How exactly does 'OR' logic works in Matlab. Getting Wrong answers.

How can the following be true?? The single and the double bar gives the following as true, but it's clearly wrong... What am I missing?

>> 'D' == 'A' | 'C' | 'I'

ans = 
      1

>> 'D'=='A'||'C'||'I'

ans =
      1

[Jan]

The answer is clearly correct, only the expectation is wrong.

[Stephen23]

It seems that people forget about operator precedence as soon as they leave high school.

Answer 1

[Walter Roberson]

'D' == 'A' | 'D' == 'C' | 'D' == 'I'
ismember('D', {'A', 'C', 'I'})

[Walter Roberson]

To expand:

The MATLAB comparison operators have higher priority than the "|" and "||" and "&" and "&&" operators. The expression

'D' == 'A' | 'C' | 'I'

that would be evaluated as

('D' == 'A') | ('C') | ('I')

The ('D' == 'A') part would be false. The ('C') part is the same as the implicit test ('C' ~= 0) and so that is true. or(false,true) is true so the expression as a whole becomes true.

The MATLAB comparison and logical operators never have implicit "chains". It is not correct in MATLAB to write

'D' == 'A' | 'C' | 'I'

with the expectation that 'D' will be tested against 'A' or 'C' or 'I' .

The MATLAB comparison and logical operators are vectorized. You can write

'D' == ['A', 'C', 'I']

which would evaluate to [false, false, false] . However, if you had this in an "if" condition then the test would be equivalent to

'D' == 'A' & 'D' == 'C' & 'D' == 'I'

which would have to be false because no value can simultaneously be equal to three values. In an "if" condition, the result is not considered true unless all of the values in the logical condition are considered true, the equivalent of

all('D' == ['A', 'C', 'I'])

To get the equivalent of testing to see if at least one of them holds you should specifically code a call to any()

any('D' == ['A', 'C', 'I'])

You might have noticed that ['A', 'C', 'I'] is the same as 'ACI' so yes the above line of code is the same as

any('D' == 'ACI')

Remember, in MATLAB strings are exactly the same as row vectors of characters and all of the row vector operations apply, same as if you had tested

any(4 == [1 3 9])

If you want to treat a string as a single object instead of as a row vector of characters, you need to use strcmp()

strcmp('D', 'A') | strcmp('D', 'C') | strcmp('D', 'I')

or in short form

strcmp('D', {'A', 'C', 'I'})

Here {'A', 'C', 'I'} is not the same as ['A', 'C', 'I'] and so is not the same as 'ACI'

Personally I am more likely to code with ismember() than with strcmp()

ismember('D', {'A', 'C', 'I'})