title: Python numpy array get first column values divide by 255 and store back
body: <p>I have following numpy array </p>
  
  <pre><code>[[[  0   0   0 255]
    [  0   0   0 255]
    [  0   0   0 255]
     ..., 
    [  255   0   0 255]
    [  0   0   0 255]
    [  0   0   0 255]]]
  </code></pre>
  
  <p><strong>and this is a large array i just gave a sort example here</strong></p>
  
  <p>I want to divide the first column first index value of every row by 255 and store it back to array,</p>
  
  <p><strong>Ex</strong> [  255   0   0 255] take first column first index value 255/255 and store it back, same for all rows</p>
  
  <pre><code>[[[  0   0   0 255]
    [  0   0   0 255]
    [  0   0   0 255]
     ..., 
    [  1   0   0 255]
    [  0   0   0 255]
    [  0   0   0 255]]]
  </code></pre>