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Problem with logical indexing

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RP
RP on 24 Apr 2019
Commented: Ajith Thomas on 19 Jul 2019
I'm trying to write a function that will take a row vector of temperatures as input and will give the number of temperatures below 32° as output. I have tried this:
function numfreeze = freeezing(temperatures)
numfreeze = numel(freezing(freezing<32))
end
...but I get the error message "Out of memory. The likely cause is an infinite recursion within the program."
Can someone help me and tell me what this means? When I just use
numfreeze = numel(freezing(freezing<32))
in the command window after setting a vector for "temperatures", it works fine.

Accepted Answer

Guillaume
Guillaume on 24 Apr 2019
Edited: Guillaume on 24 Apr 2019
freezing is the name of the function. So when you write freezing inside the function, you're telling it to call itself. Of course, once it's called itself, it reexecute that freezing call ad finitum. Hence the infinite recursion.
You probably meant to use the input variable temperatures inside that function
function numfreeze = freeezing(temperatures)
numfreeze = numel(temperatures(temperatures<32))
end
While the above will work, it's not the most efficient. You're deleting all the elements above 32 to then find out how many remain. It be simpler to just count how many true values are in your logical array and not bother with the deletion, so:
function numfreeze = freeezing(temperatures)
numfreeze = nnz(temperatures<32); %you can also use sum instead of nnz
end
  1 Comment
RP
RP on 24 Apr 2019
Thank you so much, this works and I didn't know nnz before!

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More Answers (2)

Alex Mcaulley
Alex Mcaulley on 24 Apr 2019
I think you have a typo, calling infinite times the function freezing, then:
function numfreeze = freeezing(temperatures)
numfreeze = numel(temperatures(temperatures<32))
end

Ajith Thomas
Ajith Thomas on 19 Jul 2019
function numfreeze=freezing(w)
lowerthan_32=w(w<32);
no_logical_values=lowerthan_32(lowerthan_32>=0);
numfreeze=length(no_logical_values);
end
  1 Comment
Ajith Thomas
Ajith Thomas on 19 Jul 2019
function numfreeze=freezing(w)
lowerthan_32=w(w<32);
numfreeze=numel(lowerthan_32);
end

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