Python list indexing from negative to positive indexes
Question:
I am trying to index from a list. I understand that negative indices refer to items counting backwards from the end of the list. My question is why it does not seem possible to index over a range starting from a negative index to a positive index, even though it seems intuitive.
For example, foo[-1], foo[0], foo[1] are all valid ways to index from the list foo. Why does foo[-1:1] not work?
Alternatively, since foo[-1:] selects the last element, why does foo[-1:0] not work equivalently?
foo = ['a','b','c']
print(foo[-1])
print(foo[0])
print(foo[1])
print(foo[-1:1])
It is possible to index directly from -1, 0, and 1, but not through that range -1:1.
Answers:
you can do
foo[-1:1:-1]
output : ['c']
slicing uses foo[start:end:step]
you can do foo[::-1] to reverse the list etc.
As far as I can tell, you are looking for something like this:
a = [10, 20, 30, 40]
a[-1:] + a[:1] # this is [40, 10]
You could make it behave like this automatically, but that would require a check if b > c when querying a[b:c] (keep in mind, that your -1 in the example is actually len(a) - 1).
This will print all the elements in foo
print(foo[-len(foo):])
i.e.
['a', 'b', 'c']
This will print last two elements in foo
print(foo[-2:])
i.e.
['b', 'c']
That’s how it works
I am trying to index from a list. I understand that negative indices refer to items counting backwards from the end of the list. My question is why it does not seem possible to index over a range starting from a negative index to a positive index, even though it seems intuitive.
For example, foo[-1], foo[0], foo[1] are all valid ways to index from the list foo. Why does foo[-1:1] not work?
Alternatively, since foo[-1:] selects the last element, why does foo[-1:0] not work equivalently?
foo = ['a','b','c']
print(foo[-1])
print(foo[0])
print(foo[1])
print(foo[-1:1])
It is possible to index directly from -1, 0, and 1, but not through that range -1:1.
you can do
foo[-1:1:-1]
output : ['c']
slicing uses foo[start:end:step]
you can do foo[::-1] to reverse the list etc.
As far as I can tell, you are looking for something like this:
a = [10, 20, 30, 40]
a[-1:] + a[:1] # this is [40, 10]
You could make it behave like this automatically, but that would require a check if b > c when querying a[b:c] (keep in mind, that your -1 in the example is actually len(a) - 1).
This will print all the elements in foo
print(foo[-len(foo):])
i.e.
['a', 'b', 'c']
This will print last two elements in foo
print(foo[-2:])
i.e.
['b', 'c']
That’s how it works