title: create a matrix from array of elements under diagonal in numpy
body: <p>I would like to create a matrix using a list whose elements would be the elements of the matrix under the diagonal. </p>
  
  <pre><code>import numpy as np
  x1 = np.array([0.9375, 0.75, 0.4375, 0.0, 0.9375, 0.75, 0.4375, 0.9375, 0.75, 0.9375])
  x1
  </code></pre>
  
  <p>the matrix I would like to have is </p>
  
  <pre><code>array([[ 1.    ,  0.9375,  0.75  ,  0.4375,  0.    ],
     [ 0.9375,  1.    ,  0.9375,  0.75  ,  0.4375],
     [ 0.75  ,  0.9375,  1.    ,  0.9375,  0.75  ],
     [ 0.4375,  0.75  ,  0.9375,  1.    ,  0.9375],
     [ 0.    ,  0.4375,  0.75  ,  0.9375,  1.    ]])
  </code></pre>
  
  <p>I thought you could do this with <strong>np.tril</strong> but it gives a result I do not expect. </p>
  
  <pre><code>mat = np.tril(x1, k = -1  )
  print(mat)
  </code></pre>
  
  <p>what am I missing ? </p>
  
  <p>I apologize in advance if this is a trivial question but I could not figure out how to it without looping.</p>