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MathGroup Archive 1998

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Re: strange behaviour

Jean-Marie THOMAS wrote:
> MatrixQ has a strange behaviour:
> ?MatrixQ
> "MatrixQ[expr] gives True if expr is a list of lists that can represent
> a \ matrix, and gives False otherwise. MatrixQ[expr, test] gives True
> only if \ test yields True when applied to each of the matrix elements
> in expr."
> MatrixQ[{{},{}}] gives True because its argument is a list of lists of
> equal length. No problem.
> Now:
> NumberQ[{}] gives False because {} is not a number. I can understand
> that.
> But:
> MatrixQ[{{},{}},NumberQ] gives True, and this seems to me in
> contradiction with the definition of MatrixQ[expr, test]
> Can somebody explain this to my stupid mind?
> -----------------------------------------------
> Jean-Marie THOMAS
> Conseil et Audit en Ingenierie de Calcul
> +33 (0)3 88 32 93 64
> =======================


Perhaps a rewording:

"For all e if e is an entry in {{},{}} then NumberQ[e] gives True." 

Since a implies b is false iff a is true and b is false, this is true.


" There is no entry, e, in {{},{}} for which NumberQ[e] give False"

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