question is like:
1- write a progrogram that measuring/calculating frequency of a acoustic guitars string individually
This reads more like "Homework assignment is...".
What hardware are you using to measure anything? Specifically, how are you measuring the string vibrations individually? The code needs to be written to deal with that hardware.
2- when you tune one string it should be seen the frequency on lcd, one string at one time, use a zero-crossing filter/formula
How does "it" know when you are tuning one string vs. another?
is there any one of you guys can help me on that, I am using seeeduino board , I tried to use frequency library on the above link, but I get error
Oh, well, that's easy. Delete line 87. That will fix the error.