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Re: Finding and changing strings in a matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg106786] Re: Finding and changing strings in a matrix
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Sat, 23 Jan 2010 07:34:47 -0500 (EST)
  • References: <hjbv9l$2qj$1@smc.vnet.net>

On 2010.01.22. 11:39, Jagra wrote:
> Hi everyone!
>
> I have a matrix with {500, 20} dimensions.  Some random positions in
> each column have text strings in them instead of numbers.  While it
> can happen, the text strings do not typically appear across an entire
> row of data.
>
> I need to find the text strings and replace them by the values
> immediately above them.  In the case where the matrix has consecutive
> text strings I need to replace them with the first number above the
> run of strings.
>
> Here's a part of the data file so you get the idea of what I start
> with:
>
> myData ={{5217.61, 10163.8, 5844.44}, {5196.81, 10142.1, 5831.21},
> {5293.99,
>    10183.5, 5930.53}, {5328.66, 10378., 5945.69}, {5372.38, "-",
>    5957.44}, {5402.41, 10536.9, "-"}, {"-", 10494.7, "-"}, {"-",
>    10634.2, 6002.92}, {5437.61, 10638.1, 6011.55}, {5397.86, 10546.4,
>    5957.43}, {5412.88, "-", "-"}};
>
> myData // MatrixForm
>
> I want to get rid of all "-"s and replace them with the numbers most
> directly above them.
>
> I guess I could do this with loops, but that seems like a waste of
> Mathematica's power.
>
> I've tried using Rest[myData] and Most[myData] then using Position to
> identify where one of the matrices had strings and the other didn't
> then using that to replace the strings in the original data.  But I
> run into a snag when I have consecutive strings.
>

This is a rather inefficient but simple way to do it:

Transpose[
  Transpose[myData] //. {s___, x_?NumericQ, "-", e___} :> {s, x, x, e}
]

It'll work fine unless you matrices are very big.

Hope this helps,
Szabolcs


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