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Lists and Tuples

Lists and Tuples

The list data structure is central to most Python programs; Gurobi Python programs are no exception. We'll also rely heavily on a similar data structure, the tuple. Tuples are crucial to providing efficient and convenient access to Gurobi decision variables in Gurobi Python programs. The difference between a list and a tuple is subtle but important. We'll discuss it shortly.

Lists and tuples are both just ordered collections of Python objects. A list is created and displayed as a comma-separated list of member objects, enclosed in square brackets. A tuple is similar, except that the member objects are enclosed in parenthesis. For example, [1, 2, 3] is a list, while (1, 2, 3) is a tuple. Similarly, ['Pens', 'Denver', 'New York'] is a list, while ('Pens', 'Denver', 'New York') is a tuple.

You can retrieve individual entries from a list or tuple using square brackets and zero-based indices:

gurobi> l = [1, 2.0, 'abc']
gurobi> t = (1, 2.0, 'abc')
gurobi> print(l[0])
1
gurobi> print(t[1])
2.0
gurobi> print(l[2])
abc

What's the difference between a list and a tuple? A tuple is immutable, meaning that you can't modify it once it has been created. By contrast, you can add new members to a list, remove members, change existing members, etc. This immutable property allows you to use tuples as indices for dictionaries.