) max(i));
end

% Process output
[out{1:2}] = deal(tf, loc);
nout = min(max(nargout,1), length(out));
varargout(1:nout) = out(1:nout);

end % ismemberf

function [tf loc B] = ismember1(A, S, tol)

    [Su I J] = unique(S(:),'last');
    
    % Set the tolerance
    if isempty(tol) || isnan(tol)
        maxS = max(Su);
        minS = min(Su);
        if maxS == minS
            tol = 1e-10;
        else
            tol = (maxS - minS)*1e-10;
        end
    else
        tol=tol(1);
    end
    xlo = Su - tol;
    xhi = Su + tol;
    [dummy ilo] = histc(A, [xlo; Inf]);
    [dummy ihi] = histc(A, [-Inf; xhi]);
    tf = ilo & ihi & (ilo >= ihi);
    loc = zeros(size(tf));
    loc(tf) = I(ilo(tf));
    %
    % Building a logical matrix of boolean B of size (m x n)
    % where m = numel(A), n = numel(S)
    % B(m,n) is true if two elements in A(m) and S(n) is "identical"
    %
    if nargout>=3
        left = ihi(tf);
        right = ilo(tf);
        % index in S
        % Group all the index of S when they map to the same Su
        II = accumarray(J(:), (1:numel(S)).', [length(Su) 1], @(x) {x});
        % Find all the index in the set S for each elements that belong to
        % it
        iS = arrayfun(@(l,r) cat(1,II{l:r}).', left, right, ...
                      'UniformOutput', false);
        nele = cellfun(@length, iS);
        iS = [iS{:}]; % concatenate in a long row vector
        % index in A
        iA = cumsum(accumarray(cumsum([1; nele]),1));
        iA(end)=[];
        inonly = find(tf);
        iA = inonly(iA);
        % Logical matrix, in sparse
        B = sparse(iA,iS,true,numel(A),numel(S));
    end
end % ismember1