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In most OO languages, where variables may point to objects, they may also have a null value, which is highly convenient.
In Matlab, I have a function which parses a command, and then returns a cell array, or false (which is equal to zero — which is another common pattern) if it fails:
function re = parse(s)
...
if (invalid)
re = false;
return;
end
end
The problem is that when I check the result, it gives an error:
re = parse(s);
if (false == re)
Undefined function 'eq' for input arguments of type 'cell'.
I've written a function to check it without an error: strcmp('logical', class(re)) && false == re, but that seems to be really slow for use in hot areas of the code, and also inconvenient if I have to add this function to every M file I'm writing.
Using NaN is even worse, because besides throwing that error, it also isn't equal to itself.
What's a better alternative for use with this pattern?
550
Use the null function to calculate orthonormal and rational basis vectors for the null space of a matrix. The null space of a matrix contains vectors x that satisfy Ax = 0 .
NULL and nil are equal to each other, but nil is an object value while NULL is a generic pointer value ((void*)0, to be specific). [NSNull null] is an object that's meant to stand in for nil in situations where nil isn't allowed. For example, you can't have a nil value in an NSArray.
null (MATLAB Functions) Z = null(A) is an orthonormal basis for the null space of A obtained from the singular value decomposition. That is, A*Z has negligible elements, size(Z,2) is the nullity of A , and Z'*Z = I .
Let's start out with “Nil” since it's the most common and easy-to-understand way of representing nothingness in Ruby. In terms of what it means, Nil is exactly the same thing as null in other languages.
You can use the isequal function to compare any two items without causing that error. For example:
if isequal (re, false)
%code here
end
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