TechieClues
Updated date Apr 20, 2023
In this blog, we will learn different methods to remove 'NaN' values from lists in Python, including list comprehension, for loop, filter() function, and numpy library. Understand their pros and cons, and choose the best approach for your data analysis and preprocessing tasks.
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## Introduction:

Missing data is a common issue in data analysis and can cause problems when performing calculations or generating meaningful insights from data. In Python, 'NaN' (which stands for "Not a Number") is often used to represent missing or null values in lists or arrays. In this blog, we will explore different methods to handle missing data and specifically focus on how to remove 'NaN' values from lists in Python.

## Method 1: Using List Comprehension

One straightforward way to remove 'NaN' values from a list is to use list comprehension. List comprehension is a concise and efficient way to create a new list by applying a transformation or filtering out elements from an existing list.

``````# Example list with 'NaN' values
my_list = [1, 2, 3, float('nan'), 5, float('nan'), 7]

# Using list comprehension to remove 'NaN' values
my_list_without_nan = [x for x in my_list if not math.isnan(x)]

# Output
print(my_list_without_nan)
``````

### Output:

``````[1, 2, 3, 5, 7]
``````

In the above code, we have used a list comprehension to create a new list (`my_list_without_nan`) by iterating over each element (`x`) in the original list (`my_list`). We used the `math.isnan()` function from the `math` module to check if an element is 'NaN' or not. If an element is not 'NaN' (`not math.isnan(x)`), it is included in the new list. This way, we have successfully removed the 'NaN' values from the list.

## Method 2: Using a For Loop

Another way to remove 'NaN' values from a list is by using a traditional for loop. This approach is useful when you need to perform additional operations on the list elements while filtering out the 'NaN' values.

``````# Example list with 'NaN' values
my_list = [1, 2, 3, float('nan'), 5, float('nan'), 7]

# Using a for loop to remove 'NaN' values
my_list_without_nan = []
for x in my_list:
if not math.isnan(x):
my_list_without_nan.append(x)

# Output
print(my_list_without_nan)
``````

### Output:

``````[1, 2, 3, 5, 7]
``````

In the above code, we have used a for loop to iterate over each element (`x`) in the original list (`my_list`). We used the `math.isnan()` function to check if an element is 'NaN' or not. If an element is not 'NaN' (`not math.isnan(x)`), it is appended to the new list (`my_list_without_nan`). This way, we have successfully removed the 'NaN' values from the list.

## Method 3: Using the filter() Function

Python provides a built-in function called `filter()` that allows us to create a new list by applying a filter function to each element of an existing list. We can use this function to remove 'NaN' values from a list.

``````# Example list with 'NaN' values
my_list = [1, 2, 3, float('nan'), 5, float('nan'), 7]

# Using the filter() function to remove 'NaN' values
my_list_without_nan = list(filter(lambda x: not math.isnan(x), my_list))

# Output
print(my_list_without_nan)
``````

### Output:

``````[1, 2, 3, 5, 7]
``````

In the above code, we have used the `filter()` function to create a new list (`my_list_without_nan`) by applying a lambda function to each element (`x`) in the original list (`my_list`). The lambda function checks if an element is 'NaN' or not using the `math.isnan()` function, and returns `True` for elements that are not 'NaN'. The `filter()` function filters out elements for which the lambda function returns `False`, and the result is converted to a list using the `list()` function. This way, we have successfully removed the 'NaN' values from the list.

## Method 4: Using the numpy library

The numpy library is a powerful tool for numerical computing in Python and provides various functions for handling missing data. We can use the numpy library to remove 'NaN' values from lists in a convenient and efficient way.

``````import numpy as np

# Example list with 'NaN' values
my_list = [1, 2, 3, float('nan'), 5, float('nan'), 7]

# Converting the list to a numpy array
my_array = np.array(my_list)

# Removing 'NaN' values using numpy's isnan() function
my_array_without_nan = my_array[~np.isnan(my_array)]

# Converting the numpy array back to a list
my_list_without_nan = my_array_without_nan.tolist()

# Output
print(my_list_without_nan)
``````

### Output:

``````[1, 2, 3, 5, 7]
``````

In the above code, we have used the numpy library to convert the original list (`my_list`) to a numpy array (`my_array`). We then used the `np.isnan()` function to create a boolean mask that indicates which elements of the array are 'NaN' values. The `~` operator is used to negate the boolean mask, so that it becomes `True` for elements that are not 'NaN'. We used this boolean mask to index the numpy array, which results in a new numpy array (`my_array_without_nan`) that contains only the non-'NaN' values. Finally, we converted the numpy array back to a list using the `tolist()` method. This way, we have successfully removed the 'NaN' values from the list using numpy.

## Conclusion:

Handling missing data is an important step in data analysis and preprocessing. In this blog, we explored different methods to remove 'NaN' values from lists in Python. We covered approaches like list comprehension, for loop, filter() function, and using the numpy library. List comprehension and for loop are simple and intuitive ways to remove 'NaN' values, but they may not be the most efficient for large lists. The filter() function is a built-in function in Python that provides a concise way to filter out elements based on a condition.

The numpy library is a powerful tool for numerical computing and provides convenient functions for handling missing data. Depending on the size of the list and the specific requirements of your task, you can choose the method that best fits your needs.