Append element to a list inside nested list - python

Question

I am developing a procedure add_to_index, that takes 3 inputs:

an index: [[,[url1,url2,...]],...]
a keyword: String
a url: String

If the keyword is already in the index, url is added to the list of urls associated with that keyword.

If the keyword is not in the index, a new element is to the index:

[keyword,[url]]

CODE

index = []

def add_to_index(index,keyword,url):
    flag = 0
    count = 0
    for lists in index:
        count += 1
        if(lists[0]==keyword): 
            index[count][1].append(url)

    if(flag ==0):
        index.append([keyword,url])   

#calling the function below

add_to_index(index,'google','http://google.com')
print index

output -> [['google', 'http://google.com']]

add_to_index(index,'computing','http://acm.org')
print index

output -> [['google', 'http://google.com'], ['computing', 'http://acm.org']]

add_to_index(index,'google','http://gmail.com') 
print index

error->

index[count][1].append(url)
AttributeError: 'str' object has no attribute 'append'

Expected output:

 [['google', ['http://google.com', 'http://gmail.com']], 
 ['computing', ['http://acm.org']]]

It really seems like this is the wrong data structure for this: you'd be much better off using a dictionary and a dictbuilder. — ekrah
– ekrah, Commented Dec 15, 2014 at 20:11
It was from a quiz that I got at a competitive programming competition. — akshaynagpal
– akshaynagpal, Commented Dec 15, 2014 at 20:12

Bhargav Rao · Accepted Answer · 2014-12-15 20:17:35Z

3

You have done three mistakes, Firstly you have not used the flag and secondly you are adding the url as a string. And finally as Kaivosuketaja has mentioned in the comment, count should be incremented in the end. It can be done otherwise as

index = []

def add_to_index(index,keyword,url):
    flag = 0
    count = 0

    for lists in index:

        if(lists[0]==keyword): 
            flag = 1
            index[count][1].append(url)
        count += 1

    if(flag ==0):
        index.append([keyword,[url]])   
        # Take note of append away here
#calling the function below

add_to_index(index,'google','http://google.com')
print index

add_to_index(index,'computing','http://acm.org')
print index

add_to_index(index,'google','http://gmail.com') 
print index

The output now is

[['google', ['http://google.com']]]
[['google', ['http://google.com']], ['computing', ['http://acm.org']]]
[['google', ['http://google.com', 'http://gmail.com']], ['computing', ['http://acm.org']]]

edited Dec 15, 2014 at 20:17

answered Dec 15, 2014 at 20:11

Bhargav Rao

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2 Comments

Kaivosukeltaja Over a year ago

Make that three mistakes - count should be incremented at the end of the for loop.

Bhargav Rao Over a year ago

@Kaivosukeltaja I'm sorry if you feel bad, But I took three mins to write your name as I couldn't copy and paste due to reasons. :( ..

rlms · Accepted Answer · 2014-12-15 20:16:12Z

0

I think this is what you want:

index = []

def add_to_index(index,keyword,url):
    flag = 0
    count = 0
    for lists in index:        
        if lists[0] == keyword: 
            lists[1].append(url)
            flag = 1
        count += 1

    if flag == 0:
        index.append([keyword, [url]])   

#calling the function below

add_to_index(index,'google','http://google.com')
print index

answered Dec 15, 2014 at 20:16

rlms

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Comments

Raydel Miranda · Accepted Answer · 2014-12-15 20:22:54Z

I will suggest the use of dictionaries for this:

index = {}

def add_to_index(index, keyword, url):
    if keyword not in index:
        index[keyword] = [url]
    else:
        index[keyword].append(url)


>>> add_to_index(index,'computing','http://acm.org')
>>> add_to_index(index,'google','http://gmail.com') 
>>> add_to_index(index,'google','http://gmail.com') 
>>> index
{'computing': ['http://acm.org'], 'google': ['http://gmail.com', 'http://gmail.com']}

You could even make index a non global variable by implementing a simple class(ofcourse, this is possible with nested lists too):

class Index(object):

    def __init__(self):
        self.index = {}

    def add_to_index(self, keyword, url):
        if keyword not in index:
            self.index[keyword] = [url]
        else:
            self.index[keyword].append(url)

ekrah · Accepted Answer · 2014-12-15 20:24:27Z

First, you're trying to append to a string that is inside of the list you want to append to. Then, you forgot to say flag = 1 when you've found the keyword. Try the following:

index = []

def add_to_index(index,keyword,url):
    flag = 0
    count = 0
    for lists in index:
        if(lists[0]==keyword): 
            index[count][1].append(url)
            flag = 1
    count += 1
    if(flag ==0):
        index.append([keyword,url])   

#calling the function below

add_to_index(index,'google','http://google.com')
add_to_index(index,'computing','http://acm.org')
add_to_index(index,'google','http://gmail.com') 
print index

I think you'd be much better off using a defaultdict, though. It automatically searches for a keyword and adds the item to the existing keyword, or if it isn't found, creates a new keyword.

mikeyro · Accepted Answer · 2016-03-26 22:57:42Z

0

You could simplify things a little by getting rid of the flag and count variables.

index = []

def add_to_index(index, keyword, url):
    for e in index:
        if e[0] == keyword:
            e[1].append(url)
            return

        else:
            index.append([keyword,[url]])

answered Mar 26, 2016 at 22:57

mikeyro

1

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