# channelflow.org

### Site Tools

gibson:teaching:spring-2018:math445:surfer
surfer.m
```function [U,G] = surfer(root,n)
% SURFER  Create the adjacency graph of a portion of the Web.
%    [U,G] = surfer(root,n) starts at the URL root and follows
%    Web links until it forms an adjacency graph with n nodes.
%    U = a cell array of n strings, the URLs of the nodes.
%    G = an n-by-n sparse matrix with G(i,j)=1 if node j is linked to node i.
%
%    Example:  [U,G] = surfer('http://www.harvard.edu',500);
%
%    This function currently has two defects.  (1) The algorithm for
%    finding links is naive.  We just look for the string 'http:'.
%    (2) An attempt to read from a URL that is accessible, but very slow,
%    might take an unacceptably long time to complete.  In some cases,
%    it may be necessary to have the operating system terminate MATLAB.
%    Key words from such URLs can be added to the skip list in surfer.m.

%   Copyright 2014 Cleve Moler
%   Copyright 2014 The MathWorks, Inc.

% Initialize

clf
shg
axis([0 n 0 n])
axis square
axis ij
box on
set(gca,'position',[.12 .20 .78 .78])
uicontrol('style','frame','units','normal','position',[.01 .09 .98 .07]);
uicontrol('style','frame','units','normal','position',[.01 .01 .98 .07]);
t1 = uicontrol('style','text','units','normal','position',[.02 .10 .94 .04], ...
'horiz','left');
t2 = uicontrol('style','text','units','normal','position',[.02 .02 .94 .04], ...
'horiz','left');
slow = uicontrol('style','toggle','units','normal', ...
'position',[.01 .24 .07 .05],'string','slow','value',0);
quit = uicontrol('style','toggle','units','normal', ...
'position',[.01 .17 .07 .05],'string','quit','value',0);

U = cell(n,1);
hash = zeros(n,1);
G = logical(sparse(n,n));
m = 1;
U{m} = root;
hash(m) = hashfun(root);

j = 1;
while j < n & get(quit,'value') == 0

% Try to open a page.

try
set(t1,'string',sprintf('%5d %s',j,U{j}))
set(t2,'string','');
drawnow
catch
set(t1,'string',sprintf('fail: %5d %s',j,U{j}))
drawnow
j = j+1;
continue
end
if get(slow,'value')
pause(.25)
end

% Follow the links from the open page.

for f = findstr('http:',page);

% A link starts with 'http:' and ends with the next quote.

e = min([findstr('"',page(f:end)) findstr('''',page(f:end))]);
if isempty(e), continue, end
url = deblank(page(f:f+e-2));
url(url<' ') = '!';   % Nonprintable characters
if url(end) == '/', url(end) = []; end

% Look for links that should be skipped.

skips = {'.gif','.jpg','.jpeg','.pdf','.css','.asp','.mwc','.ram', ...
'scripts','netscape','shockwave','webex','fansonly'};
skip = any(url=='!') | any(url=='?');
k = 0;
while ~skip & (k < length(skips))
k = k+1;
skip = ~isempty(findstr(url,skips{k}));
end
if skip
if isempty(findstr(url,'.gif')) & isempty(findstr(url,'.jpg'))
set(t2,'string',sprintf('skip: %s',url))
drawnow
if get(slow,'value')
pause(.25)
end
end
continue
end

% Check if page is already in url list.

i = 0;
for k = find(hash(1:m) == hashfun(url))';
if isequal(U{k},url)
i = k;
break
end
end

% Add a new url to the graph there if are fewer than n.

if (i == 0) & (m < n)
m = m+1;
U{m} = url;
hash(m) = hashfun(url);
i = m;
end

if i > 0
G(i,j) = 1;
set(t2,'string',sprintf('%5d %s',i,url))
line(j,i,'marker','.','markersize',6)
drawnow
if get(slow,'value')
pause(.25)
end
end
end

j = j+1;
end
delete(t1)
delete(t2)
delete(slow)
set(quit,'string','close','callback','close(gcf)','value',0)

%------------------------

function h = hashfun(url)
% Almost unique numeric hash code for pages already visited.
h = length(url) + 1024*sum(url);```
gibson/teaching/spring-2018/math445/surfer.txt · Last modified: 2018/03/21 09:17 by gibson