function s=prompt(str,options,deflt_option)



% s=prompt(str,options[,deflt_option]);

%

% Prompts user for input of ONE character from options string.  It uses only

% the first character of the user's response, converted to lower case, and

% returns this character which will then match at least one of the characters

% in the options string.  Each character in the options string is an acceptable

% choice.  If the user types a character not in the options string, the

% function displays a message and prompts again.

%

% INPUTs:

%

%   str          = the prompt string (the list of options and a colon are

%                  tacked on at the end)

%   options      = a character string of options 

%   deflt_option = one character from options which is the default selection

%                  when <CR> is the response to the prompt

%                  (OPTIONAL ... if not present, no default option is assumed;

%                  the <CR> response to the prompt returns an empty matrix)

%

% OUTPUT:

%

%   s            = the character chosen by the user which has been converted to

%                  lower case, is guaranteed to be only one character in

%                  length, and matches at least one of the characters in the

%                  options string

%

% EXAMPLE:

%

% ? opts=['yn'];

% ? s=prompt('  Plot this again?',opts);

%

%   Plot this again? [y/n]: 

%

%   (now the user may only enter 'y', or 'Y', or 'n', or 'N')



n=length(options);

if (n<=1)

  error('  < 2 options makes no sense')

end

options=lower(options);

if (~exist('deflt_option'))

  no_default=1;

else

  deflt_option=lower(deflt_option(1));

  id=find(deflt_option==options);

  if (length(id)==0)

    error('  "default_option" must be one of the provided "options"')

  end

  no_default=0;

end



opts=' (';

for j=1:(n-1)

  opts=[opts options(j) '/'];

end

opts=[opts options(n) ')'];

if (no_default)

  opts=[opts ': '];

else

  opts=[opts ' [' deflt_option ']: '];

end

str=['  ' str opts];



while (1)

  disp(' ')

  s=input(str,'s');

  ns=length(s);

  if (ns==0)

    if (no_default)

      s=[];

    else

      s=deflt_option;

    end

    return

  else

    s=lower(s(1));

    if (find(options==s))

      return

    else

      disp(' ')

      disp(['  Select only from the following: ' options])

    end

  end

end