Check if there is at least one element starting with specific character in a list

Question

I have a list containing a group of numbers (str) , and I want to check if there is at least one element in the list starting with '+' .

The problem with my code is that an infinite loop will occur if no number starts with '+', I need a better solution?

my_list = ['11112352', '222222003', '5682052003', '21543003', '98756003', '+004454883']
while True:
    for number in my_list:
        if number.startswith('+'):
            break

Avoid using `while` loops for such problems and stick with just for loops if possible since all you are doing is iterating through each element of the list — Melvin Abraham, Nov 17 '20 at 15:44
Hi!Welcome to SO! Could you please indicate why you need the `while` loop? Your problem requires iterating through each element of the list and break if a string starts with +. Hence the while loop is not necessary. — Ganesh Tata, Nov 17 '20 at 15:45
Hey, thanks, My problem is to check that there is at least one element starting with '+' in the list , I am asking for a better solution — Faith, Nov 17 '20 at 15:47
[Using `startswith` for a single character is inefficient](https://stackoverflow.com/q/13270888/13552470). As in my answer, simply use a slice of `[0]`. — Red, Nov 17 '20 at 15:59
we can use startswith for one or multiple characters , it's a more generic solution that we can apply for different cases , that's why I accepted it — Faith, Nov 17 '20 at 16:05

score 2 · Answer 1 · answered Nov 17 '20 at 15:45

my_list = ['11112352', '222222003', '5682052003', '21543003', '98756003', '+004454883']
for number in my_list:
    if number.startswith('+'):
        break

This:

while True:

is unnecessary, the for loop will already check every item in the list

To make it clearer, if it print every iteration:

my_list = ['11112352', '222222003', '5682052003', '21543003', '98756003', '+004454883']
for number in my_list:
    print(number)
    if number.startswith('+'):
        break

I get

so as you can see the while loop is unnecessary

Jan Christoph Terasa · Accepted Answer · 2020-11-17T16:10:19.480

To check if any element starts with a '+', use builtin functions any and str.startswith. You can nearly write down the exact words as Python code:

starts_with_plus = any(element.startswith('+') for element in my_list)

This will return a boolean value. This will also return on the first instance where the condition is True, like returning out of a loop, making this very efficient.

If you want to check for other prefixes as well, just put it in a small function:

def starts_with(lst, prefix):
    return any(element.startswith(prefix) for element in lst)

starts_with_plus = starts_with(my_list, '+')

score 1 · Answer 3 · answered Nov 17 '20 at 15:46

1

Here's a simpler solution:

my_list = ['11112352', '222222003', '5682052003', '21543003', '98756003', '+004454883']
result = [x for x in my_list if x.startswith('+')]
if result:
    # do something if result is not empty
else:
    # result is empty -- no '+' entries in my_list

answered Nov 17 '20 at 15:46

Tom Barron

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An issue with this is, for example, if the list is really really long, and say the first element starts with a `+`, you would still be checking every element in the list unnecessarily. – Sash Sinha Nov 17 '20 at 15:50

Angel Salamanca · Answer 4 · 2020-11-18T16:30:33.293

1

If you just need a True/False result, you can try this:

any(number for number in my_list if number[0] == "+")

Function any delivers True if at least one element of the list passed as argument is True.

Edited as suggested by Ann Zen. See also How exactly any works

edited Nov 18 '20 at 16:30

answered Nov 17 '20 at 15:48

Angel Salamanca

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This is inefficient as it will need to process *all* the strings before returning `True`/`False`. Please refer to my answer for the proper way of using `any`. – Red Nov 17 '20 at 15:55
1

The enclosing `[ ]` are actually counter-productive here since they'll force the whole list to be evaluated. – tzaman Nov 17 '20 at 15:55

score 0 · Answer 5 · answered Nov 17 '20 at 15:47

0

Use the any method:

my_list = ['11112352', '222222003', '5682052003', '21543003', '98756003', '+004454883']
print(any(s[0] == '+' for s in my_list))

Output:

True

answered Nov 17 '20 at 15:47

Red

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score 0 · Answer 6 · answered Nov 17 '20 at 15:48

You can do this with the help of a for loop and a flag variable:

flag = 0

for item in my_list:
    if item.startswith('+'):
        flag = 1
        break

print(flag)

If you get an output of 0, then it's obvious that no element starts with a +. On the other hand, if the output of the print statement is 1, there's at least one element that starts with a +.

Check if there is at least one element starting with specific character in a list

6 Answers6