References and copies

[Thomas Philips]

I want to represent an NxN matrix by a list containing N lists, each
of which has N elements. Initially the elements are set to " ". For
N=2, I write

x = [" "][:]*2 #assignment creates references, not copies!
x [' ', ' ']
y = [x[:]]*2
y

[[' ', ' '], [' ', ' ']]
But if I assign y[0][0], I y[1,0] changes as well

y[0][0]=1
y

[[1, ' '], [1, ' ']]

I am clearly creating references in spite of trying not to. The
problem is completely solved by using copy.copy()

x = [" "][:]*2
y=[]
for i in range(2): y.append(copy.copy(x))
y [[' ', ' '], [' ', ' ']]
y[0][0]=1
y

[[1, ' '], [' ', ' ']]

I fail to see the error in my first attempt. Where is the fly in the
ointment?

Thomas Philips

 

[Yermat]

Thomas Philips a écrit :

I want to represent an NxN matrix by a list containing N lists, each
of which has N elements. Initially the elements are set to " ". For
N=2, I write

x = [" "][:]*2 #assignment creates references, not copies!
x

[' ', ' ']

y = [x[:]]*2
[...]

I fail to see the error in my first attempt. Where is the fly in the
ointment?

Thomas Philips

Your trboules come from this line: y = [x[:]]*2 because you do not copy
the x twice but just assign a copy of x twice...
see the following:

>>> x = [" "]*2
>>> x [' ', ' ']
>>> x = [" "]*2
>>> y = [x]*2
>>> y [[' ', ' '], [' ', ' ']]
>>> id(y[0]) 8303600
>>> id(y[1]) 8303600
>>>
>>> y = [[" "] *2, [" "]*2]
>>> id(y[0]) 8306544
>>> id(y[1]) 8305424
>>>

 

[Larry Bates]

You might try:

x=n*[n*[' ']]

But I always question creating arrays this
way in Python. It is normally much better
to start with an empty list and build it as
you go using .append() method.

HTH,
Larry Bates
Syscon, Inc.

I want to represent an NxN matrix by a list containing N lists, each
of which has N elements. Initially the elements are set to " ". For
N=2, I write

x = [" "][:]*2 #assignment creates references, not copies!
x [' ', ' ']
y = [x[:]]*2
y

[[' ', ' '], [' ', ' ']]
But if I assign y[0][0], I y[1,0] changes as well

y[0][0]=1
y

[[1, ' '], [1, ' ']]

I am clearly creating references in spite of trying not to. The
problem is completely solved by using copy.copy()

x = [" "][:]*2
y=[]
for i in range(2): y.append(copy.copy(x))
y [[' ', ' '], [' ', ' ']]
y[0][0]=1
y

[[1, ' '], [' ', ' ']]

I fail to see the error in my first attempt. Where is the fly in the
ointment?

Thomas Philips

 

[Duncan Booth]

You might try:

x=n*[n*[' ']]

But I always question creating arrays this
way in Python. It is normally much better
to start with an empty list and build it as
you go using .append() method.

Its just as well you question this as it creates a list containing n copies
of the same list.

A better way to write your example:

x = [ n*[' '] for i in range(n) ]

 

[Thomas Philips]

1. Aaaargh.........!

2. I see, said the blind man, (as he picked up his hammer and
nail).......................

3. Thanks for the insight!

Thomas Philips