python: return False if there is not an element in a list starting with 'p='

Question

having a list like

lst = ['hello', 'stack', 'overflow', 'friends']

how can i do something like:

if there is not an element in lst starting with 'p=' return False else return True

?

i was thinking something like:

for i in lst:
   if i.startswith('p=')
       return True

but i can't add the return False inside the loop or it goes out at the first element.

Since you only know it is `False` once you have tested all of the elements, you should return `False` *after* the loop finishes. — Scott Hunter, Jan 20 '18 at 17:09
Then put `return False` *outside* the loop, or in the `else` block. You can't be sure there are none until the end. Or just use `return any(...)`. — jonrsharpe, Jan 20 '18 at 17:09

score 10 · Accepted Answer · edited Jan 05 '20 at 00:58

10

This will test whether or not each element of lst satisfies your condition, then computes the or of those results:

any(x.startswith("p=") for x in lst)

edited Jan 05 '20 at 00:58

Martijn Pieters

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answered Jan 20 '18 at 17:08

Scott Hunter

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4

Why the list comprehension inside `any` instead of a generator comprehension `any(x.startswith("p=") for x in lst)`? – Patrick Haugh Jan 20 '18 at 17:09
1

I removed the list comprehension. You were wasting memory and tests. With a generator expression `any()` can *short-circuit the loop*; when the first element passes the test there is no need to test everything that follows any more. – Martijn Pieters Jan 05 '20 at 01:01

mehrdadep · Answer 2 · 2020-02-23T08:41:43.300

5

use any conditions to check all the elements in a list with the same condition:

lst = ['hello','p=15' ,'stack', 'overflow', 'friends']
return any(l.startswith("p=") for l in lst)

edited Feb 23 '20 at 08:41

answered Jan 20 '18 at 17:08

mehrdadep

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score 5 · Answer 3 · answered Jan 20 '18 at 17:14

You can use builtin method any to check it any element in the list starts with p=. Use string's startswith method to check the start of string

>>> lst = ['hello', 'stack', 'overflow', 'friends']
>>> any(map(lambda x: x.startswith('p='),lst))
>>> False

An example which should result in True

>>> lst = ['hello', 'p=stack', 'overflow', 'friends']
>>> any(map(lambda x: x.startswith('p='),lst))
>>> True

score 4 · Answer 4 · answered Jan 20 '18 at 17:09

I suggest using an iterator such as

output = next((True for x in lst if x.startswith('p=')),False)

This will output True for the first lst element that starts with 'p=' then stop searching. If it reaches the end without finding 'p=', it returns False.

kyriakosSt · Answer 5 · 2018-01-20T17:40:13.860

3

Well, let's do it in two parts:

First of all, you could create a new list in which each element would be a string containing only the first 3 characters of each original item. You can use map() to do so:

newlist = list(map(lambda x: x[:2], lst))

Then, you only need to check if "p=" is one of those elements. That would be: "p=" in newlist

Combining both of the above in a function with a single statement should look like this:

def any_elem_starts_with_p_equals(lst):
    return 'p=' in list(map(lambda x: x[:2], lst))

edited Jan 20 '18 at 17:40

answered Jan 20 '18 at 17:06

kyriakosSt

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`return 'p=' in list(map(lambda x: x[:2], lst))` this is not valid python syntax. Unless you put this in a function – Sohaib Farooqi Jan 20 '18 at 17:21
@GarbageCollector I think the OP really means that the code is in a function, judging by the wording and the example posted – kyriakosSt Jan 20 '18 at 17:25
Yes I guess so. It would be better if you write the complete function in your answer to avoid any ambiguity. – Sohaib Farooqi Jan 20 '18 at 17:32

score 3 · Answer 6 · answered Jan 20 '18 at 17:08

3

Try this

if len([e for e in lst if e.startwith("p=")])==0: return False

answered Jan 20 '18 at 17:08

whackamadoodle3000

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python: return False if there is not an element in a list starting with 'p='

6 Answers6