% lect9_4: Converting units of energy

% Convert a quantity of energy given in Joules J), ft-lbs (ft-lb), calories (cal), 
% or electron volts (eV) to another unit.

% Obtain the energy value, current units, and output units
Ein = input('Enter the value of energy to be converted: ');
EinUnits = input('Enter the current units (J, ft-lb, cal, eV): ','s');
EoutUnits = input('Enter the new units (J, ft-lb, cal, eV): ','s');
% Clear the error flag
ErrorFlag = 0;
% Convert Ein to Joules
switch EinUnits
    case 'J'
        EJ = Ein;
    case 'ft-lb'
        EJ = Ein/0.738;
    case 'cal'
        EJ = Ein/0.239;
    case 'eV'
        EJ = Ein/6.24e18;
    otherwise
        ErrorFlag = 1;
end
% Convert Ein (J) to new units
switch EoutUnits
    case 'J'
        Eout = EJ;
    case 'ft-lb'
        Eou = EJ*0.738;
    case 'cal'
        Eout = EJ*0.239;
    case 'eV'
        Eout = EJ*6.24e18;
    otherwise
        ErrorFlag = 1;
end
% Check to see if there was an error. If not, print results.
if ErrorFlag
    disp('Error: current or new units are typed incorrectly.');
else
    fprintf('E = %g %s\n', Eout, EoutUnits);
end