NaN error

[John W. Eaton]

On 11-Jan-2011, Los Griegos Engineering wrote:

| Hello all,
| 
| I came across the 196 algorithm at:
| http://mathworld.wolfram.com/196-Algorithm.html
| 
| I decided to code a version of it, and it works... until I increase the
| iterations past a limit. I think the error is in the memory, but I'm
| just not sure where the error is.
| 
| function [ a ] = rev (a,iter)
| 
| 
| for k=1:iter
| 
| a=num2str(a);
| b=[];
| 
| for i=length(a):-1:1
| j=length(a)-i+1;
| b(j)=a(i);
| endfor
| 
| b=str2double(b);
| a=str2double(a);
| 
| a=b+a;
| 
| endfor
| endfunction
| 
| The results I get are:
| octave-3.2.3> rev(123,1)
| ans =  444
| octave-3.2.3> rev(123,10)
| ans =  1354353
| octave-3.2.3> rev(123,11)
| ans = NaN
| 
| octave-3.2.3> rev(12,11)
| ans =  175857
| octave-3.2.3> rev(12,14)
| ans = NaN
| octave-3.2.3> 
| 
| Is the error due to the memory address of the character array?
| GNU Octave, version 3.2.3
| Octave was configured for "i486-pc-linux-gnu".

You can write your function as

  function a = rev (a, iter)
    for k = 1:iter
      a = num2str (a);
      b = fliplr (a);
      b = str2double (b);
      a = str2double (a);
      a = b + a;
    endfor
  endfunction

to avoid the inner loop.  If I modify it as follows to see what the
arguments to str2double are, and what the result of the call to
str2double is

  function a = rev (a, iter)
    for k = 1:iter
      a = num2str (a);
      b = fliplr (a);
      bs = char (b)
      b = str2double (b)
      as = char (a)
      a = str2double (a)
      a = b + a;
    endfor
  endfunction

I see

  octave> rev (123, 11)
  bs = 627627
  b =  627627
  as = 726726
  a =  726726
  bs = 60+e53453.1
  b = NaN
  as = 1.35435e+06
  a =  1354350
  ans = NaN

so the problem appears to bee that num2str starts returning numbers
formatted as floating point constants, and when the characters are
reversed you get something like "60+e53453.1" which is not a number.

Octave doesn't have arbitrary precision integers, so using built-in
data types here, you will hit a limit at some point no matter what you
do.

jwe