estebanpdl - 4 months ago 17

Python Question

I want to create a list of multiple lists, considering one list in particular.

For example: I want to add items from

`a`

`b`

`c`

`x`

`x`

`mainlist = []`

x = [1, 2, 3] # items will be added hear.

a = ['a1', 'b1', 'c1']

b = ['a2', 'b2', 'c2']

c = ['a3', 'b3', 'c3']

First I need to create a list of lists for

`x`

`x = [[i] for i in x]`

It returns:

`out[1]: [[1], [2], [3]]`

Now, I want to add items to those lists:

`for item in range(0, len(x)):`

x[item].insert(1, a[item])

out[2]: [[1, 'a1'], [2, 'b1'], [3, 'c1']]

And then append to

`mainlist`

`mainlist.append(x)`

out[3]: [[[1, 'a1'], [2, 'b1'], [3, 'c1']]]

My question is how can I add items from

`b`

`c`

`a`

`[[[1, 'a1'], [2, 'b1'], [3, 'c1']], [1, 'a2'], [2, 'b2'], [3, 'c2']], [1, 'a3'], [2, 'b3'], [3, 'c3']]]`

I tried this, and I got the result, however i think this code could be improved.

`item1 = [[i] for i in x]`

item2 = [[i] for i in x]

item3 = [[i] for i in x]

for i in range(0, len(item1)):

item1[i].insert(1, a[i])

for i in range(0, len(item2)):

item2[i].insert(1, b[i])

for i in range(0, len(item3)):

item3[i].insert(1, c[i])

mainlist.append(item1)

mainlist.append(item2)

mainlist.append(item3)

Any suggestions to improve it are appreacited. Thanks!

Answer

Just *zip* *x* with each of the lists *a,b,c* using a *list comp*:

```
x = [1, 2, 3] # items will be added hear.
a = ['a1', 'b1', 'c1']
b = ['a2', 'b2', 'c2']
c = ['a3', 'b3', 'c3']
main_list = [zip(x, y) for y in a,b,c]
```

That will give you:

```
[[(1, 'a1'), (2, 'b1'), (3, 'c1')], [(1, 'a2'), (2, 'b2'), (3, 'c2')], [(1, 'a3'), (2, 'b3'), (3, 'c3')]]
```

If you really want sublists instead of tuples you can call *map* to map the tuples to lists:

```
[list(map(list, zip(x, y))) for y in a,b,c]
```

If you were going to use a regular loop without the *zip* logic, you would do something like:

```
l = []
for y in a, b, c:
l.append([[x[i], ele] for i, ele in enumerate(y)])
print(l)
```

Or a list comp version of the same:

```
[[[x[i], ele] for i, ele in enumerate(y) ] for y in a, b, c]
```

Presuming all are the same length as `x`

, each `i`

is the index of the current element in `y`

and we match it up with the corresponding element from `x`

which is also exactly what zip is doing.