title: Tensorflow, Change placeholder and pass values
body: <p>I want to do this:</p>
  
  <p>I have 2D-tensor, matrix with shape=(None, 4), so N examples with 4 element, 
  [x, y, a, b].</p>
  
  <pre><code>X = tf.placeholder(tf.float32, shape=(None, 4))
  </code></pre>
  
  <p>In next step, I want to have matrix m x n with value b at coordinates (x, y)</p>
  
  <p>Shape of Y is also given. e.g. shapeY = (5, 5)</p>
  
  <p>Example:</p>
  
  <p>Input:</p>
  
  <pre><code>X = np.array([[0, 0, 10, 10], 
                [1, 1, 20, 20], 
                [2, 2, 30, 30], 
                [3, 3, 40, 40], 
                [4, 4, 50, 50]])
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>Y = [[10, 0, 0, 0, 0],
       [0, 20, 0, 0, 0],
       [0, 0, 30, 0, 0],
       [0, 0, 0, 40, 0],
       [0, 0, 0, 0, 50]]
  </code></pre>
  
  <p>This example is with actual values, but I need this with placeholders as part of tensorflow graph, before I pass real values. Result must be like this.</p>