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While or not to While that is the question :)

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  • Subject: [mg46839] While or not to While that is the question :)
  • From: János <janos.lobb at>
  • Date: Tue, 9 Mar 2004 19:57:58 -0500 (EST)
  • Sender: owner-wri-mathgroup at


Inside a While loop I slide on a nested list like 
{a,{g,c},{t,a},c,g,a,{g,c},{a,t},c,,g,a,t,t}.  Let's say I start after 
{t,a} and {a,t} parallel - positions 4 and 9, that is element c and c.  
If I find a single element then I pair it with its corresponding DNA 
complement, a->t, t->a, c->g, g->c and replace the element with a 
sublist of the element and its complement.  So, after the first step my 
list will look like 
{a,{g,c},{t,a},{c,g},g,a,{g,c},{a,t},{c,g},g,a,t,t}.  If I find a 
double, like the {g,c} on position 7, then first I create a triplet 
from it {g,c,c} - the original c in the double is now on the third 
position inside the triplet -, and then after some housekeeping I take 
the third element of the triplet off and put it into another bucket, so 
the triplet becomes a double again and in the While I slide happily 
further right on the nested list.   In the above example I would hit 
{g,c} after 4 steps.  If I run the While, let say for 5 or more steps, 
it errors out complaining that I want to select some parts of the empty 
{} list.  If I ran it for 4 steps and then I manually execute ALL the 
statements inside the While for step 5, I do not see any complains and 
I can go up to as many steps manually, by increasing the loop variable, 
as my heart desires.

As being a newbie I have not learned yet to use Modules or Blocks , - I 
am still just at page 320 in the book - so all my variables are 

So my first question is why I am getting different results if I execute 
the while programatically, for 5 steps, compared to executing it for 4 
steps and do the 5th step manually?  I am using 5.0.1 on OS X.

The first error message I get is this:
   RowBox[{\(ReplacePart::"partw"\), \(\(:\)\(\ \)\), "\<\"Part 
\\!\\({1, 2}\\)
      of \\!\\({}\\) does not
       exist. \\!\\(\\*ButtonBox[\\\"More?\\\", \
ButtonStyle->\\\"RefGuideLinkText\\\", ButtonFrame->None, \

So, my second question is how to use Trace to determine where in the 
code do I get the first  "partw" ?  I use ReplacePart in 4 places 
inside the While, but at manual execution none of them complain.

I am sure my code is far from perfect, but still it should give the 
same result on both cases.


clear perl code is better than unclear awk code; but NOTHING comes 
close to unclear perl code

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