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I am unable to interpret the following command in a MATLAB code

while (fa > 0) == (fb > 0)

I thought it says: if fa>0 , fb > 0 and both are equal to each other then do some commands.

However, while running in debug mode, at I found that even then fa and fb were < 0 and not equal to each other, the commands were still executed. May someone kindly help in correct interpretation of the command

This is the beginning of a while loop

Thanks

gt6989b
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d'kar
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2 Answers2

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fa>0 is a boolean value, meaning it evaluates to either true or false.
The same is true for fb>0
So this is stating that if both fa>0 and fb>0 evaluate to true, then do the stuff in the loop.
Likewise if both fa>0 and fb>0 evaluate to false, then you still do the stuff in the loop.
This is because true=true and false=false

gd1035
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  • what if fa and fb are vectors? – gt6989b Oct 09 '18 at 20:14
  • I'm not familiar with matlab, but it seemed that OP was confused about fa>0 being boolean, and how to evaluate such expressions. Their comment seems to indicate that they thought they were comparing the values of fa and fb. @gt6989b – gd1035 Oct 09 '18 at 20:33
  • You are, of course, correct, but $fa$ and $fb$ are most likely to be vectors, not scalars, as that is the most common instance of Matlab operations... – gt6989b Oct 10 '18 at 14:27
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fa > 0 returns a true/false vector for each entry, which is true iff the entry is positive.

Therefore, (fa > 0) == (fb > 0) is true iff all corresponding entries of fa and fb ahve the same signs.

In other words, $fa = [1,-1]$ and $fb = [-1,1]$ would cause it to be false, but $fa = [-1,-1]$ and $fb = [-2,-3]$ would make it true.

gt6989b
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