Accessing field names in struct

I've the following struct

test = struct(...
            'a',false,...
            'b',false,...
            'c',false,...
            'd',false,...
            'e',false,... 
            'f',false,...
            'g',true...
        );

How do get the field name that has a value true ?

Expected result:

g

Any suggestions?

Answer 1

[Walter Roberson]

F = fieldnames(test) ;
F(cell2mat(struct2cell(test)))

Answer 2

[Stephen23]

test = struct(...
    'a',false,...
    'b',false,...
    'c',false,...
    'd',false,...
    'e',false,... 
    'f',false,...
    'g',true...
    );
F = fieldnames(test);
R = F{structfun(@(a)a,test)}

R = 'g'

[Stephen23]

Timing tests:

test = struct(...
    'a',false,...
    'b',false,...
    'c',false,...
    'd',false,...
    'e',false,... 
    'f',false,...
    'g',true...
    );
timeit(@()f1(test))

ans = 6.6802e-05

timeit(@()f2(test))

ans = 2.1892e-05

function f1(test)
F = fieldnames(test);
R = F(cell2mat(struct2cell(test)));
end

function f2(test)
F = fieldnames(test);
R = F{structfun(@(a)a,test)};
end