Rather than using loops you should learn how to use neat comma-separated lists:
and vectorized code:
and some simple logical indexing. For your data and formula, something like this:
% Fake data (replace with your own):
S = struct(...
'Name',{'Sun','Mercury','Venus','Earth','Mars','Jupiter','Saturn','Uranus','Neptune','Pluto'},...
'Mass',{1989000,0.33,4.87,5.97,0.642,1899,568,86.8,102,0.0125},...
'Dist',{0,0.39,0.723,1,1.523,5.203,9.539,19.18,30.06,39.53});
% Code:
U = 'venus'; % user selection.
X = strcmpi(U,{S.Name});
D2 = [S(~X).Dist];
M2 = [S(~X).Mass];
D1 = S(X).Dist;
M1 = S(X).Mass;
G = 6.673e-11; % change to suit your units
F = G*(M1*M2)./((D1-D2).^2);
C = [{S(~X).Name};num2cell(F)];
fprintf('%-11s %g\n',C{:})
Which prints (note that you will need to provide the correct gravitational constant value for G):
Sun 0.00123654
Mercury 9.67109e-10
Earth 2.52851e-08
Mars 3.25991e-10
Jupiter 3.07482e-08
Saturn 2.37495e-09
Uranus 8.28033e-11
Neptune 3.8514e-11
Pluto 2.69737e-15
