Pseudocode For Looking Up A Function
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initialisation:

   open file
   while not feof :
      read x
      read y
      store y in array position x
   close file

interpolation:

   select value of x
   if we know y(x)
      set y = y(x)
   otherwise
   if x is lower than lowest x in array
      set s = complaint
      set y = nan
   otherwise
   if x is higher than highest x in array
      set s = complaint
      set y = nan
   otherwise
      get highest value of x in array which is lower than x (x1)
      get lowest value of x in array which is higher than x (x2)
      evaluate y = y(x1) + (y(x2)-y(x1))/(x2-x1) * (x-x1)
   if not y = nan,
      print y
   otherwise
      print complain