How do I use if elseif to find indices of elements that meet a statement and assign NaN to elements that don't?

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NA 2021-6-16

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https://ww2.mathworks.cn/matlabcentral/answers/857620-how-do-i-use-if-elseif-to-find-indices-of-elements-that-meet-a-statement-and-assign-nan-to-elements

编辑： NA 2021-6-17

Hi All,

I have a 3D matrix (a2).

I want to find the index of elements in a2 <= -0.1 in the first dimension for all trials (trl) and experiments (exp). Some of my trials (trl) don't have values <=-0.1, and for these I would like to assign them with NaN.

I've had a go at writing code, but my if elseif statement is completely off. Can someone please give me some pointers on what I should do.

Many thanks

onsetT=zeros(trl,anim); %preallocate
for exp = 1:12 %experiments
    for trl=1:50 %trials
        if a2(:,trl,exp)<=-0.1 %if there are elements <=0.1
            [row] = find(a2(:,trl,exp)<=-0.1,1); %find index of these elements
            onsetT(trl,exp)=lfpTime(row); %use index to locate onset time
        elseif a2(:,trl,exp)>=0 %if elements >0 i.e. positive
           a2(:,trl,exp)=NaN; %assign them with NaN
           onsetT(trl,exp)=a2(:,trl,exp); %assign onset time for these trials NaN
        end
    end
end

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Answer 1

KSSV 2021-6-16

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If a is your matrix, to replace the values which are less than -0.1 just use:

idx = a < -0.1 ;
a(idx) = NaN ; 

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NA 2021-6-16

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Thanks for this. The issue I am facing now, is that I get an error message at the point were the for loop comes across a trial where all the values are NaN: "Unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is 1-by-0."

I tried to overcome this by adding the following line of code but was not successful:

if isnan(a2) 
    onsetT(trl,exp)=NaN;
else

What I'm trying to do is assign NaN in the matrix 'onsetT' (50x12) when all elements in the trial are NaN.

To put things into prespective my code looks like this:

onsetT=zeros(trl,exp);
idx=a2>0;
a2(idx)=NaN;
for exp =1:12
    for trl=1:50
        if isnan(a2) 
        onsetT(trl,exp)=NaN;
        else
        [row] = find(a2(:,trl,exp)<=-0.1,1);
        onsetT(trl,exp)=lfpTime(row);
        end
    end
end