what does it mean by writing [~,idx] in code?

for p= 4:4:population

dists= total_dist(rand_pair(p-3:p));

[~,idx]=min(dists);

best = routes(idx,:);

what idx, ~ means??

[the cyclist]

Yogesh, I don't know why you posted this question four times, but I deleted the other three.

[merlin toche]

Hello

anyone can help me with my code below? I want to do a classification using fuzzyknn.please, I tried to do what I can but as I'm still a beginner, I need your help to finalize this work. thank you for your advance here attached my code

[Steven Lord]

Since this doesn't sound like it's related to the original question about the [~, idx] syntax you should ask it as a separate question using the Ask link at the top of this page.

[merlin toche]

@Steven Lord

please I posted the question as you asked so well but no answer. I went through this page because for 2 months my concerns have been ignored, can you explain this to me? thank you

[merlin toche]

i have this error when i plot my code

attached

Answer 1

[James Tursa]

The ~ represents an output that is discarded. It is essentially equivalent to:

[dummy,idx]=min(dists);
clear dummy

For this example, the code wants to work with the index of the minimum value, not the value itself, so the minimum value that is returned is discarded and only the index is retained.

http://www.mathworks.com/help/matlab/matlab_prog/ignore-function-outputs.html?searchHighlight=tilde

[Dyuman Joshi]

Hello @James Tursa, do you have any source for the equivalence mentioned?

The webpage cited in the link doesn't refer to any such relation.

[Adam Danz]

@Dyuman Joshi, the page James shared in his answer is a good resource but doesn't contain James' analogous explanation.

When an output is suppressed with a tilde, the output is computed from within the function but isn't returned and therefore isn't stored in the caller's workspace. The only detail that differs from James' anaology is that the dummy variable is never returned to begin with, but it is computed within the min function.

[Dyuman Joshi]

Thank you for the response, Adam.

I now know that while a variable that is suppressed is computed, but it is not returned and not stored in the caller's workspace.

I was stuck on an approach on how to show/understand it via code, but it was quite simple -

[~,~,out]=yo(5)

a = 5

b = 25

c = -5

out = -5

function [a,b,c]=yo(x)
    a=x
    b=x.^2
    c=-x
end

[Adam Danz]

Right, and if you suppress the lines within the function,

[~,~,out]=yo(5)

out = -5

function [a,b,c]=yo(x)
    a=x;
    b=x.^2;
    c=-x;
end

Answer 2

[the cyclist]

When you see

>> [a,b,c] = function(...)

then a,b, and c are the output of a function. If you do not want one of the outputs of a function, then you can replace it with the ~ symbol:

>> [a,~,c] = function(...)

and then b will not be output.

[James Tursa]

To clarify, the syntax doesn't actually prevent the function from producing the output ... it just causes MATLAB to ignore the output and automatically clear it instead of assigning it to a workspace variable. So using the syntax makes your code cleaner looking but the function will still use the same resources (time & memory) to run.