Why is the conditional statement not working as per the condition given?

Question

Subhanu 2024-4-20

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2109411-why-is-the-conditional-statement-not-working-as-per-the-condition-given

编辑： Cris LaPierre 2024-4-20

horizon=0.3;
spacing=0.1;
length=1;
numberOfParticlesAlongX=floor(length/spacing);
numberOfParticlesInDomain=numberOfParticlesAlongX;    %Number of particles in X-direction attributed to domain
xComponent(1:numberOfParticlesAlongX)=((1:numberOfParticlesAlongX)-1)*spacing; %Co-ordinate of each particle stored in xComponent
CORD=xComponent;
for i=1:numberOfParticlesInDomain
    p=1;
    for j=1:numberOfParticlesInDomain
        if i~=j    %Condition for checking undesired presence of a particle in its own neighbourhood 
            d=abs(CORD(i)-CORD(j));  %Distance between the two particles is stored in'd' for comparison with the horizon value
            if d<=horizon
                neighbourhoodArray(i,p)=j;
                p=p+1;     %'p' is incremented at every step to store only the particles within the horizon distance of a particular particle.
            end
        end
    end
end
disp(neighbourhoodArray);
     2     3     0     0     0
     1     3     4     0     0
     1     2     4     5     6
     2     3     5     6     0
     3     4     6     7     0
     3     4     5     7     8
     5     6     8     9    10
     6     7     9    10     0
     7     8    10     0     0
     7     8     9     0     0

This code is part of a project on material failure modelling. I need to store all the particles (labelled as 'j' in the for loop) within a certain distance (lablled as 'horizon') from a particular particle (labelled as 'i' in the for loop). With the given data of horizon=0.3, spacing (between particles)=0.1, and length=1, number of particles in the domain will come out to be 10. The x-component of 1st particle is '0' and that of the last one is '0.9', as in the 'xComponent' array.

Now, with the given data, for the first particle (with CORD(1)=0), the neighbourhood particles should be those at CORD(2)=0.1 (Particle-2), CORD(3)=0.2 (Particle-3), CORD(3)=0.3 (Particle-4). But, in the neighbourhoodArray array, for the 1st particle, only Particle-2 and Particle-3 are being shown to be in the neighbourhood of Particle-1, which is incorrect.

I ran the code statement-by-statement using 'Step' and found that the compiler is not executing the statement under "if d<=horizon" when 'd' and 'horizon' have the same value (=0.3). I really don't understand what the problem is with the conditional statement, as I think the logic that I have used is correct.

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Answer 1

Cris LaPierre 2024-4-20

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https://ww2.mathworks.cn/matlabcentral/answers/2109411-why-is-the-conditional-statement-not-working-as-per-the-condition-given#answer_1444906

编辑：Cris LaPierre 2024-4-20

在 MATLAB Online 中打开

The issue is that some of your values in CORD have some small error in them due to floating point arithmatic.

horizon=0.3;
spacing=0.1;
length=1;
numberOfParticlesAlongX=floor(length/spacing);
numberOfParticlesInDomain=numberOfParticlesAlongX;    %Number of particles in X-direction attributed to domain
xComponent(1:numberOfParticlesAlongX)=((1:numberOfParticlesAlongX)-1)*spacing; %Co-ordinate of each particle stored in xComponent
CORD=xComponent;
CORD(4) - 0.3
ans = 5.5511e-17