FIND function with multiple conditions

Hello!

I have a dataset, state, that is 500x6 data. This dataset presents position and velocity for a circular orbit. As a result, there are cases where an x-position might have 2 options for y-location (1 positive, 1 negative).

The second data, p, set is 1x6: the final position and velocity at the end of an arc. This data may have either positive or negative for y-location for each computation.

The goal is to search the 500x6 data set to find the location (index) where p(1,1:3) = state(i,1:3). The goal is to match position only.

I have tried the following:

temp = min(abs(p(1,1:3) - state(:,1:3))) % tolerance

location = find(abs(p(1,1:3) - state(:,1:3)) == temp)

This method works when the y-location is positive for both z and state. It correctly finds the location in state.

Any recommendations on how to modify the search to correctly find the index when the y-location is negative in both sets? Thank you!

Answer 1

[Mathieu NOE]

hello Nadia

I have tried another approach , by looking at the 2 nearest points to zero crossing points of the vector defined as the difference between position and state .

the index of the 2 points is stored in ind_close

I have tested my code on dummy data

hope it works for you

% dummy data 

theta = 0:0.1:2*pi;
p = sin(theta);
s = 0.5*cos(theta);
figure(1),plot(theta,p,'b',theta,s,'r');
legend(' data : position', 'data : state')

%%%%%%%%%%%%%%%%%%% main loop %%%%%%%%%%%%%%%%%%%%%%%%%
dd = p  - s; % difference 
[ind,theta0,dd0,ind_close,theta0_close,dd0_close] = crossing_V6(dd,theta,0,'linear');        % Zero-Crossing 

figure(2)
plot(theta, dd, '*r')
hold on
plot(theta0_close, dd0_close, 'bp')
hold off
grid on
legend('p  - s  difference', 'nearest points to Zero-Crossings points')

% save data
p(ind_close)
s(ind_close)


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

function [ind,t0,s0,indclose,t0close,s0close] = crossing_V6(S,t,level,imeth)

% [ind,t0,s0,t0close,s0close] = crossing_V6(S,t,level,imeth)

% CROSSING find the crossings of a given level of a signal
%   ind = CROSSING(S) returns an index vector ind, the signal
%   S crosses zero at ind or at between ind and ind+1
%   [ind,t0] = CROSSING(S,t) additionally returns a time
%   vector t0 of the zero crossings of the signal S. The crossing
%   times are linearly interpolated between the given times t
%   [ind,t0] = CROSSING(S,t,level) returns the crossings of the
%   given level instead of the zero crossings
%   ind = CROSSING(S,[],level) as above but without time interpolation
%   [ind,t0] = CROSSING(S,t,level,par) allows additional parameters
%   par = {'none'|'linear'}.
%	With interpolation turned off (par = 'none') this function always
%	returns the value left of the zero (the data point thats nearest
%   to the zero AND smaller than the zero crossing).
%
%	[ind,t0,s0] = ... also returns the data vector corresponding to 
%	the t0 values.
%
%	[ind,t0,s0,t0close,s0close] additionally returns the data points
%	closest to a zero crossing in the arrays t0close and s0close.
%
%	This version has been revised incorporating the good and valuable
%	bugfixes given by users on Matlabcentral. Special thanks to
%	Howard Fishman, Christian Rothleitner, Jonathan Kellogg, and
%	Zach Lewis for their input. 

% Steffen Brueckner, 2002-09-25
% Steffen Brueckner, 2007-08-27		revised version
% M Noé            , 2020-08-20		revised version

% Copyright (c) Steffen Brueckner, 2002-2007
% [e-mail]

% check the number of input arguments
error(nargchk(1,4,nargin));

% check the time vector input for consistency
if nargin < 2 | isempty(t)
	% if no time vector is given, use the index vector as time
    t = 1:length(S);
elseif length(t) ~= length(S)
	% if S and t are not of the same length, throw an error
    error('t and S must be of identical length!');    
end

% check the level input
if nargin < 3
	% set standard value 0, if level is not given
    level = 0;
end

% check interpolation method input
if nargin < 4
    imeth = 'linear';
end

% make row vectors
t = t(:)';
S = S(:)';

% always search for zeros. So if we want the crossing of 
% any other threshold value "level", we subtract it from
% the values and search for zeros.
S   = S - level;

% first look for exact zeros
ind0 = find( S == 0 ); 

% then look for zero crossings between data points
S1 = S(1:end-1) .* S(2:end);
ind1 = find( S1 < 0 );

% bring exact zeros and "in-between" zeros together 
ind = sort([ind0 ind1]);

% and pick the associated time values
t0 = t(ind); 
s0 = S(ind);

if strcmp(imeth,'linear')
    % linear interpolation of crossing
    for ii=1:length(t0)
        %if abs(S(ind(ii))) > eps(S(ind(ii)))    % MATLAB V7 et +
        if abs(S(ind(ii))) > eps*abs(S(ind(ii)))    % MATLAB V6 et -    EPS * ABS(X)
            
            % interpolate only when data point is not already zero
            NUM = (t(ind(ii)+1) - t(ind(ii)));
            DEN = (S(ind(ii)+1) - S(ind(ii)));
            slope =  NUM / DEN;
            t0(ii) = t0(ii) - S(ind(ii)) * slope;
            s0(ii) = level;
        end
    end
end

% Addition:
% Some people like to get the data points closest to the zero crossing,
% so we return these as well
[CC,II] = min(abs([S(ind-1) ; S(ind) ; S(ind+1)]),[],1); 
indclose = ind + (II-2); %update indices 

t0close = t(indclose);
s0close = S(indclose);
end

[Nadia Numa]

I found a work around but thank you! I will give this a try.

[Mathieu NOE]

you're welcome