CODEWITHSUNDEEP
pythondictionaryfor-loop
Malaikatu Kargbo
Malaikatu Kargbo

Reputation: 313

Add multiple values to one key in dictionary with for loop

I'm using a similar method in a function I'm using. Why do I get a key error when I try to do this?

def trial():
     adict={}
     for x in [1,2,3]:
          for y in [1,2]:
               adict[y] += x
print(adict)

Upvotes: 0

Views: 1447

Answers (4)

Yang
Yang

Reputation: 888

There is no value assigned to adict[y] when you first use it. 

def trial():
     adict={}
     for x in [1,2,3]:
          for y in [1,2]:
               if y in adict: # Check if we have y in adict or not 
                   adict[y] += x
               else: # If not, set it to x
                   adict[y] = x
     print(adict)

Output:

>>> trial()
{1: 6, 2: 6}

Upvotes: 0

Miriam Farber
Miriam Farber

Reputation: 19634

You did not initialize adict for each key. You can use defaultdict to solve this issue:

from collections import defaultdict
def trial():
    adict=defaultdict(int)
    for x in [1,2,3]:
        for y in [1,2]:
            adict[y] += x
    print(adict)
trial()

The result is defaultdict(<class 'int'>, {1: 6, 2: 6})

Upvotes: 1

Superluminal
Superluminal

Reputation: 977

You should modify youre code like this: 

def trial():
   adict={0,0,0}
   for x in [1,2,3]:
      for y in [1,2]:
           adict[y] += x
print(adict)

"adict" has no entries. So adict[1] fails because it is accessing a variable that does not exist.

Upvotes: 0

Daniel Roseman
Daniel Roseman

Reputation: 599530

adict starts off empty. You can't add an integer to a value that doesn't already exist.

Upvotes: 1

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