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MathGroup Archive 1992

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Re: Table[ ...If[...i...] ,{i,...}]

  • To: mathgroup-adm at yoda.physics.unc.edu
  • Subject: Re: Table[ ...If[...i...] ,{i,...}]
  • From: David Sibley <sibley at math.psu.edu>
  • Date: Fri, 17 Jul 92 15:18:51 -0400

The original problem was with

     Table[ v[i]'[t] == If[ x[i]>0,ex1,ex2] , {i,2,6} ]

I fooled with this a bit and think I see what's going on.  Apparently
mma does indeed substitute correctly for all the i's, but when it
discovers that the If evaluates to neither True nor False, sticks back
in the form of the If with i's in it rather than numbers.  Thus, this
seems to be a bug.  It occurs in an anomolous situation, to be sure,
but it is certainly not the expected behavior.

To see how I reached the above conclusion, try:

    Table[ If[ x[i], i, -i ], {i,2,6} ]
    Table[ If[ x[i], i, -i , i^2 ], {i,2,6} ]

and

    x[3] = True
    Table[ If[ x[i], i, -i ], {i,2,6} ]

Finally, as a workaround the following seems OK, though Lord knows why.

    Table[ If[ x[t], t, -t ]/.t->i, {i,2,6} ]

David Sibley
sibley at math.psu.edu






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