How can I make horizontal bands in a grayscale image?

"Give the formula that would create a 400×400 image with 5 horizontal black bands separated by white bands. Write the MATLAB code to make this image and display it."

I have figured out how to make the correct number of bands, but they are vertical instead of horizontal. Anyone know how to make them horizontal? My code so far is below.

colormap(gray(400))

yy = ones(400,1)*cos(2*pi*(0:399)/80);

show_img(yy);

Answer 1

[Jeff E]

yy = (ones(400,1)*cos(2*pi*(0:399)/80))';