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Re: Help with While Loop Function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg115948] Re: Help with While Loop Function
  • From: DrMajorBob <btreat1 at austin.rr.com>
  • Date: Thu, 27 Jan 2011 03:39:48 -0500 (EST)

Clearall should be ClearAll, but you probably need only Clear.

You cleared x, y, t, w -- which don't need clearing -- and didn't Clear f,  
which might.

You have several parentheses () where you need brackets [] in the  
definition of f.

You have List1 most places, but list1 (no capital) is the first argument  
to Append.

Append is slow; there are better ways. One is using list1 = {list1, t},  
then list1 = Flatten@list1 after the While is complete. Better yet, use  
Sow and Reap.

You have If[ f[t, y] = 0, (other arguments)] where "=" is an assignment,  
not a test for equality. Ditto for the other If.

You have f[t_, y_] == (whatever), which does not set f equal to anything;  
it tests for equality.

You don't need Floor to find out if something is a perfect square. Just  
test whether Sqrt gives an Integer.

In the text, you mentioned testing if "t*((y^2+1)/2)^2 + y^2 is a perfect  
square" is a perfect square, but that's not what you're testing. Instead,  
you're testing (t/4) (y^2 - 1)^2 + y^2.

w is the number of items added to the list, but that would be Length@list1  
- 2.

If[w == 3, t = limit, x++] is intended to exit the loop early while you're  
debugging? But you could use Break, making the code more clear (although I  
don't approve of using Break, Return, GoTo, and all that). Better yet, use  
a reasonable limit on t.

If I've guessed right what you actually meant (not likely), the result is  
this:

ClearAll[x, y, t, w, f]
x = 2; y = 3; t = 1; w = 0;
f[t_, y_] := IntegerQ@Sqrt [(t/4) (y^2 - 1)^2 + y^2]
limit = 100000000000; (* 100 billion *)
Timing[
  list1 = Join[
    {0, 1},
    First@Last@Reap@While[
        t < limit,
        t += x;
        f[t, y] && (Sow@t; w++);
        x++]
    ]
  ]

{71.5941, {0, 1, 10, 45, 351, 1540, 11935, 52326, 405450, 1777555,
   13773376, 60384555, 467889345, 2051297326, 15894464365,
   69683724540}}

That counts how many are added, but it doesn't Break. If you want the  
break, it could look like

w == 3 && Break[];

as a new line before x++.

Here's a longer run, not incrementing w:

ClearAll[x, y, t, w, f]
x = 2; y = 3; t = 1; w = 0;
f[t_, y_] := IntegerQ@Sqrt [(t/4) (y^2 - 1)^2 + y^2]
limit = 1000000000000;
Timing[
  list1 = Join[
    {0, 1},
    First@Last@Reap@While[
        t < limit,
        t += x;
        f[t, y] && Sow@t;
        x++]
    ]
  ]

{294.936, {0, 1, 10, 45, 351, 1540, 11935, 52326, 405450, 1777555,
   13773376, 60384555, 467889345, 2051297326, 15894464365, 69683724540,
    539943899076}}

Solve[{n (n + 1)/2 == #, n > 0}, n, Integers] & /@ list1

{{}, {{n -> 1}}, {{n -> 4}}, {{n -> 9}}, {{n -> 26}}, {{n ->
     55}}, {{n -> 154}}, {{n -> 323}}, {{n -> 900}}, {{n ->
     1885}}, {{n -> 5248}}, {{n -> 10989}}, {{n -> 30590}}, {{n ->
     64051}}, {{n -> 178294}}, {{n -> 373319}}, {{n -> 1039176}}}

0 is not.

Bobby

On Wed, 26 Jan 2011 04:04:26 -0600, KenR <ramsey2879 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
>


-- 
DrMajorBob at yahoo.com


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