Representing Logical true and False:

It has been stated that expressions which are theoretically true really have the integer value of 1, and expressions which are theoretically false really have the integer value of 0.Exhibiting the concepts of logical true and false in the MATLAB is somewhat different: the concept of false is presented by the integer value of 0, but the concept of true can be presented by any non-zero value (not merely the integer 1). This can lead to a few strange Boolean expressions. For illustration, consider the following if statement:

>> if 5

disp('Yes, this is true!')

end

Yes, this is true!

As 5 is a nonzero value, it is a way of telling true. And hence, when this Boolean expression is computed, it will be true, therefore the disp function will be executed and "Yes, this is true" is exhibited. Obviously, this is a pretty bizarre if the statement, one that hopefully would not ever be encountered!

Though, a simple mistake in an expression can lead to this type of answer. For illustration, let's state that the user is prompted for the choice of Y or N for a yes/no question:

letter = input('Choice (Y/N): ','s');

In the script we might want to execute a specific action if the user responded with 'Y'. Nearly all scripts would permit the user to enter either lower- or uppercase (example, either 'y' or 'Y') to specify yes. The appropriate expression that would return true if the value of letter was 'y' or 'Y' would be

letter == 'y' || letter == 'Y'

Though, if by fault this was written as:

letter == 'y' || 'Y'

this expression would  forever be true, regardless of the value of the variable letter. This is as 'Y' is a nonzero value; therefore it is a true expression. The first section of the expression might be false, but as the second expression is true the whole expression would be true.

Posted Date: 10/19/2012 3:17:47 AM | Location : United States

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