How to count data in a struct?

I have a struct with 61,460 lines of data that span a time period of nearly 7 years. I'm plotting a range of that data from say 4314 to 5600 using a

for i = 4314:5600, 
    plot(centers(i).lon, centers(i).lat, 'b*') 
end

command. The struct has 6 fields of day, type, lat, lon, j and i. Lat and Lon plot the data with their respective coordinates but in the struct, there are some data sets that don't have any values of Lat or Lon, instead, in the struct, these empty spaces are filled in as "[]" or Not A Number.

Basically, I'm able to plot the range of data I want but I don't know how to count the data that's plotted on the map because between lines 2,000 and 4,000, there aren't 2,000 lines of data. How do I count the number of real data sets between a range of data and exclude non numbers or [] values.

Thanks!

[per isakson]

See the functions

1. isnan Array elements that are NaN
2. isempty Determine whether array is empty

and try

sum(double( isnan( [centers.lat] )))

Proposal: Upload a small sample of the data

[Marco Barron]

Wow, thanks, I'll give this a shot over the weekend