Python - Identify a negative number in a list

Question

I need help doing a program which should receive ten numbers and return me the number of negative integers I typed.

Example:

If I enter:

1,2,-3,3,-7,5,4,-1,4,5

the program should return me 3.

How can I do it?

@D.Shawley That question has been banned: http://meta.stackexchange.com/a/172760/182887 — jamylak, Apr 14 '13 at 05:35

score 51 · Accepted Answer · answered Apr 13 '13 at 22:24

Break your problem down. Can you identify a way to check if a number is negative?

if number < 0:
    ...

Now, we have many numbers, so we loop over them:

for number in numbers:
    if number < 0:
        ...

So what do we want to do? Count them. So we do so:

count = 0
for number in numbers:
    if number < 0:
        count += 1

More optimally, this can be done very easily using a generator expression and the sum() built-in:

>>> numbers = [1, 2, -3, 3, -7, 5, 4, -1, 4, 5]
>>> sum(1 for number in numbers if number < 0)
3

score 31 · Answer 2 · edited Mar 03 '22 at 08:46

31

sum(n < 0 for n in nums)

This is the most Pythonic way to do it.

edited Mar 03 '22 at 08:46

Peter Mortensen

answered Apr 14 '13 at 00:31

jamylak

7

I think `any` is more clear than `sum`. – jds Feb 17 '15 at 19:30
4

@gwg The question asks for the total number of negative integers. `any` would return `True` or `False`, which isn''t the desired answer – jamylak Feb 18 '15 at 00:36
`numbers = [1, 2, -3, 3, -7, 5, 4, -1, 4, 5] sum(number < 0 for number in numbers)` The result is 3. – Boštjan Mejak Jun 19 '19 at 21:52
@BoštjanMejak Just like OP said yeah – jamylak Jun 20 '19 at 13:29
The OP’s proposal is very beautiful! Python’s idioms are supreme beauty. – Boštjan Mejak Jun 20 '19 at 14:04

score 2 · Answer 3 · answered Apr 13 '13 at 22:32

2

Or you could use filter to "filter" out the negatives.

total = len(filter(lambda x: x < 0, my_list))

answered Apr 13 '13 at 22:32

squiguy

3

Using `lambda` and `filter()` together is slower and less readable than a generator expression. – Gareth Latty Apr 13 '13 at 22:41
@Lattyware Well `filter` takes a `lambda`. I just like a functional approach :). I agree with you 100%. I even +1 your answer. – squiguy Apr 13 '13 at 22:45
`filter` takes a function - creating it using `lambda` is just useful in some cases. I was just commenting to say that this is generally a worse way to do this task. – Gareth Latty Apr 13 '13 at 22:54
most of the people behind python hate the use of `lamda`s, however they've gained a large following. I try to avoid them whenever possible. – jamylak Apr 14 '13 at 00:34
@jamylak Sure, just thought I would share a different way than putting up a duplicate answer. – squiguy Apr 14 '13 at 00:35
1

@squiguy you are also relying on `filter` building a `list` which wastes space and also means you have to add an enclosing call to `list(...)` under python 3 – jamylak Apr 14 '13 at 00:37
@squiguy The problem with this answer now is that a large portion of people use python 3 where this doesnt work unless you add `len(list(filter(lambda x: x < 0, my_list))` – jamylak Feb 14 '18 at 02:37

3 Answers3