Nested If-ELSEIF statements

Question

Hans123 2020-6-16

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/549273-nested-if-elseif-statements

评论： Hans123 2020-6-16

I tried looking up this question in litreature and the forum and I couldn't find anything pertaining to my situation.

I have the following nested if-elseif statements, and I observed by using breakpoints, the code skips after processing the first if-elseif statement. It skips the caulations at the ellipsis too. I want it to run through the rest of the elseif and check the logic and calculate results - any help is appreciated

This excerpt is supposed to change the extreme points out of Nx and -Nx.

                if X<-Nx
                    X=Nx;
                elseif X>Nx
                    X=-Nx;
                    
                    if Y<-Ny
                        Y=Ny;
                    elseif Y>Ny
                        Y=-Ny;
                        
                        if Z<-Nz
                            Z=Nz;
                        elseif Z>Nz
                            Z=-Nz;
                            
                            ...
                                
                        
                        end
                    end 
                end

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Hans123 2020-6-16

在 MATLAB Online 中打开

I think I figured out the issue, the reason the code skipped the rest of the if-elseif is because my end commands were clustered together at the end of the code. I should have end my if-elseif statement right afterwards before starting a new one.

Thanks^

FIX

                if X<-Nx
                    X=Nx;
                elseif X>Nx
                    X=-Nx;
                end
                
                    if Y<-Ny
                        Y=Ny;
                    elseif Y>Ny
                        Y=-Ny;
                    end
                    
                        if Z<-Nz
                            Z=Nz;
                        elseif Z>Nz
                            Z=-Nz;
                        end      
                            ...
                                
                       

Hans123 2020-6-16

@Bjorn, thank you for your reply - this is meant for a periodic boundary condtion statement. It is not intuitive took me a minute too

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Answer 1

Geoff Hayes 2020-6-16

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https://ww2.mathworks.cn/matlabcentral/answers/549273-nested-if-elseif-statements#answer_452043

在 MATLAB Online 中打开

Hans123 - do you really need nested if-elseif blocks to change the extreme points? Wouldn't you just do them for x, y, and z seperately? Otherwise you might see this "skipping" depending upon which condition evaluates to true. For example,

if X<-Nx
    X=Nx;
elseif X>Nx
    X=-Nx;
    
    if Y<-Ny
        Y=Ny;
    elseif Y>Ny
    % etc.

if the first condition evaluates to true i.e. X < -Nx then the code to check Y and Z will not be evaluated since they are nested within the elseif body. Instead try

if X<-Nx
    X=Nx;
elseif X>Nx
    X=-Nx;
end
    
if Y<-Ny
    Y=Ny;
elseif Y>Ny
    Y=-Ny;
end
        
if Z<-Nz
    Z=Nz;
elseif Z>Nz
    Z=-Nz;
end

Note how the above code is duplicated for x, y, and z. This might be okay for just three variables, but if you have more, you may want to use arrays to manage this.

bounds = [Nx Ny Nz];
values = [X Y Z];
for k = 1:length(bounds)
    if values(k) < -bounds(k)
        values(k) = bounds(k);
    elseif values(k) > bounds(k)
        values(k) = -bounds(k);
    end
end