List Comprehension and traversing through a list

Question

[k for k in range (1,42) if k%2 != 0]

output: [1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41]

Now, I want to make it to:

[-1, 3, -5, 7, -9, 11, -13, 15, -17, 19, -21, 23, -25, 27, -29, 31, -33, 35, -37, 39, -41]

So I tried:

def test(N):
    k = []
    for i in range (1,N+1):
        if(i%2 != 0):
            k.append(i)
    for b in k[::2]:       <--- it changes the value but doesn't update the list
        b = -b
    return k
test(43)

Any ideas or suggestions on how to approach this problem? Or the other question is, how could I traverse the even numbered index in the list while updating the element to a negative value

Looks like an alternating series if you've taken Calculus. Try multiplying by `(-1)**(k + 1)` — Malik Brahimi, Jan 26 '16 at 01:13

Mike Müller · Accepted Answer · 2016-01-26T01:20:28.693

2

This works:

numbers = (k for k in range(1, 42) if k%2 != 0)
[x * [-1, 1][n % 2] for n, x in enumerate(numbers)]

Output:

[-1, 3, -5, 7, -9, 11, -13, 15, -17, 19, -21, 23, -25, 27, -29, 31, -33, 35, -37, 39, -41]

Performance

The approach using modulo is a bit faster than the one for alternating series as suggest by Malik Brahimi in a comment:

%%timeit
numbers = (k for k in range(1, 420000) if k%2 != 0)
[x * (-1)**(n + 1) for n, x in enumerate(numbers)]
1 loops, best of 3: 263 ms per loop

%%timeit
numbers = (k for k in range(1, 420000) if k%2 != 0)
[x * [-1, 1][n % 2] for n, x in enumerate(numbers)]
1 loops, best of 3: 210 ms per loop

edited Jan 26 '16 at 01:20

answered Jan 26 '16 at 01:11

Mike Müller

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1

Looks like an alternating series if you've taken Calculus. Try multiplying by `(-1)**(k + 1)` – Malik Brahimi Jan 26 '16 at 01:14
@MalikBrahimi Thanks for the hint. Added some performance measurements for the different approaches. – Mike Müller Jan 26 '16 at 01:21
@MikeMüller That's a very complicated list comprehension! But thank you! – misheekoh Jan 26 '16 at 01:38

score 1 · Answer 2 · answered Jan 26 '16 at 01:10

1

[k if k%4==3 else -k for k in range (1,42) if k%2 != 0]

answered Jan 26 '16 at 01:10

Karoly Horvath

94,607
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score 1 · Answer 3 · answered Jan 26 '16 at 01:10

b is a reference to the (immutable) integer stored in k. By updating b to equal -b, you're not modifying the list, you're simply changing the thing that b referecnes, for the int 2 object to the int -2 object. You might be better off just doing the negation as you build the list:

def test(N):
    k = []
    for i in range (1,N+1, 2): # i increases in steps of 2
        if i % 4 == 1:
            i = -i
        k.append(i)
    return k

test(43)

List Comprehension and traversing through a list

3 Answers3

Performance