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Re: Help with While Loop Function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg115954] Re: Help with While Loop Function
  • From: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>
  • Date: Thu, 27 Jan 2011 03:40:54 -0500 (EST)
  • References: <ihorj6$hfh$1@smc.vnet.net>

Hi,

I am afraid you have created a long list of errors. May I suggest you
try to read somewhat more documentation before going on? Anyways,
here's the list with errors and tips and a working version:

1. Clearall should be ClearAll (capital A)
2. you might consider suppressing output on the next 5 lines using ;
3. "f[t_,y_] ==" should be "f[t_,y_] =". Single = is assignment
(function Set[ ]), double = (==) is equality test (function Equal[])
4. functions need square brackets to hold their arguments, not round
parenthesis (Floor[], Sqrt[])
5. Use := (SetDelayed) instead of = (Set) in the definition of f. = o=
r
Set uses the values of your variables at the time of the function
definition if they have a value. This is not what you want here.
6. "If[f[t,y] = 0" must be "If[f[t,y] == 0"; a Set/Equal confusion
7. "List1 = Append[list1,t]" is OK, but you could use the more concise
"AppendTo[list1,t]" instead. Both are relatively slow, so for long
list other constructions would be better.
8. "If [w = 3" should be "If [w == 3"; a Set/Equal confusion
9. "t = 1000000000000" is used to get out of the loop. Break[] is
invented to do just this in a more elegant way. (It's ugly
nevertheless)

So, we end up with:

In[108]:= ClearAll[x, y, t, w]
x = 2;
y = 3;
t = 1;
w = 0;
List1 = {0, 1};
f[t_, y_] := (Floor[
      Sqrt[(t/4) (y^2 - 1)^2 + y^2]])^2 - ((t/4) (y^2 - 1)^2 + y^2);
While[t < 1000000000000, t = t + x;
  If[f[t, y] == 0, List1 = Append[List1, t]; w = w + 1];
  If[w == 3, Break[], x++]];
List1

Out[116]= {0, 1, 10, 45, 351}

Cheers -- Sjoerd

On Jan 26, 11:04 am, KenR <ramsey2... at msn.com> wrote:
> Please Help me to Get the following to work as desired
> t is the triangular number. I increment it repeated ly by adding the
> counter x to it. When t*((y^2+1)/2)^2 + y^2 is a perfect square i want
> to append t to my list of trangular numbers with the list starting
> with {0,1}, and exit the loop when 3 more triangular numbers have been
> added or when t >= 1000000000000. It is not working for me. I am new
> to Mathematica. Thanks
>
> Clearall[x,y,t,w]
> x = 2
> y = 3
> t = 1
> w = 0
> List1 = {0,1}
> f[t_,y_] ==(Floor(Sqrt ((t/4) (y^2-1)^2 + y^2)))^2 - ((t/4) (y^2-1)^2
> + y^2)
> While[t<1000000000000,t = t+x;If[f[t,y] = 0, List1 = Append[list1,t=
];w
> = w +1];
> If [w = 3,t = 1000000000000, x++]];
> List1



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