Removing an item from a list of lists based on each of the lists first element

by

Reverse delete a, modifying it in-place:

for i in reversed(range(len(a))):
    if a[i][0] == 3:
        del a[i]

An in-place modification means that this is more efficient, since it does not create a new list (as a list comprehension would).

Since OP requests a performant solution, here’s a timeit comparison between the two top voted answers here.

Setup –

a = np.random.choice(4, (100000, 2)).tolist()

print(a[:5])
[[2, 1], [2, 2], [3, 2], [3, 3], [3, 1]]

List comprehension –

%timeit [x for x in a if x[0] != b]
11.1 ms ± 685 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

Reverse delete –

%%timeit
for i in reversed(range(len(a))):
    if a[i][0] == 3:
        del a[i]

10.1 ms ± 146 µs per loop (mean ± std. dev. of 7 runs, 1 loop each)

They’re really close, but reverse delete has a 1UP on performance because it doesn’t have to generate a new list in memory, as the list comprehension would.

More Related Contents:

  1. Is there a way to ordinate numbers in a list?
  2. How do you find min and max without using min and max function [duplicate]
  3. How to create the following lists with list comprehensions? [closed]
  4. Unable to use convert strings into integers for a for loop
  5. Printing the number from a list and its square on the same line in python3 [closed]
  6. Python3 – create global variable(list) from function
  7. How to return dictionary keys as a list in Python?
  8. Access multiple elements of list knowing their index
  9. Count frequency of words in a list and sort by frequency
  10. Calling filter returns [duplicate]
  11. Python 3 turn range to a list
  12. Getting only element from a single-element list in Python?
  13. Unsupported operand type(s) for +: ‘int’ and ‘str’ [duplicate]
  14. Python – TypeError: ‘int’ object is not iterable
  15. Check if all elements of a list are of the same type
  16. One liner: creating a dictionary from list with indices as keys
  17. How to make a custom object iterable?
  18. Understand Python swapping: why is a, b = b, a not always equivalent to b, a = a, b?
  19. Python range() and zip() object type
  20. Why do I get “List index out of range” when trying to add consecutive numbers in a list using “for i in list”? [duplicate]
  21. Initializing 2D array in Python
  22. How do I return dictionary keys as a list in Python?
  23. “TypeError: ‘type’ object is not subscriptable” in a function signature
  24. How to sort a list of strings in reverse order without using reverse=True parameter?
  25. Deleting consonants from a string in Python
  26. Python – How to convert only numbers in a mixed list into float?
  27. How does str.startswith really work?
  28. Nested lambda statements when sorting lists
  29. How to convert only numbers in a mixed list into float
  30. Converting string to tuple without splitting characters

Leave a Comment