How to plot - Histogram Processing

Histogram Processing

h(rk)= nk

p(rk)=nk/n

I try to perform the Matlab programming with formula equation for the image processing as below

However I am not sure my answer is correct or not

Trial>> clear;

Trial>> x=('cameraman.tif');

Trial>> I=double(x);

Trial>> subplot(1,2,1),imshow(x);

Trial>> subplot(1,2,2),hist(I);

Kindly please provide your opinion and suggestion thus I will be able to improve my computing skills