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MathGroup Archive 2004

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Re: What's wrong with this small code?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50051] Re: [mg49928] What's wrong with this small code?
  • From: Omega Consulting <info at omegaconsultinggroup.com>
  • Date: Fri, 13 Aug 2004 05:55:47 -0400 (EDT)
  • References: <200408060709.DAA21469@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On Aug 6, 2004, at 2:09 AM, Justin wrote:

> I made a 4*4 IdentityMatrix, named "test". Then, I wanna show the
> diagnol "1" in RED. After running the following code, I got the red
> diagnol opposite to the "1" diagnol. What's wrong with this code? Can
> anybody find it out?
>
> Thanks.
>
> test = IdentityMatrix[4];
> {numRows, numColumns} = Dimensions[test];
> Show[
>   Graphics[
>     RasterArray[
>       Table[
>         If[
>           test[[i, j]] > 0,
>           RGBColor[1, 0, 0],
>           RGBColor[0, 1, 0]
>           ],
>         {i, 1, numRows}, {j, 1, numColumns}
>         ]
>       ]
>     ]
>   ]

Raster and RasterArray are upside-down compared to most raster formats. 
That is, the first row in the matrix is the bottom row. The row number 
is supposed to match the y-axis of the coordinate system (which is 0 at 
the bottom and increases upward). The following does what you want:

test = Reverse[IdentityMatrix[4]];
{numRows, numColumns} = Dimensions[test];
Show[
   Graphics[
     RasterArray[
       Table[
         If[
           test[[i, j]] > 0,
           RGBColor[1, 0, 0],
           RGBColor[0, 1, 0]
           ],
         {i, 1, numRows}, {j, 1, numColumns}
         ]
       ]
     ]
   ]

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