How to detect string when pitch-tracking on electric guitar?

This might be a little bit late because the post is one years old. But here's a solution, which I found out after long research for pitch detecting a guitar.

THIS IS WHY FFT DOESN'T WORK :

You cannot use FFT since the result gives you a linear array, and the sound is calculated logarithmically (exponential distance between notes). Plus, FFT gives you an array of bins in which your frequency COULD BE, it doesnt give you the precise result.

THIS IS WHAT I SUGGEST :

Use dywapitchtrack. it's a library that uses a wavelet algorythm, which works directly on your wave instead of calculating large bins like FFT.

description: The dywapitchtrack is based on a custom-tailored algorithm which is of very high quality: both very accurate (precision < 0.05 semitones), very low latency (< 23 ms) and very low error rate. It has been thoroughly tested on human voice. It can best be described as a dynamic wavelet algorithm (dywa):

DOWNLOAD : https://github.com/inniyah/sndpeek/tree/master/src/dywapitchtrack

USE(C++): put the .c and .h where you need it and import it in your project

include the header file

//Create a dywapitchtracker Object
dywapitchtracker pitchtracker;

//Initialise the object with this function
dywapitch_inittracking(&pitchtracker);

When your buffer is full (buffer needs to be at 44100 resolution and power of 2 of length, mine is 2048):

//use this function with your buffer
double thePitch = dywapitch_computepitch(&pitchtracker, yourBuffer, 0, 2048);

And voilà, thePitch contains precisely what you need. (feel free to ask question if something is unclear)

An simple FFT peak estimator is not a good guitar pitch detector/estimator, due to many potentially strong overtones. There exist more robust pitch estimation algorithms (search stackoverflow and DSP.stackexchange). But if you require the players to pre-characterize each string on their individual instruments, both open and fretted, before starting the game, an FFT fingerprint of those characterizations might be able to differentiate the same note played on different strings on some guitars. The thicker strings will give off slightly different ratios of energy in some of the higher overtones, as well as different amounts of slight inharmonicity.