Graphing sensor data using sd card

I'm gonna have to spend some time studying how to use the character array and nested while function and I will post beck here if I get stuck again with that. What would be the function used inside the second while loop for reading the ith to the nth character in the file? I assume it is with the myFile.read() function but I'm not sure how to tell it to read a certain character. So, I guess the library has a code for every character so I need to save the readings as a character and convert later? My plan is to record the data at certain rate and keep 24 hr of data so the file can be fixed length however, I am unsure of how to create a fixed length .txt file. Can you explain to me, or point me in the direction of how to limit my text files to only allow a certain number of recordings? I have attached a copy of the humidity and temperature files.

HUMIDITY.TXT (1.39 KB)

TEMP.TXT (1.39 KB)