Problem in updating a dictionary: all the keys have the same list

Question

I am facing a problem in assigning values to dictionary. I want a changing list (which is being updated by every iteration of a loop) assigned to the keys of my dictionary by using a for loop. But the problem is the lists are eventually same for all of the keys.

Let me show it with example:

months = ['jan', 'feb', 'mar', 'apr']  # this is the list of keys of the Dictionary
numbers = []
sqrDict = {}
i = 1
for m in months:
    numbers.append(i**2)
    sqrDict[m] = numbers
    i += 1

print('sqrDict =', sqrDict)

I actually want my dictionary to be as:

sqrDict = {'jan': [1], 'feb': [1, 4], 'mar': [1, 4, 9], 'apr': [1, 4, 9, 16]}

But what I am having is:

sqrDict = {'jan': [1, 4, 9, 16], 'feb': [1, 4, 9, 16], 'mar': [1, 4, 9, 16], 'apr': [1, 4, 9, 16]}

I don't understand what the problem is. What is going wrong with my code?

What's wrong with your code is already a part of your title: all keys have the same list — Pranav Hosangadi, Jan 20 '23 at 18:38
https://stackoverflow.com/questions/13498453/changing-one-dict-value-changes-all-values — Pranav Hosangadi, Jan 20 '23 at 18:41

3ddavies · Answer 1 · 2023-01-20T18:40:14.620

0

The issue are having is that you are setting sqrDict[m] = numbers. At the end, all the values in the dict point to numbers. This should fix your code, as it assigns each position in sqrDict to the value of numbers at the time, instead of pointing to it, which is achieved by changing numbers[] to numbers[:] as shown below:

months = ['jan', 'feb', 'mar', 'apr'] #this is the list of keys of the Dictionary
numbers = []
sqrDict = {}
i = 1
for m in months:
    numbers.append(i**2)
    sqrDict[m] = numbers[:]
    i += 1

print('sqrDict = ',sqrDict)

edited Jan 20 '23 at 18:40

answered Jan 20 '23 at 18:39

3ddavies

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@mkrieger1 updated answer to explain! – 3ddavies Jan 20 '23 at 18:41
1

_"assigns each position in sqrDict to the value of numbers"_ a more correct way to say this would be that it copies `numbers` and creates a new list that is assigned to that key – Pranav Hosangadi Jan 20 '23 at 18:42

Problem in updating a dictionary: all the keys have the same list

1 Answers1