Removing matching items from a list?

[kevin4fong at gmail.com]

On Saturday, August 3, 2013 3:37:41 PM UTC-7, Steven D'Aprano wrote:
> On Sat, 03 Aug 2013 13:12:55 -0700, kevin4fong wrote:
> 
> 
> 
> > Basically, I'm trying to find out how to remove matching items from a
> 
> > list. But there doesn't seem to be any information on how to go about
> 
> > doing this specific function.
> 
> 
> 
> The documentation cannot possibly cover every single one of the infinite 
> 
> number of things somebody might want to do with Python. Programming is 
> 
> about figuring out how to do the things you want from the tools provided 
> 
> by the language.
> 
> 
> 
> 
> 
> > For example, what I want is:
> 
> > 
> 
> > let's say there is a list:
> 
> > 
> 
> > pHands[0] = ['ad', 'ac', 'as', 'ah', '7d', '8s', '9d', 'td', 'js', 'jd']
> 
> > 
> 
> > So up there, my list, which is named pHands[0] has ten items in it.
> 
> 
> 
> Ten items? I wouldn't have known that if you didn't say so! *wink*
> 
> 
> 
> But seriously... you don't need to repeat obvious things like that. We 
> 
> can count as well as you can.
> 
> 
> 
> But more importantly, it is irrelevant. Is "ten items" important to this 
> 
> problem? No. What if there were nine, or thirty five, or seven? Would the 
> 
> problem of "removing matching items" change? No, the problem remains the 
> 
> same regardless of how many items there are.
> 
> 
> 
> 
> 
> > I'm trying to make a function where a search is initiated into the list
> 
> > and any matching items with a first matching number/letter reaching four
> 
> > are removed
> 
> 
> 
> In general, rather than removing items from a list, it is faster and 
> 
> safer (less error-prone) to create a new list with the items not removed. 
> 
> But either way, when trying to program, start by thinking about how *you* 
> 
> would do this task by hand:
> 
> 
> 
> 1) I would start by counting how many items start with each initial 
> 
> letter: in your example I would count those starting with 'a' (4), 
> 
> '7' (1), and so on.
> 
> 
> 
> 2) Then go through those initial letters, and pick out the ones equal to 
> 
> 4 (or should that be "four or more"?). In this case, only 'a' is 4 or 
> 
> more, but there could be multiple examples, say, 4 'a's and 6 'b's.
> 
> 
> 
> 3) Then, either delete those items from the original list, *or* make a 
> 
> new list with the remaining items. Making a new list is usually better.
> 
> 
> 
> Now start turning each step into Python code.
> 
> 
> 
> Step 1:
> 
> 
> 
> Iterate over each item, grab the first character of that item, and add 
> 
> one to a counter.
> 
> 
> 
> counts = {}  # Start with an empty dict.
> 
> for item in pHands[0]:
> 
>     c = item[0]  # Grab the first character.
> 
>     if c in counts:
> 
>         counts[c] += 1
> 
>     else:
> 
>         counts[c] = 1
> 
> 
> 
> 
> 
> This sets the counter to 1 the first time each character is seen, 
> 
> otherwise increments the counter by 1. There's a ready-made tool for 
> 
> doing that:
> 
> 
> 
> from collections import Counter
> 
> counts = Counter(item[0] for item in pHands[0])
> 
> 
> 
> but under the hood, it's essentially doing the same thing as I wrote 
> 
> above.
> 
> 
> 
> 
> 
> Steps 2 and 3 can be done together:
> 
> 
> 
> new_list = []
> 
> for item in pHand[0]:
> 
>     c = item[0]
> 
>     # Look up the character in the counter.
> 
>     if counts[c] < 4:
> 
>         new_list.append(item)
> 
> 
> 
> pHand[0] = new_list
> 
> 
> 
> 
> 
> 
> 
> -- 
> 
> Steven

Thank you. I found all the replies useful but yours the most useful because of the descriptions and indepth steps so I could understand what the code did.

Would you also happen to know how I could go about setting up a list that keeps track of the removed sets?

For example, if there were 4 a's removed, that would be one set. And the list would be:

['a']

but if there was also 4j's in addition to the a's. It would be:

['a', 'j']

And so forth....