title: Create new array from a defined array
body: <p>If I have an array, <code>a = [1,2,3,4,2,1]</code>, how can I create another array, which shows the number of times each number in array a has been repeated, for example from array a then the new array would be <code>b = [2,2,1,1]</code>? Is this possible using a command in the NumPy library? </p>