       RE: checking for a particular value in a matrix

• To: mathgroup at smc.vnet.net
• Subject: [mg31467] RE: [mg30933] checking for a particular value in a matrix
• From: Au Han Bin <auhb3 at cz3.nus.edu.sg>
• Date: Wed, 7 Nov 2001 05:29:58 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```does anyone understand what does Flatten does? the help file just tells me
that it flattens. somehoww, it does not actually define what flatten does?

regards, joshua_au at hotmail.com

On Thu, 27 Sep 2001, David Park wrote:

> This code will check that there is exactly one entry in a matrix of value 1.
>
> ExactlyOneOne[(mat_)?MatrixQ] :=
>   Count[mat, 1, Infinity] == 1
>
> testmat1 = {{1, 0}, {2, 3}};
> testmat2 = {{1, 0}, {2, 1}};
> testmat3 = {{-1, 0}, {2, 3}};
>
> ExactlyOneOne /@ {testmat1, testmat2, testmat3}
> {True, False, False}
>
> As for your second question, use a DelayedSet (:=). When you use Set (=)
> Mathematica calculates the right hand side of the definition immediately -
> even though the symbol a appears on the left hand side. This is a confusing
> aspect of Mathematica definitions that trips up many users. I don't know why
> Mathematica can't check to see if the symbol appears on the lhs. There is
> probably some logic to it.
>
> David Park
> djmp at earthlink.net
>
>
> > From: Au Han Bin [mailto:auhb3 at cz3.nus.edu.sg]
To: mathgroup at smc.vnet.net
> >
> > Hi, I am a programming newbie and I need advice on how to check that a
> > table or matrix , for all nonzero entries in a given row, has only one
> > entry of value 1.
> >
> >
> >
> > i have another query on why the code below does not work, when i call
> > convert{1,0,0,0} again, the results given are from an earlier question,
> > e.g. {1,1} is it initialised wrongly?
> >
> >
> > Clear[a,convert];
> > convert[a___] = Sum[a[[i]]*2^(Length[a]-i),{i,1,Length[a]}]
> >
> > regards, thanks,
> >
> > joshua
> >
> >
>
>

```

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