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Section 9.7 Deleting elements
There are several ways to delete elements from a list. If you know the index of the element you want, you can use
pop:
pop modifies the list and returns the element that was removed. If you don’t provide an index, it deletes and returns the last element.
Activity 9.7.1.
What is printed by the following statements?
alist = [4, 2, 8, 6, 5]
temp = alist.pop(2)
temp = alist.pop()
print(alist)
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[4, 8, 6]
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pop(2) removes the item at index 2, not the 2 itself.
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[2, 6, 5]
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pop() removes the last item, not the first.
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[4, 2, 6]
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Yes, first the 8 was removed, then the last item, which was 5.
Activity 9.7.2.
What is printed by the following statements?
alist = [4, 2, 8, 6, 5]
alist = alist.pop(0)
print(alist)
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[2, 8, 6, 5]
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alist is now the value that was returned from pop(0).
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[4, 2, 8, 6, 5]
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pop(0) changes the list by removing the first item.
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4
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Yes, first the 4 was removed from the list, then returned and assigned to alist. The list is lost.
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None
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pop(0) returns the first item in the list so alist has now been changed.
If you don’t need the removed value, you can use the
del operator:
If you know the element you want to remove (but not the index), you can use
remove:
The return value from
remove is
None.
To remove more than one element, you can use
del with a slice index:
As usual, the slice selects all the elements up to, but not including, the second index.
Activity 9.7.3.
What is the value of alist after the following code executes?
alist = ['a', 'b', 'c', 'd', 'e', 'f']
del alist[1:5]
print(alist)
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[’a’, ’b’, ’c’, ’d’, ’e’, ’f’]
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The del method removes part of the list, so it will be shorter.
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[’a’, ’f’]
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’a’ is the 0th element of the list and ’f’ is the 5th element of the list, so these are the values that remain after deleting [1:5].
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[’f’]
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Remember that lists start at 0, not 1, and that a slice stops before the second element - not after.
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[’b’, ’c’, ’d’, ’e’]
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These are the values that will be deleted with the slice [1:5]
Activity 9.7.4.
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