problem using setdiff to check if a vector is present in a loop

Question

Stephen Devlin 2018-2-9

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评论： Stephen Devlin 2018-2-10

Hi,

I have a problem with some code I have written and hope someone has a pointer or two. The issue is with the variable "matchingThing",derived from using setdiff on two different arrays. The problem is when I use it in a loop, on the second pass through the loop matching drop seems to double in length, taking the variables from the previous run through the loop instead of seeing the difference between the two current sets.

Any ideas?

Steve

close all
clear all
clc
% *******************
NozPitch=0.08406666;
Delay2=10.05;
delayTime=(Delay2/1000000);
% *******************
data=     [ 1           15       12.162        0.488        6.897
    2           15       12.165        0.581        7.016
    3           15       12.343        0.578        6.913
    4           15       12.366        0.522         6.79
    5           15       12.535        0.605        6.805
    6           15       12.537        0.461        6.991]
scatter(data(:,3),data(:,4),'filled','k')
box on
grid minor
% *******************
Used_ThingsArray=nan(length(data),5);
% *******************
dataCopy=data;
dataCopy(:,2)=[];
FrameThingCoords=dataCopy
% check the Thing you select as first Thing to test is not in the Used_ThingsArray
selectThing=dataCopy(1,:)
% *******************
IndexAllThingsInROI= data(:,3)<= data(1,3)+(NozPitch)
TotalThingsWithinRange=sum(IndexAllThingsInROI(:) == 1)
ThingsInRange=dataCopy(IndexAllThingsInROI,:)
% *******************
for k=1:2:length(dataCopy)
    %
    Thing=k
    ThingSelected=dataCopy(k,:)
    if  ismember(ThingSelected,Used_ThingsArray)==0 ;%has the Thing selected already been measured
        ThingSelectedCoords=ThingSelected(:,2:3)
        matchingThing = setdiff(ThingsInRange,ThingSelected,'stable')'
        %%Checks, unneeded to run normally
        CheckThingsInRange{k}=ThingsInRange
        CheckThingSelected{k}=ThingSelected
        CheckmatchingThing{k}=matchingThing
          ThingsDistanceLength=sqrt((((matchingThing(1,3)-ThingSelected(1,3))).^2) + (((matchingThing(1,2)-ThingSelected(1,2))).^2))/1000;
          Velocity=ThingsDistanceLength/delayTime
          %trajectory_mR=(tan((matchingThing(1,2)-ThingSelected(1,2) ) /(  matchingThing(1,3)-ThingSelected(1,3))))*1000%mrads
          trajectory_D=(tand((matchingThing(1,2)-ThingSelected(1,2) ) /(  matchingThing(1,3)-ThingSelected(1,3))))
      end
end

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Guillaume 2018-2-9

编辑：Guillaume 2018-2-9

I personally find the code very confusing with no explanation why sometimes data being used and other times datacopy, one being the other with one less column.

Never ever use length on a matrix. In your case, you probably mean size(matrix, 1). Be aware that length could return size(matrix, 2). However, I doubt it's the reason for your problem.

In fact, never ever use length. Use numel for vector and size with an explicit dimension for matrices.

A common reason for arrays not being the correct size is the lack of preallocation and the growing of arrays in a loop.

I haven't looked at the code in enough details to understand the issue. I would recommend reducing the number of variables to make the code easier to understand. datacopy and and selectThing serve no purpose at all. datacopy(r, c) is either data(r, c) for c == 1 or data(r, c+1) for c >= 1.

Stephen Devlin 2018-2-9

Hi,

I don’t have a computer nearby, just my phone. dataCopy is just a copy of data with a column taken out but I wanted to preserve the original data for when I get past the problem I have at the moment.

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Answer 1

Greg 2018-2-9

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/381760-problem-using-setdiff-to-check-if-a-vector-is-present-in-a-loop#answer_304396

编辑：Greg 2018-2-9

在 MATLAB Online 中打开

I think all you need is the 'rows' input to setdiff. Otherwise, setdiff includes an implicit unique, so any duplicate data points in ThingsInRange will be removed, regardless of their appearance in ThingSelected.

matchingThing = setdiff(ThingsInRange,ThingSelected,'stable','rows');

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Stephen Devlin 2018-2-10

Hi Greg, yes just tried that and removed the apostrophe that made the output into a column vector, now works perfectly.

Very much appreciated.

Have a good weekend,

Steve

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problem using setdiff to check if a vector is present in a loop

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