How to use not equal to '~=' operator in if statement?

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Sohail Ahmed il 21 Feb 2017
Commentato: Walter Roberson il 28 Nov 2017
My code works fine with '==' but not with '~='.I expect it not to display 'error' if user enters A or B
x=input('input x','s')
if(x~='A')||(x~='B')
disp('error');
end
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John D'Errico il 21 Feb 2017
Modificato: John D'Errico il 21 Feb 2017
A problem of elementary logic?
You want an error to return only if A is not in the set {'A','B'}. So a call to ismember might be a good alternative.
Regardless, given the approach you have followed, if x is equal to 'A', then the second half of the clause will be true, even though the first part of the clause is false. And the logical statement
false || true
is TRUE.
You are asking for a result that is only true when BOTH parts of the clause are true. Use a logical and, NOT a logical or.
if(x~='A') && (x~='B')
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Jan il 21 Feb 2017
Remember, that the negation of
(x=='A') || (x=='B')
is:
~((x == 'A') || (x == 'B')) ==>
~(x == 'A') && ~(x == 'B') ==>
(x ~= 'A') && (x ~= 'B')
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Zhuoying Lin il 28 Nov 2017
Modificato: Walter Roberson il 28 Nov 2017
Hi I have a similar question:
when I type:
if (x ~= 'a') && (x ~= 'p') && (x ~= 'T')
fprintf('ERROR:You entered incorrect choice.')
end
but it also shows that:
Operands to the || and && operators must be convertible to
logical scalar values.
Error in term (line 4)
if (x ~= 'a') && (x ~= 'p') && (x ~= 'T')
Walter Roberson il 28 Nov 2017
if any((x ~= 'a') & (x ~= 'p') & (x ~= 'T'))
fprintf('ERROR:You entered incorrect choice.')
end
or
if ~all( ismember(x, {'a', 'p', 'T'}) )
fprintf('ERROR:You entered incorrect choice.')
end

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Torsten il 21 Feb 2017
Modificato: Torsten il 21 Feb 2017
If user enters A, then x~=B is true, so (x~='A')||(x~='B') is true, thus "error" is displayed.
Same for B.
Best wishes
Torsten.
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