使用 for 循环迭代嵌套列表以编辑内部列表时遇到问题

Question

I just can't figure out how to manage some arp tables. The only data that is relevant to me is the ip address and the mac address listed. I don't know if I should,

1. iterate through the nested_list , removing [0, 2, 4, 5]

2. iterate through the lists, assigning [1] and [3] to new lists.

I have tried both ways and I'm just not getting there. My syntax knowledge is clearly lacking.

Before any of the code you're about to see I start by accessing a file using with / as and use .readlines() to create a list of strings I call read_output .

Here is my code as it stands now:

nested_list = []

for index in read_output: 
    nested_list.append(index.split()) #this creates the nested list.

print(nested_list)

Which works fine and gives me this output as example:

[['Internet', '10.220.88.1', '135', '0062.ec29.70fe', 'ARPA', 'FastEthernet4'],
 ['Internet', '10.220.88.40', '144', '001c.c4bf.826a', 'ARPA', 'FastEthernet4']]

The arp list can be a lot bigger than this but I thought I would save some space for our example. I want to be able to go through the list and add/remove/extract data as I see fit but every time I try I am met with varying degrees of failure.

If i just:

print(nested_list[0][3])

I get the expected result of 0062.ec29.70fe but when I try to use a second for loop to access every [n][0] , [n][2] , etc., things go horribly awry.

I'm starting with:

for index in read_output:
    nested_list.append(index.split())
    for indx in nested_list:
    # --->insert some horrible excuse for python here

My intended output is taking the nested list above and trimming it down to:

[['10.220.88.1', '0062.ec29.70fe'],
 ['10.220.88.40', '001c.c4bf.826a']]

Answer 1

Don't append the whole sub-list in the first place but only the two items you want:

nested_list = []

for index in read_output: 
    inner = index.split()
    nested_list.append([inner[1], inner[3]]) 

Answer 2

Alright so I hope I'm understanding this correctly, but from what I understand, you're getting your data correctly, which is this:

[['Internet', '10.220.88.1', '135', '0062.ec29.70fe', 'ARPA', 
'FastEthernet4'],
 ['Internet', '10.220.88.40', '144', '001c.c4bf.826a', 'ARPA', 
'FastEthernet4']]

So you're correct in seeing that this is actually a list, containing two sub-lists. We can take advantage of this in our code below.

list = [['Internet', '10.220.88.1', '135', '0062.ec29.70fe', 'ARPA', 'FastEthernet4'],['Internet', '10.220.88.40', '144', '001c.c4bf.826a', 'ARPA', 'FastEthernet4']]

So now that we've contained that list, similar to how you did, we can loop through that list.

for item in list:
    print ("IP: {0}\nMac Address:{1}\n".format(item[1],item[3]))

This gave me the output that I hope you're expecting which is:

IP: 10.220.88.1
Mac Address: 0062.ec29.70fe

IP: 10.220.88.40
Mac Address: 001c.c4bf.826a

So what we did is run a loop through the outer list, and since we knew that the IP address was always going to be at index of 1 and 3 of the inner list, we just accessed them through their index.

You don't need to run another loop through the inner lists since you already have this knowledge of their placement.

To store these addresses you'd probably want to store them in a dictionary since you have a key, value pair kind of setup.

address = {}
count += 1
for item in list:
    count += 1
    address["list: {0}".format(count)] = [item[1], item[3]]
    print ("IP: {0}".format(item[1])
    print ("Mac Address: {0}".format(item[3]))
    print ()
print (address)

With this we set a count to increase our key name, and with each new list, we saved the two specific values we wanted in another list. A good challenge would be to see how you could convert it to a form where your key is the ip and mac and your values are, well the values. I thought about doing that but it even boggled my mind a bit and I wanted to get my answer out to you quickly. Hope this helps!