Python List of Lists Incrementing

Question

x  = 0

y = []

for i in range (5):
    y.append(x)
    x+=1

print y

Gives:

[0, 1, 2, 3, 4]

But,

x  = range (2)

y = []

for i in range (5):
    y.append(x)
    x[0]+=1

print y

I would have thought (hoped) that this gives me:

[[0, 1], [1, 1], [2, 1], [3, 1], [4, 1]]

But instead it just gives me:

[[5, 1], [5, 1], [5, 1], [5, 1], [5, 1]]

I've tested it by putting 'print y' inside the for loop, and obviously what is happening is that when x[0]+=1, all the 'x's that are already inside 'y' are being updated with the new value.

How do I write the program so that it 'forgets' that the 'x's already in list 'y' are still variables (and so doesn't keep updating them)?

I've realized I could just write this one specifically as:

y = []

for i in range (5):
    x = [i, 1]
    y.append(x)

But what if I wanted to define x outside of the for loop? I can't think of a time when I would want to do it this way but I just want to know anyway.

"How do I write the program so that it 'forgets' that the 'x's already in list 'y' are still variables" - this makes no sense. Pyton doesn't really have "variables", it has names that are references to objects (see e.g. nedbatchelder.com/text/names.html). You fill y with multiple references to the same list. To fix it, copy; y.append(x[:]). — jonrsharpe
– jonrsharpe, Commented Dec 8, 2014 at 22:54

simopopov · Accepted Answer · 2014-12-08 22:59:53Z

2

@tzaman gives the perfect solution, but you want your own code:

x  = range (2)

y = []

for i in range (5):
    y.append(x[:]) #You should append copy of x
    x[0]+=1

print y

gives:

[[0, 1], [1, 1], [2, 1], [3, 1], [4, 1]]

answered Dec 8, 2014 at 22:59

simopopov

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2 Comments

Padraic Cunningham Over a year ago

or just append(range(2))

Patrik333 Over a year ago

Thanks, I'll try this one. tzaman's solution was alright but not really what I was asking, so this one's better.

tzaman · Accepted Answer · 2014-12-08 22:50:39Z

2

If you define x outside the for loop, it's a reference to the same list every time. If you want each sub-list to be independent you have to create it inside the loop. Your final code-block works.

However, a more concise and pythonic way of doing what you want is a list comprehension:

y = [[i, 1] for i in range(5)]

answered Dec 8, 2014 at 22:50

tzaman

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