Re: put some elements to zero

• To: mathgroup at smc.vnet.net
• Subject: [mg101481] Re: put some elements to zero
• From: pfalloon <pfalloon at gmail.com>
• Date: Wed, 8 Jul 2009 07:12:10 -0400 (EDT)
• References: <h2vcj8\$qq7\$1@smc.vnet.net>

```On Jul 7, 9:46 pm, Luigi B <L.Balz... at gmail.com> wrote:
> Dear All,
>  I have a matrix with only two columns and many rows. For instance
> {xi,yi}.
> For those xi<x0, I would like to set yi=0.
> I can only think of a very inefficient loop...
>
> thanks
> -Luigi-

One solution is the following:

x0=0;
pairs = RandomReal[{-1,1}, {10000,2}];
pairs = Transpose[pairs];
pairs = Transpose[{pairs[[1]], UnitStep[pairs[[1]]-x0]*pairs
[[2]]}];

The key is to operate on the entire list rather than individual rows.

Cheers,
Peter.

```

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