       Re: matrix problem

• To: mathgroup at smc.vnet.net
• Subject: [mg107114] Re: matrix problem
• From: Peter Pein <petsie at dordos.net>
• Date: Wed, 3 Feb 2010 06:09:38 -0500 (EST)
• References: <hk93ca\$f68\$1@smc.vnet.net>

```Am 02.02.2010 12:47, schrieb Piotr Munik:
> Dear Math Group,
>
> I have matrix M and matrix N
> for example
> M={{1,2},{3,4},{5,6},{7,8}}
> N={{1,8},{5,0},{9,7}}
> (dimension is diffrent)
> I need to make new matrixes which include only this rows from matrix M and
> matrix N for which firsts elements are the same in both matrix (M and N).
> so I need
> M1={{1,2},{5,6}}
> N1={{1,8},{5,0}}
>  Elements of matrix are real (not always integer)
> Peter
>
>

Hi Peter,

1.) do not use reserved function names ("N" in this case) as variables;

2.) In:= mM={{1,2},{3,4},{5,6},{7,8}}; mN={{1,8},{5,0},{9,7}};

The following call to Pick[] works on both matrices at once:

In:= {mM1,mN1}=
Pick[#,Map[First,#,{2}],
Alternatives@@Intersection@@([[All,All,1]])]&[{mM,mN}]
Out= {{{1,2},{5,6}},{{1,8},{5,0}}}

Greetings,
Peter

```

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