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I am looking to randomly generate a list and change one element in the list to create a new list. Whilst keeping track of all the generated lists.

For example:

values_possible = [0,0.5,1]
combinations = []
combinations.append([random.choice(values_possible) for i in range(8)])

lets say this generates 

[0,0.5,0.5,0,0.5,0.5,1.0,1.0]

then if I copy this list and change one element, say combinations[1][1]

# set a new list to equal the first and then change one element
combinations.append(combinations[0])
combinations[1][1] = 0.33

print(combinations[0])
print(combinations[1])

this returns

[0,0.33,0.5,0,0.5,0.5,1.0,1.0]
[0,0.33,0.5,0,0.5,0.5,1.0,1.0]

it seems both combinations[0][1] and combinations[1][1] have changed to 0.33. Is there a way to solve this? May I ask why this happens and what I am misunderstanding about lists? I had expected the following output:

[0,0.5,0.5,0,0.5,0.5,1.0,1.0]
[0,0.33,0.5,0,0.5,0.5,1.0,1.0]
Zakoff
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  • `combinations.append([random.choice(values_possible) for i in range(8)])` generates `[[0,0.5,0.5,0,0.5,0.5,1.0,1.0]]`. That is because `append` appends one single element to the list. In this case the element is a list. Use `.extend` instead, otherwise, if that is what you want to get, correct the typo and add `[` `]` around the generated list – Pynchia Aug 09 '15 at 18:16

2 Answers2

2

The short answer to your question is using colon:

 combinations.append(combinations[0][:])

Why? The reason is that in python when you append a variable into your list, the variable is appended by its reference. In your example, it means that the two elements in the list are the same. They are pointing to the same address in the memory and if you modify either of them, both value will change as they are one and using the same chunk of memory.

If you want to copy the values of a variable, in your case, combinations[0], you need to use colons to make a copy of values and put them in another part of memory (it will occupy another chunk of memory with different address) In this case, you can modify them separately.

You can also take a look at this question and answer: python list by value not by reference

I hope this helps.

Community
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Mahsa2
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1

use 

combinations.append(list(combinations[0]))

to append a copy of the first element.

yurib
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