Matlab OR | operator not working

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bsriv 2019-2-21

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评论： Steven Lord 2019-2-22

Hi I have a dataset (x.data) of 90,000 or so indices with values at each index. I am trying to pull the indices at certain values 21009 and 21003

When I sum(x.data==21009) I get 80, which is correct (ie, there are 80 indices with values of 21009). When I sum(x.data==21003) I get 150, also expected.

However when I try to pull both sum(x.data== 21009 | 21003) I get 90,000 and when I look at the output for x.data==21009 | 21003) it's just a column of 1s. It's probably something obvious but what am I missing?

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Stephen23 2019-2-22

在 MATLAB Online 中打开

Following the documented rules of operator precedence

https://www.mathworks.com/help/matlab/matlab_prog/operator-precedence.html

the syntax that you invented

x.data== 21009 | 21003

is equivalent to

(x.data== 21009) | 21003

which, because the term in the parentheses can be either false or true, is therefore equivalent either of these:

(false) | 21003
(true)  | 21003

which, because any non-zero values is considered to be true, are equivalent to these:

(false) | true
(true)  | true

which (using standard rules of logic) is clearly always true.

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Answer 1

Bob Thompson 2019-2-21

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To the best of my knowledge the or operator doesn't work quite like that. You need to set separate conditions on each side of the or, rather than multiple values to check for.

x.data == 21009 | x.data == 21003;

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bsriv 2019-2-21

Thanks!

Steven Lord 2019-2-22

If you later decide that you want to select elements that match any of a larger set of values (3, 4, or more) consider switching to ismember instead of chaining together 3, 4, or more expressions with the or operator | between them.

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