how can i use image which is save in variable?

in the following code i wanted to use gray image which saved in variable b for further use,but it gives an error "undefined b" on second line... kindly sort this out

CODE:

i=imread('C:\Users\Romesha\Desktop\eye.jpg');b=rgb2gray(i);imshow(b); im = imread(b); imshow(im);

[Walter Roberson]

duplicates http://www.mathworks.co.uk/matlabcentral/answers/113197-how-can-i-use-image-which-is-save-in-variable