You could also use a structure named g to store the value of g for each planet instead of a matrix. For example, you create your structure as follows:
g.Earth = …
g.Mars = …
g.Uranus = …
Then, when the user inputs a planet, you can use dynamic field naming to access the appropriate g as follows:
user_planet = 'Earth';
user_g = g.(user_planet);
The following article gives a short introduction on dynamic field naming in structures:
