I have created a 2 dimension array like:

```
rows =3
columns= 2
mylist = [[0 for x in range(columns)] for x in range(rows)]
for i in range(rows):
for j in range(columns):
mylist[i][j] = '%s,%s'%(i,j)
print mylist
```

Printing this list gives an output:

```
[ ['0,0', '0,1'], ['1,0', '1,1'], ['2,0', '2,1'] ]
```

where each list item is a string of the format 'row,column'

Now given this list, i want to iterate through it in the order:

```
'0,0'
'1,0'
'2,0'
'0,1'
'1,1'
'2,1'
```

that is iterate through 1st column then 2nd column and so on. How do i do it with a loop ?

This Question pertains to pure python list while the question which is marked as same pertains to numpy arrays. They are clearly different

Use `zip`

and `itertools.chain`

. Something like:

```
>>> from itertools import chain
>>> l = chain.from_iterable(zip(*l))
<itertools.chain object at 0x104612610>
>>> list(l)
['0,0', '1,0', '2,0', '0,1', '1,1', '2,1']
```

same way you did the fill in, but reverse the indexes:

```
>>> for j in range(columns):
... for i in range(rows):
... print mylist[i][j],
...
0,0 1,0 2,0 0,1 1,1 2,1
>>>
```

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