CODEWITHSUNDEEP
python
user9546355
user9546355

Reputation: 43

Add element from list to dictionary and increase value

I have dictionary and a list. List only contains items, dictionary contains items and how many of a given item do I have. What I'm trying to do is to iterate through a dict and list, and if the key in a dict is the same as item in a list I only want to update its value by 1 but if there are no the same items I want to add the item form list to dict and set its value to 1.

stock = {apple: 1, banana: 4, orange: 10}
delivery = [apple, apple, grapefruit]

    for k, v in stock.items():
        for item in delivery:
            if item == stock[k]:
                stock[v] += 1
        else:
            stock.update({item:1})

Output should look sth like this:

stock = {apple: 3, banana: 4, orange: 10, grapefruit: 1}

However I'm getting message: dictionary changed size during iteration

Upvotes: 1

Views: 565

Answers (7)

Ali Shahrivarian
Ali Shahrivarian

Reputation: 311

Also possible like this:

stock = {'apple': 1, 'banana': 4, 'orange': 10}
delivery = ['apple', 'apple', 'grapefruit']

stock.update({item: delivery.count(item) + stock.get(item,0) 
               for item in set(delivery)}) 

print(stock)
# {'apple': 3, 'banana': 4, 'orange': 10, 'grapefruit': 1})

Upvotes: 0

user9145571
user9145571

Reputation:

You can actually use Counter from collections which does exactly what you want.

from collections import Counter
stock = Counter({'apple': 1, 'banana': 4, 'orange': 10})
delivery = ['apple', 'apple', 'grapefruit']
stock.update(delivery)

Upvotes: 3

Sunitha
Sunitha

Reputation: 12015

You can use collections.counter for this purpose

from collections import Counter
stock = Counter({'apple': 1, 'banana': 4, 'orange': 10})
delivery = ['apple', 'apple', 'grapefruit']
stock.update(delivery)

stock
# Counter({'orange': 10, 'banana': 4, 'apple': 3, 'grapefruit': 1})

Upvotes: 0

mseehafer
mseehafer

Reputation: 1

The best idea in this case will be to use a Counter collection instead of a standard dict (https://docs.python.org/2/library/collections.html) Check out the basic example given there:

# Tally occurrences of words in a list
cnt = Counter()
for word in ['red', 'blue', 'red', 'green', 'blue', 'blue']:
    cnt[word] += 1

When using the standard dict you can loop over the list and along the following lines:

for v in delivery:
    try:
        stock[v] += 1
    except KeyError:
        stock[v] = 1

Alternatively you can use the *get * method of the dict and test for None.

Upvotes: 0

Leo K
Leo K

Reputation: 5354

You might want to reverse the order of enumeration and iterate over the list, adding items to the dict or increasing their value by one, as appropriate:

for item in delivery:
    if item in stock:
        stock[item] += 1
    else:
        stock[item] = 1

(note you can also use defaultdict(int) for your 'stock' dict object, so you don't even need the if/else and can do just:

for item in delivery:
    stock[item] += 1

Upvotes: 0

RoadRunner
RoadRunner

Reputation: 26325

Using dict.get():

stock = {'apple': 1, 'banana': 4, 'orange': 10}
delivery = ['apple', 'apple', 'grapefruit']

for d in delivery:
    stock[d] = stock.get(d, 0) + 1

print(stock)

Output:

{'apple': 3, 'banana': 4, 'orange': 10, 'grapefruit': 1}

Upvotes: 1

Taohidul Islam
Taohidul Islam

Reputation: 5414

You shouldn't change dict or list while iterating over it. You can write your code like:

stock = {'apple': 1, 'banana': 4, 'orange': 10}
delivery = ['apple', 'apple', 'grapefruit']

for item in delivery:
    if item in stock:
        stock[item] +=1
    else:
        stock[item] = 1

print(stock)

Upvotes: 4

Related Questions