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allensimbul
3,065 PointsReversing the order via negative int
I don't seem to understand the concept of reversing the order via negative int numbers = list(range(21)) print(numbers[2::2]) gives me: [2, 4, 6, 8, 10, 12, 14, 16, 18, 20] but why doesnt print(numbers[2::-1]) give me the reverse of gives me: [2, 4, 6, 8, 10, 12, 14, 16, 18, 20] but rather [2, 1, 0]
Can someone explain please?
2 Answers
Ryan S
27,276 PointsHi Allen,
The parameters in a slice are:
numbers[start:stop:step]
In your first example, numbers[2::2], you are telling it to start at index 2 (which happens to be the number 2), and stop at the end (since there is no 'stop' parameter given), and to increment by 2.
In your second example, numbers[2::-1], you are again telling it to start at index 2, and to decrement by 1 until you reach the end (which will be the beginning in this case because you are stepping in reverse). This is why you end up with [2, 1, 0].
If you wanted to reverse the whole list so it is the exact opposite of your first example, you'd need to do the following:
>>> numbers[-1:1:-2]
[20, 18, 16, 14, 12, 10, 8, 6, 4, 2]
Here we start at the very end (-1), we want it to finish at index 1 (so we don't miss index 2), then we step backwards by -2.
Hope this clears things up.
allensimbul
3,065 PointsThank you that makes a lot more sense now :)
allensimbul
3,065 Pointsallensimbul
3,065 Points"In your second example, numbers[2::-1], you are again telling it to start at index 2, and to decrement by 1 until you reach the end (which will be the beginning in this case because you are stepping in reverse). This is why you end up with [2, 1, 0]." I understand this logic, by why does numbers[::-1] give the reverse of numbers[:] rather than [] because woudnt it start from 0 to the end so if decrement by -1 it just be []?
Ryan S
27,276 PointsRyan S
27,276 PointsOh ok, I think I may have misunderstood your original question.
Yes you are correct that you will still start at index 0, but remember that there are essentially 2 sets of indexes, there are the positive ones that count from the beginning of the iterable, and the negative ones that count from the end. And when you are using a negative step from 0, you will be stepping through the negative ones.
Back to your numbers example, say we had the following slice:
Since no start parameter is specified, we start at index 0. When you count backwards from zero with a step of -1, your first value will be the value at
numbers[-1], thennumbers[-2], and so on, until you reachnumbers[-21], which is the end of the slice, but equivalent to the beginning of the iterable.