       Re: Thread::tdlen: Objects of unequal length

• To: mathgroup at smc.vnet.net
• Subject: [mg121887] Re: Thread::tdlen: Objects of unequal length
• From: Sseziwa Mukasa <mukasa at gmail.com>
• Date: Thu, 6 Oct 2011 04:19:42 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <201110050804.EAA07154@smc.vnet.net>

```The operator for matrix multiplication is . not *, * is for element by element multiplication.

Incidentally, . actually does tensor multiplication (read the help) so the braces are unnecessary you can do:

{x1,x2,x4,x4}.{{...}}.{x1,x2,x3,x4}

which will result in a scalar, unless you really want a two dimensional 1x1 result in which case s must be two dimensional.

Regards,
Sseziwa

On Oct 5, 2011, at 4:04 AM, SysInv wrote:

> This problem is driving me crazy. I'm trying to multiply matrices, where one is 1x4 (s) and the other is 4x4 (t) and the multiplication is s*t*Transpose[s]
>
> Despite this I get the error message that the objects is of unequal length. The matrices are:
>
> s={{x1,x2,x3,x4}}
>
> =
t={0.0284435,0.00395759,0.000211963,0.0357403},{0.00395759,0.0113862,-0.000199939,-0.000556137},{0.000211963,-0.000199939,0.00118147,-0.00043913},{
> 0.0357403,-0.000556137,-0.00043913,0.0649449}
>
> I must use double level in the 1 row matrix, since otherwise Transpose[] complains that it needs at least 2 levels. I tried without this as well, but I keep getting:
>
> Thread::tdlen : Objects of unequal length in {{x1,x2,x3,x4}}{{0.0284435,0.00395759,0.000211963,0.0357403},<<2>>,{<<10>>,<<3>>}}{{x1},{x2},{x3},{x4}} cannot be combined. >>
>
> . when I run s*t*Transpose[s].
>
> I've spent hours trying to figure this simple problem out, but without any luck. Any pointers guys? It doesn't help if I change the order of the transpose...
>

```

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