i want to take input as
3 10 20 30
i.e, n array elements
I tried -
n , arr = int(input()), list(map(int(input).split()[:n]))
It shows error as n is not defined
2 Answers
Think about how Python interprets your statement:
- it evaluates the r-value (the right-hand-side part of the assignment), which is a
tuple, then - assigns that
tupleto the l-value:(n, arr).
But in step 1, it encounters a yet-unknown quantity: n.
You would therefore do, instead (and assuming s = '3 10 20 30', as it isn't a good idea to shadow the built-in function input()):
arr = [int(x) for x in s.split()]
n = len(arr) # if you really need it
Edit
It has been pointed out that the expected result may be n: 3 and arr: [10, 20, 30]. If that is indeed the case:
n, *arr = [int(x) for x in s.split()]
6 Comments
DeepSpace
This gives the wrong output. If
s = '3 10 20 30' then OP wants n = 3, arr = [10, 20, 30]
Pierre D
It's not clear from the question, but I can see that would be a plausible way to interpret it.
Jan Christoph Terasa
asterisk tuple packing/unpacking of your edit is the best solution, if you need to check
n against the number of elements after that.Pierre D
you should keep your answer as well: it's fun to see the walrus operator in action!
Jan Christoph Terasa
Yes, but in this case it's overkill and very hard to read. So far the only "useful" cases of assignment expressions I have found are conditionals.
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Using Python 3.8+ this can be solved quite elegantly by using assignment expressions:
n, arr = int((v := input().split())[0]), list(map(int, v[1:]))
2 Comments
DeepSpace
Nice! but I believe OP meant to
len(v := input().split()[1:]), list(map(int, v))Jan Christoph Terasa
Oh, yes, sorry.
arr = list(map(int, input().split()))[1:]nandarr?