Python : How to Remove multiple keys from Dictionary while Iterating ?
In this article we will discuss how to remove multiple key / value pairs from dictionary while iterating.
Suppose we have a dictionary of string and int i.e.
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# Dictionary of strings wordFreqDic = { "Hello": 51, "at" : 23 , "test" : 33, "here" : 63, "start" : 13 } |
Now we want to remove all key/value pairs from dictionary whose value is divisible by 3. Let’s see how to do that,
Removing elements from Dictionary while Iterating
We can not change the size of dictionary while iterating over it. If we try to remove any element from dictionary while iterating over it then it will throw error.
For example,
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for (key, value) in wordFreqDic.items() : if value % 3 == 0: del wordFreqDic[key] |
Will through following Error,
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RuntimeError: dictionary changed size during iteration |
So, let’s see how to delete multiple elements from dictionary.
Removing keys from dictionary while iterating by creating a list of keys
As we cannot iterate and remove elements from dictionary at same time. Therefore will, first iterate over the dictionary and create a list of keys that need to be deleted from dictionary. Now, we will iterate over this list of keys and delete their entries from dictionary i.e.
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''' Removing multiple keys from dictionary by creating a list of keys to be deleted and then deleting them one by one ''' # List of keys to be deleted from dictionary selectedKeys = list() # Iterate over the dict and put to be deleted keys in the list for (key, value) in wordFreqDic.items() : if value % 3 == 0: selectedKeys.append(key) # Iterate over the list and delete corresponding key from dictionary for key in selectedKeys: if key in wordFreqDic : del wordFreqDic[key] |
Now dictionary contents will be,
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Modified Dictionary {'start': 13, 'at': 23} |
Removing keys from dictionary while iterating by creating a copy of dictionary
Create a copy of dictionary and iterate over this temporary dictionary, check element to be deleted and delete it from original dictionary i.e.
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''' Removing multiple keys from dictionary by creating a copy of dictionary and iterating over it ''' # Create a temporary copy of dictionary copyOfDict = dict(wordFreqDic) # Iterate over the temporary dictionary and delete corresponding key from original dictionary for (key, value) in copyOfDict.items() : if value % 3 == 0: del wordFreqDic[key] |
Now dictionary contents will be,
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Modified Dictionary {'start': 13, 'at': 23} |
Learn more about Dictionaries in Python,
- The Modern Python 3 Bootcamp
- The Python Bible Everything You Need to Program in Python
- Programming for Everybody (Getting Started with Python)
Complete example is as follows,
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def main(): # Dictionary of strings wordFreqDic = { "Hello": 51, "at" : 23 , "test" : 33, "here" : 63, "start" : 13 } print("Original Dictionary ", wordFreqDic) ''' Removing multiple keys from dictionary by creating a list of keys to be deleted and then deleting them one by one ''' # List of keys to be deleted from dictionary selectedKeys = list() # Iterate over the dict and put to be deleted keys in the list for (key, value) in wordFreqDic.items() : if value % 3 == 0: selectedKeys.append(key) # Iterate over the list and delete corresponding key from dictionary for key in selectedKeys: if key in wordFreqDic : del wordFreqDic[key] print("Modified Dictionary " , wordFreqDic) # Dictionary of strings wordFreqDic = { "Hello": 51, "at" : 23 , "test" : 33, "here" : 63, "start" : 13 } print("Original Dictionary ", wordFreqDic) ''' Removing multiple keys from dictionary by creating a copy of dictionary and iterating over it ''' # Create a temporary copy of dictionary copyOfDict = dict(wordFreqDic) # Iterate over the temporary dictionary and delete corresponding key from original dictionary for (key, value) in copyOfDict.items() : if value % 3 == 0: del wordFreqDic[key] print("Modified Dictionary ", wordFreqDic) if __name__ == '__main__': main() |
Output:
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Original Dictionary {'Hello': 51, 'test': 33, 'here': 63, 'start': 13, 'at': 23} Modified Dictionary {'start': 13, 'at': 23} Original Dictionary {'Hello': 51, 'test': 33, 'here': 63, 'start': 13, 'at': 23} Modified Dictionary {'start': 13, 'at': 23} |
Python Resources
- Best Python Tutorials on lists, dict, functions, iterators & many more.
- Data Analysis in Python using Pandas Dataframe - Top Tutorials
C++11 / C++14 Resources
- Best C++11 Tutorials on Topics like Smart Pointers, tuples, Unordered map / set, lambda function etc.
- C++11 Multi-threading Series
- C++ - Boost Library Tutorials
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