combine for loop and ~isinf,but there is an error in every loops except the result in the first loop

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yang-En Hsiao 2019-3-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/453365-combine-for-loop-and-isinf-but-there-is-an-error-in-every-loops-except-the-result-in-the-first-loop

编辑： Stephen23 2019-3-30

I use ~isinf and a for loop to make a code,in this code ,i want to let the un-Inf value to show me in each loop,however,i found that there is a error here,but i can't find where do i wrong

c=[ 3 6 1 Inf]
fg=c(~isinf(c))
for k=1:4
   ad=c(~isinf(c(k))) 
end

So in fact ad should show me as the same as each element in fg,like this

fg =  3 6 1 
ad =  3
ad =  6
ad =  1
ad =  []<-----maybe won't be shown

However,the window show me

fg =  3 6 1 
ad =  3
ad =  3
ad =  3
ad =  []<-----maybe won't be shown

Where i am wrong with the code?

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采纳的回答

Stephen23 2019-3-29

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/453365-combine-for-loop-and-isinf-but-there-is-an-error-in-every-loops-except-the-result-in-the-first-loop#answer_368181

编辑：Stephen23 2019-3-29

在 MATLAB Online 中打开

v = [3,6,1,Inf];
for k = 1:numel(v)
    n = v(k);
    z = n(~isinf(n)) 
end

displays:

z =  3
z =  6
z =  1
z = []

"i can't find where do i wrong"

Your code uses a scalar logical index:

~isinf(c(k))

Logical indexing is positional (it is not like subscript or linear indexing), so when you use a scalar logical index it acts only on the first element of the array, i.e. you either select the first element or you do not. That is exactly what your example output shows.

You cannot use a scalar logical index to work like a subscript and also expect it to select different elements of an array (i.e. not the first element). For that you would need to create a non-scalar logical index (normally these are the same size as the array being indexed) or use subscript/linear indexing first (which is what my answer does).

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yang-En Hsiao 2019-3-29

在 MATLAB Online 中打开

you mean like this ?

v = [3,6,1,Inf ]; 
z=nan(1,length(k))
for k = 1:length(v) 
n = v(k)
z(k) = n(~isinf(n )) 
end 
meann=mean(z(~isinf(z )))

but the error show to me

In an assignment  A(I) = B, the number of
elements in B and I must be the same.
Error in Untitled2 (line 6)
z(k) = n(~isinf(n ))

Stephen23 2019-3-30

编辑：Stephen23 2019-3-30

在 MATLAB Online 中打开

"you mean like this ?"

No. You told us that you generate "the result from every loop", so that is what my code requires.

Also your length(k) does not make much sense for array preallocation, because k is a scalar.

You should do something like this:

v = [3,6,1,Inf];
N = numel(v); % or however you define the number of loops.
y = nan(1,N); % preallocate!
for k = 1:N
    y(k) = v(k); % your value (calculated...)
end
m = mean(y(~isinf(y)))

Where, clearly, in your actual code you generate that value, rather than selecting elements of the vector v like we are using here for these examples.

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