I want this code to show output like this using loops

I have written this matlab code to return an image with 50*1000 pixels strip now I want to make my pixel strip black and white using loop I can do it the other way but I want this through loop. This code that I have written show different results.

clc, clear, close all
height=50;
width=1000;
I=ones(height,width);
I(:,1:50)=0;
for i=1:50:length(I)
    I(:,i)=0;
end
imshow(I);

I want this code to show output like this using loops

Answer 1

[Walter Roberson]

for i=1:100:width
  I(:,i:i+49)=0;
end

[Ralph]

thanks alot