title: in python, compare two array in element and change the lower value to zero
body: <p>now we have two arrays</p>
  
  <pre><code>a = [[1.,2.,3.,4.,5.,6.,7.,8.,9.],[9.,8.,7.,6.,5.,4.,3.,2.,1.]]
  
  b = [[2.,2.,1.,2.,0.,2.,1.,2.,0.],[2.,2.,1.,2.,0.,2.,1.,2.,0.]]
  </code></pre>
  
  <p>Is there any fast way to achieve the following function: 
  compare these two arrays by element, and change the array a[] in the following rule: </p>
  
  <pre><code>if a[i,j] &gt; b[i,j]: 
  
      a[i,j] = a[i,j] 
  
  else   
  
  a[i,j] = 0
  </code></pre>
  
  <p>I know that a loop can do it, but I am wondering if there is any other faster way to do it.</p>