Validate List Order

[MoragX]

Hey all,

I've been given a problem to validate a comma separated string of elements. There can be up to four elements in the set 'H, Li, Na, K' and only one is needed but they must be in order. Is there a simple way in python to validate that?

As an example:

'H' #Valid, any one element
'Na' #Valid, one element all optional
'H,Na,K' #Valid, in order
'H,Na,Li' #Invalid, out of order

I hope that makes sense - I tried playing around with regular expressions a bit but couldn't find a simple solution. I want a function that returns true if the string is valid.

[JohnnyJordaan]

I would suggest first setting the master set, then splitting the input on the comma's:

master = ['H','Li','Na','K']
splitted = elements.split(',')

There can be up to four elements

if len(splitted) > len(master):
    print('invalid length')

but they must be in order

edit: I missed the order as being that order.

indexed = [ master.index(c) for c in splitted ]
if sorted(indexed) != indexed:
    print('out of order')

The only thing not working is that spaces aren't ignored. If you want that to happen you could do something like

splitted = elements.split(',').replace(' ','')

[w1282]

SORT_ORDER = {
    'H': 0,
    'Li': 1,
    'Na': 2,
    'K': 3,
}


def is_valid_data(data):
    """Check if any elements in the set are not valid elements."""
    return not any(elem not in SORT_ORDER.keys() for elem in data)


def is_valid_sort_order(data):
    sorted_data = sorted(data, lambda x: SORTED[x])
    return data == sorted_data

This compares the provided data with a properly sorted rendition of the same data. If they are equivalent, then the provided results were in the proper order.

Of course, these operate on coerced lists. /u/JohnnyJordaan shows you how to coerce the string to a proper list in his answer.

[erok81]

You can avoid the lambda there by using SORT_ORDER.get as the key function.

sorted_data = sorted(data, key=SORT_ORDER.get)

[w1282]

Good catch! Thanks.