Help with Start-Stop-Step slicing

Question

Hi

I'm having a bit of difficulty to understand slicing with a negative step.

Could somebody help me understanding these two examples?

EXAMPLE ONE
print('game of dice'[::-3])
#result: edom

EXAMPLE TWO
a=['a','d','b','c']
a=a[2:0:-1]
print(a)
#result: ['b','d']

Thank you in advance!

Answer 1

Slice syntax is something like this:
list[start:stop:step]
So the slice begins with the element at index "start" (if negative, then count from the end, -1 meaning the last element)
The index "stop" is non-inclusive, so never actually selected. Same as with the range() function.
The "step" is the direction and how many elements are skipped.

So in summary,
a[2:0:-1] means
start with element at index 2: 'b'
stop before you reach index 0: 'a'
step backwards by one element
So index 2 and 1 are included in the slice, giving ['b', 'd']

Answer 2

game of dice
In reverse slicing, started from last index value as -1
step count is -3, so
Index at -1 is e
-1 + -3 = -4 is d
-4 + -3 = -7 is o
-7 + -3 =-10 is m

Output is edom

Or ex2:
Simply like
'a' index at 0, - 4
'd' 1 - 3
'b' 2 - 2
'c' 3 -1
a[[2,0,-1] means extract reverse starting at index 2 to 0(not included)
So 'b' 'd' output

Answer 3

Tibor Santa and Jayakrishna🇮🇳,

Thank you both very much for your clear explanations and for the time spent writing them.

While I was sure the slicing was moving backwards, I had them wrong for two very stupid reasons... (I'm even ashamed of saying them😄)

Example1: forgot to count the spaces.
Example2: forgot that the "end" position is not included.

Thank you again!