Re: Working with arrays

• To: mathgroup at smc.vnet.net
• Subject: [mg131208] Re: Working with arrays
• From: amannucci <Anthony.J.Mannucci at jpl.nasa.gov>
• Date: Mon, 17 Jun 2013 06:28:52 -0400 (EDT)
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• References: <kpjvhm\$9n8\$1@smc.vnet.net>

```On Sunday, June 16, 2013 2:12:22 AM UTC-7, amannucci wrote:
> I thought I understood variables. This sequence completely mystifies me:
>
> Clear[lA, xtest]
>
> lA = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
>
> xtest = {4, 5, 6};
>
> lA\[CenterDot]xtest (* First case *)
>
> {{a, b}, {c, d}} . {x, y} (* Second case *)
>
>
>
> The output is:
>
>
>
> {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}\[CenterDot]{4, 5, 6}
>
>
>
> {a x + b y, c x + d y}
>
>
>
> How do I force matrix multiplication to actually occur, as in the second answer? Why does Mathematica do the matrix multiply in the second case but not the first?
>
>
>
> Thanks for any help.
>
>
>
> -Tony

I appreciate the replies I have received. Yes, CenterDot is the wrong operator! I am a little rusty here. I appreciate the helpful comments.

```

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