www.pudn.com > NumericalComputingwithMatlabCode.zip > threenplus1.m, change:2001-12-31,size:1118b

```function threenplus1(n)
%"Three n plus 1".
%   Study the 3n+1 sequence.
%   threenplus1(n) plots the sequence starting with n.
%   threenplus1 with no arguments starts with n = 1.
%   uicontrols decrement or increment the starting n.
%   Is it possible for this to run forever?

if ~isequal(get(gcf,'tag'),'3n+1')
shg
clf reset
uicontrol( ...
'position',[260 5 25 22], ...
'string','<', ...
'callback','threenplus1(''<'')');
uicontrol( ...
'position',[300 5 25 22], ...
'string','>', ...
'callback','threenplus1(''>'')');
set(gcf,'tag','3n+1');
end

if nargin == 0
n = 1;
elseif isequal(n,'<')
n = get(gcf,'userdata') - 1;
elseif isequal(n,'>')
n = get(gcf,'userdata') + 1;
end
if n < 1, n = 1; end
set(gcf,'userdata',n)

y = n;
while n > 1
if rem(n,2)==0
n = n/2;
else
n = 3*n+1;
end
y = [y n];
end

semilogy(y,'.-')
axis tight
ymax = max(y);
ytick = [2.^(0:ceil(log2(ymax))-1) ymax];
if length(ytick) > 8, ytick(end-1) = []; end
set(gca,'ytick',ytick)
title(['n = ' num2str(y(1))]);
```