How not to reload persistent variables twice?

I wnat to use persistent variables to speed up, so I want to genrate data, and make it persistent at the first call, but how to check next time, that this is not the first call, and I dont want to genrate the data again?

Answer 1

[Walter Roberson]

if isempty(VariableName)
  ... calculate initial value for variable
end

Answer 2

[Jan]

This is explained in the documentation: doc persistent:

function arrayToXLS(A, xlsfile, x1, x2)
persistent dblArray;

if isempty(dblArray) 
  disp 'Writing spreadsheet file ...'
  xlswrite(xlsfile, A);
end

Undefined persistent variables are empty. If your real data might be empty also, use a persistent flag in addition:

persistent Data initialized
if isempty(initialized)
  initialized = true;
  Data = <what eve you want, even empty array>
end