[Numpy-discussion] Quick array value assignment based on common values

Hey folks, I've one array, x, that you could define as follows: [[1, 2.25], [2, 2.50], [3, 2.25], [4, 0.00], [8, 0.00], [9, 2.75]] Then my second array, y, is: [[1, 0.00], [2, 0.00], [3, 0.00], [4, 0.00], [5, 0.00], [6, 0.00], [7, 0.00], [8, 0.00], [9, 0.00], [10,0.00]] Is there a concise, Numpythonic way to copy the values of x[:,1] over to y[:,1] where x[:,0] = y[:,0]? Resulting in, z: [[1, 2.25], [2, 2.50], [3, 2.25], [4, 0.00], [5, 0.00], [6, 0.00], [7, 0.00], [8, 0.00], [9, 2.75], [10,0.00]] My current task has len(x) = 25000 and len(y) = 350000 and looping through is quite slow unfortunately. Many thanks, -paul