更新二维列表

Question

lis is a list.Trying to manipulate it as a 2d list and update with some values.

lis = [0 for i in range(3)]
print "1st time list",lis

for i in range(3):
    lis[i]=[0]*2  # replaced and tried with  lis[i]=[0,0]

print "2nd time list ",lis

for i in  range(3):
    for j in lis[i]:
        print "i:",i
        print "j:",j
        lis[i][j] = i+j
        print "value entered: ", lis[i][j]   # here showing correct o/p             

print lis                   #should be [[0,0],[1,1],[2,2]] , but not coming
                            #as expected.

Could anybody explain it and correct me?

Answer 1

I'm assuming what you want to do is have every inner list contain the index number as the elements [[0,0],[1,1],[2,2],[3,3] ...]

Keeping with the current structure of your code, this should work:

lis = [0 for i in range(3)]
print "1st time list",lis

for i in range(3):
    lis[i]=[0]*2  # replaced and tried with  lis[i]=[0,0]

print "2nd time list ",lis

for i in range(len(lis)):
    lis[i] = [i, i]

print lis

The reason yours wasn't working is because of how you're doing the inner loop. for j in lis[i] doesnt do what you expect. What that line does is it takes lis[i] which is a list in itself, and loops through every element in that list. At the time of that loop, the last element is 0 so you're always getting a 0 in the final inner list position.

This is probably a simpler way to achieve what you want (although I don't know if your example is part of a larger project which requires you to do it the way you're currently doing it):

numElements = 3

lis = []

for i in range(numElements):
    lis.append([i,i])

print lis

Answer 2

Here's a way to populate the list so it ends up with the contents you say you want:

lis = range(3)
print "1st time list", lis

for i in range(len(lis)):
    j = lis[i]
    lis[i] = [j, j]

print lis  # -> [[0, 0], [1, 1], [2, 2]]

There are shorter and faster ways to do this, but since it's uclear what you're trying to accomplish, I haven't shown them.

Answer 3

If you are looking to create something like [[0, 0], [1, 1], [2, 2], ...]

I would do it so,

print [[i, i] for i in xrange(n)]

Where n is the number of such entries that you want. In your first example, n = 3

You can read more about list comprehensions https://docs.python.org/2/tutorial/datastructures.html