How can I create a variable changing with index number?

how can I create a variable N such that it's value changing with the index and then do the summation, i.e:

∑_Ni from i=0 to i=255 ,

where Ni is the number of pixels that have intensity value Ri

For knowledge, I have an image with its histogram.

Answer 1

[Image Analyst]

You can do

i = 0:255;

though you don't need to do that to take the sum. To sum the histogram of the image you can do

[pixelCounts, grayLevels] = imhist(grayscaleImage);
theSum = sum(pixelCounts(:));