Find Integers in list

Question

I have a list like this, named x (which I have already split):

['16','bob','2440', '34']

I want to write a code that checks to see if any of the numbers are negative. The code I tried does not work. This is what I have tried:

for num in x:
    if num < 0:
        print ("Negative number")

see the function try_parse_int https://stackoverflow.com/questions/2262333/is-there-a-built-in-or-more-pythonic-way-to-try-to-parse-a-string-to-an-integer — Golden Lion, Jun 15 '21 at 20:06

Hyperboreus · Answer 1 · 2013-08-01T19:09:58.103

6

Your list contains only strings. So you should cast them to floats (or integers, whatever you need) first:

a = ['"16','bob','2440', '-2', '34"']
for x in a:
    try:
        if float (x) < 0: print ('negative')
    except: continue

EDIT: I changes int to float as OP is asking for numbers and not exclusively integers.

edited Aug 01 '13 at 19:09

answered Aug 01 '13 at 19:02

Hyperboreus

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1

Possibly also removing the spurious quotes. or splitting better in the first place. – Wooble Aug 01 '13 at 19:02
3

There I am not sure where OP's data comes from. Maybe `"16` is a valid element, but surely no integer. – Hyperboreus Aug 01 '13 at 19:03

score 3 · Answer 2 · answered Aug 01 '13 at 19:03

You need to turn your numbers into integers first; use a predicate function to try to do this:

def negative(string):
    try:
        return int(string.strip('"')) < 0
    except ValueError:
        return False

The predicate function here also removes quotes; your input list looks like it was not cleaned up properly and you may want to do so first before testing for negative values.

Then use that to test for negative values:

negative_values = [v for v in a if negative(v)]

or test if there are any negative values:

if any(negative(v) for v in a):
    print "No negative values please!"

Or using Martijn's code, to check if a negative number is contained (which I think is what OP is looking for): `any (negative (x) for x in a)` — Hyperboreus, Aug 01 '13 at 19:04

alecxe · Answer 3 · 2013-08-01T19:15:40.823

2

How about checking for - sign in the beginning of an item and for the rest of an item to consist of digits? One-liner:

>>> a = ["-1", "aa", "3"]
>>> any(s.startswith('-') and s[1:].isdigit() for s in a)
True

Using any, because you've said that you want to write a code that checks to see if any of the numbers are negative.

Note: if there can be negative floats, then just replace s[1:] with s[1:].replace(".", "").

Hope that helps.

edited Aug 01 '13 at 19:15

answered Aug 01 '13 at 19:06

alecxe

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Interesting approach, but depending on OP's input data, this could give unwanted results such as the problem documented in: http://stackoverflow.com/questions/10604074/python-isdigit-function-return-true-for-non-digit-character-u-u2466 (though this could also be desirable, so +1 for that). – Zigsaz Aug 01 '13 at 19:14
Thank you, good point. Yeah, trying `int()` or `float()` may be safer. – alecxe Aug 01 '13 at 19:17

score 0 · Answer 4 · answered Aug 01 '13 at 19:07

First, you need to understand that neither '"16' nor '2440' are numbers - they are strings.

Secondly, you need to figure out what you want to do with '"16' - it doesn't represent a number, but I assume you want it to. You could alter these strings, but you should probably just use an appropriate method of splitting in the first place.

That said, you can do this:

x = ['"16','bob','2440', '34"']

def int_is_negative(s)
    try:    
       return int(s) < 0
    except ValueError:
       return False

is_negative_num = [int_is_negative(s) for s in x]

Find Integers in list

4 Answers4