Counting by 10 in a Text File

Hey I need some help creating a code that is able to read a string in a text file and pull every 10th,20th,30th,40th,and so on and so forth. So I am currently trying to use the size function but isn't quite working.Also, for example the text file I am using has data points under different labels such as A, B, C, or D with 200 hundred of each. So I want to get every 10th B in the text file. Any suggestions?

[Image Analyst]

Isn't this the same as http://www.mathworks.com/matlabcentral/answers/29464-counting-by-10 ????

[Frandy]

Yes I just elaborated better