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MathGroup Archive 1995

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Re: LISTS and more LISTS

  • To: mathgroup at smc.vnet.net
  • Subject: [mg2840] Re: LISTS and more LISTS
  • From: wagner at bullwinkle.cs.Colorado.EDU (Dave Wagner)
  • Date: Sat, 23 Dec 1995 03:19:58 -0500
  • Organization: University of Colorado, Boulder

In article <4bancs$cqu at dragonfly.wri.com>,
cjkelly at bu.edu <cjkelly at bu.edu> wrote:
>Enclosed is a little ditty I've been trying to get to work but as is
>clear, it has a bug and I have no clue as to what is wrong. 
>
>It is just supposed to extract elements out of a LIST then peform 
>various operations on these elements one of which is to plot a
>series of lines (or rays) starting from the point {25,5}
>
>
>
>Clear [f,x]
>x={10,8,6,5.5,5.4,5.3,5.2,5.1,5.01,5.001,5.0001,5.0001}
>f[x_]=x^2
>Do [Line[{{5,25},{x[[i]],f[x[[i]]]}},{i,Length [x]}]]
>

You had a number of problems.  The first one is,

    Clear[f]
    f[x_] := x^2

Using "=" instead of ":=" caused f[x_] to be defined as {10,8,6,...}^2,
which isn't what you wanted.

Next, your square brackets weren't balanced correctly.  To identity
problems like this, see if your version of the front end has a
"Balance" command in one of its submenus (on the Mac, it's in the
"Nesting" submenu of the "Edit" menu).

    Do[Line[{{5,25},{x[[i]],f[x[[i]]]}}],{i,Length[x]}]
                                       ^ this got moved

Finally, when you get those problems fixed, you don't get any output
from Do.  To get output, use Table:

    Table[Line[{{5,25},{x[[i]],f[x[[i]]]}}],{i,Length[x]}]

    {Line[{{5, 25}, {10, 100}}], Line[{{5, 25}, {8, 64}}], Line[{{5, 25},
    {6, 36}}], Line[{{5, 25}, {5.5, 30.25}}], Line[{{5, 25}, {5.4,
    29.16}}], Line[{{5, 25}, {5.3, 28.09}}], Line[{{5, 25}, {5.2, 27.04}}],
    Line[{{5, 25}, {5.1, 26.01}}], Line[{{5, 25}, {5.01, 25.1001}}],
    Line[{{5, 25}, {5.001, 25.01}}], Line[{{5, 25}, {5.0001, 25.001}}],
    Line[{{5, 25}, {5.0001, 25.001}}]}

You can plot the result using

    Show[Graphics[%]]


		Dave Wagner
		Principia Consulting
		(303) 786-8371
		dbwagner at princon.com
		http://www.princon.com/princon



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