What is the output of this code ?

Question

a = [ x for x in range(4) ]
print(sum(a[1::-2] + a[::-3]))

unable to understand negative steps...
plz explain it in step by step manner..
Thanks

Answer 1

Take note that:

a[start:end:step] ---> since the `step` is negative, the slice starts at index `start` and goes up to `end+1` (in other words, `end` is not inclusive, just like when `step` is positive and goes up to `end-1`).

With that knowledge, what the slices are doing:

a[1::-2] ---> the start is 1 and the default end is -1 not inclusive or 0 inclusive, with a step -2.
Indexes the slice reaches: 1
Thus, a[1::-2] == a[1] == [1]

a[::-3] ---> the default start is len(a)-1 and the default end is -1 not inclusive or 0 inclusive, with a step -3.
Indexes the slice reaches: 3, 0
Thus, a[1::-2] == a[3] + a[0] == [3, 0]

If you still don't understand, my advice is you should try with a bigger list so you have more data to analyze. Right now your list has only 4 elements so it is difficult to understand what the slices are really doing.

Answer 2

0123 [1::-2]
begin at index 1....1
go two steps back as long as possible... not possible... first sum is 1

0123 [::-3] since you go backward start at the end.
3

go back 3steps ...0.
no more step possible
second sum is 3+0=3

Answer 3

I got it...
Thanks

print(a[1::-2])
print(a[::-3)

output ----
[1]
[3,0]

print(a[1::-2] + a[::-3])
output ---
[1,3,0]

print(sum(a[1::-2] + a[::-3]))

output---
1+3+0 = 4

Answer 4

Thanks Eduardo Petry
so what is the output after executing print statement ??

Answer 5

Oma Falk
If we run these two print statement,
print(a[1::-2])
print(a[::-3)

output ----
[1]
[3,0]

Then what is the output of
print(sum(a[1::-2] + a[::-3]))

????

If we run above print statement in Anaconda jupyter IDE, we get the output 4...How ??