Indexerror List Index Out Of Range
In this tutorial, you'll learn how all about the Python list index out of range error, including what it is, why information technology occurs, and how to resolve it.
The IndexError is ane of the most mutual Python runtime errors that you'll see in your programming journey. For the most function, these these errors are quite easy to resolve, once you understand why they occur.
Throughout this tutorial, you'll acquire why the fault occurs and walk through some scenarios where y'all might see it. Y'all'll as well larn how to resolve the error in these scenarios.
The Quick Reply:
What is the Python IndexError?
Permit's take a lilliputian scrap of time to explore what the Python IndexError is and what it looks like. When y'all see the error, you'll come across an error message displayed as beneath:
IndexError: list alphabetize out of range
We tin can break downwardly the text a niggling bit. We tin meet here that the bulletin tells us that the alphabetize is out of range. This ways that nosotros are trying to admission an index item in a Python list that is out of range, meaning that an item doesn't have an index position.
An detail that doesn't have an index position in a Python list, well, doesn't exist.
In Python, like many other programming languages, a list index begins at position 0 and continues to n-1, where north is the length of the list (or the number of items in that list).
This causes a fairly common error to occur. Say we are working with a list with 4 items. If we wanted to admission the fourth item, you lot may attempt to exercise this by using the index of 4. This, yet, would throw the error. This is because the 4th item actually has the index of three.
Let's accept a expect at a sample listing and try to access an item that doesn't exist:
# How to raise an IndexError a_list = ['welcome', 'to', 'datagy'] print(a_list[iii]) # Returns: # IndexError: listing index out of range
We can see here that the index error occurs on the last detail we try to access.
The simplest solution is to just not try to access an particular that doesn't be. But that'southward easier said than done. How do we prevent the IndexError from occurring? In the adjacent two sections, you'll learn how to fix the fault from occurring in their most common situations: Python for loops and Python while loops.
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Python IndexError with For Loop
You may encounter the Python IndexError while running a Python for loop. This is especially common when you lot try to loop over the list using the range() function.
Let'south take a look at the situation where this mistake would occur:
# How to enhance an IndexError with a For Loop a_list = ['welcome', 'to', 'datagy'] for i in range(len(a_list) + ane): impress(a_list[i]) # Returns: # welcome # to # datagy # Traceback (most recent telephone call concluding): # IndexError: list alphabetize out of range
The way that nosotros can fix this error from occurring is to simply end the iteration from occurring before the list runs out of items. The way that we tin can do this is to change our for loop from going to our length + one, to the list's length. When nosotros do this, we stop iterating over the list'south indices before the lengths value.
This solves the IndexError since information technology causes the list to terminate iterating at position length - one, since our index begins at 0, rather than at 1.
Let'southward see how we can alter the code to run correctly:
# How to prevent an IndexError with a For Loop a_list = ['welcome', 'to', 'datagy'] for i in range(len(a_list)): impress(a_list[i]) # Returns: # welcome # to # datagy
Now that you have an agreement of how to resolve the Python IndexError in a for loop, permit'southward see how we can resolve the error in a Python while-loop.
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Python IndexError with While Loop
Yous may also run into the Python IndexError when running a while loop.
For example, it may be tempting to run a while loop to iterate over each alphabetize position in a listing. You lot may, for example, write a program that looks like this:
# How to raise an IndexError with a While Loop a_list = ['welcome', 'to', 'datagy'] i = 0 while i <= len(a_list): print(a_list[i]) i += 1 # Returns: # welcome # to # datagy # IndexError: listing alphabetize out of range
The reason that this program fails is that nosotros iterate over the listing one too many times. The reason this is true is that nosotros are using a <= (greater than or equal to sign). Considering Python list indices begin at the value 0, their max index is actually equal to the number of items in the list minus 1.
We can resolve this by simply changing the operator a less than symbol, <. This prevents the loop from looping over the index from going out of range.
# How to prevent an IndexError with a While Loop a_list = ['welcome', 'to', 'datagy'] i = 0 while i < len(a_list): print(a_list[i]) i += 1 # Returns: # welcome # to # datagy
In the next section, you'll learn a better way to iterate over a Python list to prevent the IndexError.
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How to Fix the Python IndexError
There are 2 unproblematic ways in which you can iterate over a Python list to prevent the Python IndexError.
The first is actually a very evidently language fashion of looping over a list. Nosotros don't actually demand the list index to iterate over a list. We can simply access its items direct.
# Prevent an IndexError a_list = ['welcome', 'to', 'datagy'] for give-and-take in a_list: print(word) # Returns: # welcome # to # datagy
This directly prevents Python from going beyond the maximum index.
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But what if you need to access the listing'southward index?
If you need to admission the list'southward alphabetize and a list item, and then a much safer alternative is to use the Python enumerate() function.
When you pass a list into the enumerate() office, an enumerate object is returned. This allows you to access both the index and the item for each detail in a listing. The role implicitly stops at the maximum index, but allows you to get quite a bit of data.
Permit's take a look at how we tin use the enumerate() part to prevent the Python IndexError.
# Forbid an IndexError with enumerate() a_list = ['welcome', 'to', 'datagy'] for idx, discussion in enumerate(a_list): print(idx, word) # Returns: # 0 welcome # 1 to # 2 datagy
We can run into here that we the loop stops before the alphabetize goes out of range and thereby prevents the Python IndexError.
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Determination
In this tutorial, yous learned how to empathise the Python IndexError: list item out of range. Yous learned why the error occurs, including some common scenarios such as for loops and while loops. You learned some meliorate means of iterating over a Python list, such as by iterating over items implicitly besides as using the Python enumerate() function.
To learn more nearly the Python IndexError, check out the official documentation here.
Source: https://datagy.io/python-indexerror-list-index-out-of-range/
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