2.2. Data structures 9
2.2 Data structures
Data structures are able to contain more than one data in it. There are four built-in data structures in
Python: list, tuple, dictionary and set. Apart from these built-in data structure, you can define
your own data type also like numpy.array defined by numpy, which is very useful. I did not feel
any need to use the set in hydrology, so I am skipping set here, if you are interested you can learn
about it from some other source.
2.3 Data structures
2.3.1 List
A list is a sequence of items (values). The items in it could belong to any of data type, and could be
of different data type in the same list.
>>> a = ['Ram', 'Sita', 'Bangalore', 'Delhi']
>>> b = [25, 256, 2656, 0]
>>> c = [25, 'Bangalore']
The items in the list are accessed using the indices. The variable a and b hold items of similar data
types, while c holds items of different data types. In Python, the indices starts at 0. So, to get the
first and third item, the indices should be 0 and 2.
>>> a = ['Ram', 'Sita', 'Bangalore', 'Delhi']
>>> print a[0]
Ram
>>> print a[2]
Bangalore
Negative indices are also allowed in Python. The last item in the list has -1 indices, similarly second
last item has indices of -2 and so on.
>>> a = ['Ram', 'Sita', 'Bangalore', 'Delhi']
>>> print a[-1]
Delhi
Likewise, second last item in the list can be accessed by using the indices -2.
2.3.2 Dictionary
In the list, the indices are only integers. Dictionary has the capability to take any data type as indices.
This feature of dictionary makes it very suitable, when the indices are name etc. For example, in
hydrology the name of field stations and their corresponding variables are given for each station. Let
us try to retrieve the value of variable by using list first, and then by using dictionary. We can use
one list to store the name of stations, and one for the variable. First, we need to find the indices of
station, and then use this indices to access the variable from the list of variables.
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