How To Start Array Index From 1 In Python

In Python, the index of an array (or list) starts from 0 by default. However, in some situations, you may want to start the index from 1—for example, when you work with matrices or when the indices represent a real-world quantity.

In this tutorial, we’ll show you how to start an array index from 1 in Python by creating a custom data structure with your desired indexing behavior.

Step 1: Create a Custom List Class

We’ll start by creating a custom class that inherits from Python’s native list class. This custom class will be used to initialize your desired index offset (in this case, 1).

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class IndexList(list):
    def __init__(self, start_index, *args):
        self.start_index = start_index
        super(IndexList, self).__init__(*args)

In the code snippet above, we define a class IndexList that inherits from Python’s native list class. The constructor method __init__ takes the start_index parameter, which defines the desired start index for the list. We are using super() to call the parent list class constructor method (__init__) with the arguments passed.

Step 2: Overriding Indexing Methods

Next, we need to override the native Python list indexing methods to incorporate our custom index offset. In this step, we’ll override methods __getitem__ and __setitem__:

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class IndexList(list):
    # The existing constructor method from Step 1
 
    def __getitem__(self, index):
        return super(IndexList, self).__getitem__(index - self.start_index)
 
    def __setitem__(self, index, value):
        super(IndexList, self).__setitem__(index - self.start_index, value)

Here, __getitem__ and __setitem__ are methods that are internally handling the list indexing behavior. By overriding them, we are modifying the index value with our custom offset before passing it to the respective parent methods using super().

Step 3: Test the Custom List Class

Now let’s create an instance of our custom IndexList class to see if it works as expected:

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def main():
    my_list = IndexList(1, [10, 20, 30, 40, 50])
 
    print(f"Original List: {my_list}")
 
    # Accessing elements using custom index
    print(f"Element at index 1: {my_list[1]}")
    print(f"Element at index 5: {my_list[5]}")
 
    # Modifying elements using custom index
    my_list[1] = 99
    my_list[5] = 100
    print(f"Modified List: {my_list}")
 
if __name__ == "__main__":
    main()

In this code, we create an instance of IndexList with a starting index of 1 and a given list of elements. We then print the elements of the list using our custom indexing and modify some values before printing the updated list.

Full Code

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class IndexList(list):
    def __init__(self, start_index, *args):
        self.start_index = start_index
        super(IndexList, self).__init__(*args)
 
    def __getitem__(self, index):
        return super(IndexList, self).__getitem__(index - self.start_index)
 
    def __setitem__(self, index, value):
        super(IndexList, self).__setitem__(index - self.start_index, value)
 
def main():
    my_list = IndexList(1, [10, 20, 30, 40, 50])
 
    print(f"Original List: {my_list}")
 
    # Accessing elements using custom index
    print(f"Element at index 1: {my_list[1]}")
    print(f"Element at index 5: {my_list[5]}")
 
    # Modifying elements using custom index
    my_list[1] = 99
    my_list[5] = 100
    print(f"Modified List: {my_list}")
 
if __name__ == "__main__":
    main()

Output

Original List: [10, 20, 30, 40, 50]
Element at index 1: 10
Element at index 5: 50
Modified List: [99, 20, 30, 40, 100]

Conclusion

In this tutorial, we demonstrated how to start an array index from 1 in Python by creating a custom list class that inherits from Python’s native list class and overrides its indexing methods. This custom class will allow you to use lists with custom indexing and provide better control while working with data structures.