2 different ways to initialise a 2d list give different results

Python Help
1

I was initialising a 2d list and was getting strange results.
After some furtling around I found that the way I was initialising the list had an unexpected impact.

rows =2
columns= 2
Array=[[0]*columns]*rows
mylist = [[0 for x in range(columns)] for x in range(rows)]
print (Array)
print (type(Array))
print (type(Array[0]))
print (type(Array[0][0]))
print (mylist)
print (type(mylist))
print (type(mylist[0]))
print (type(mylist[0][0]))
x=1
for i in range(rows):
    for j in range(columns):
        Array[i][j] = '%s,%s,%s'%(i,j,x)
        mylist[i][j] = '%s,%s,%s'%(i,j,x)
        x+=1
print (Array)
print (mylist)

Gives output:

[[0, 0], [0, 0]]
<class 'list'>
<class 'list'>
<class 'int'>
[[0, 0], [0, 0]]
<class 'list'>
<class 'list'>
<class 'int'>
[['1,0,3', '1,1,4'], ['1,0,3', '1,1,4']]
[['0,0,1', '0,1,2'], ['1,0,3', '1,1,4']]

I am a bit puzzled by this. It seems both outer list entries for the Array are being treated as the same object. Any explanation would be welcome.

2

Python works with references. It won’t copy objects unless you do so explicitly. When you multiply a list, you’re not duplicating objects, you’re duplicating references to objects.

1 Like
3

That worked. The site I read that showed initialising a 2d array with

array=[[0]*rows]*columns]

without mentioning this gotcha was obviously incomplete.

Thanks for the help.