### Question :

Iterating over a 2 dimensional python list [duplicate]

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

##
Answer #1:

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']
```

##
Answer #2:

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
>>>
```

##
Answer #3:

This is the correct way.

```
>>> x = [ ['0,0', '0,1'], ['1,0', '1,1'], ['2,0', '2,1'] ]
>>> for i in range(len(x)):
for j in range(len(x[i])):
print(x[i][j])
0,0
0,1
1,0
1,1
2,0
2,1
>>>
```

##
Answer #4:

```
>>> mylist = [["%s,%s"%(i,j) for j in range(columns)] for i in range(rows)]
>>> mylist
[['0,0', '0,1', '0,2'], ['1,0', '1,1', '1,2'], ['2,0', '2,1', '2,2']]
>>> zip(*mylist)
[('0,0', '1,0', '2,0'), ('0,1', '1,1', '2,1'), ('0,2', '1,2', '2,2')]
>>> sum(zip(*mylist),())
('0,0', '1,0', '2,0', '0,1', '1,1', '2,1', '0,2', '1,2', '2,2')
```

##
Answer #5:

`zip`

will transpose the list, after that you can concatenate the outputs.

```
In [3]: zip(*[ ['0,0', '0,1'], ['1,0', '1,1'], ['2,0', '2,1'] ])
Out[3]: [('0,0', '1,0', '2,0'), ('0,1', '1,1', '2,1')]
```

##
Answer #6:

`zip()`

in conjunction with the `*`

operator can be used to `unzip`

a list

```
unzip_lst = zip(*mylist)
for i in unzip_lst:
for j in i:
print j
```

##
Answer #7:

```
>>> [el[0] if i < len(mylist) else el[1] for i,el in enumerate(mylist + mylist)]
['0,0', '1,0', '2,0', '0,1', '1,1', '2,1']
```