# join lists?

Hello! Is there a way to join two lists? For example, v=[1,2,3]; w=[4,5,6];

how can I obtain the list z=[1,2,3,4,5,6]? Thank you very much, Francesco

join lists?

**
asked 2012-02-03 17:21:13 +0200 **

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Hello! Is there a way to join two lists? For example, v=[1,2,3]; w=[4,5,6];

how can I obtain the list z=[1,2,3,4,5,6]? Thank you very much, Francesco

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1

**
answered 2012-02-03 17:31:26 +0200 **

This post is a wiki. Anyone with karma >750 is welcome to improve it.

You can simply add them:

```
sage: a = [1,2,3]
sage: b = [4,5,6]
sage: z = a+b
sage: z
[1, 2, 3, 4, 5, 6]
```

Note that you can't subtract them, though:

```
sage: a-b
Traceback (most recent call last):
...
TypeError: unsupported operand type(s) for -: 'list' and 'list'
```

This is basically because it's not obvious what to do with [1,2,3]-[4,5], and a fundamental Python principle is to resist the temptation to guess in the face of ambiguity.

It might be helpful to work through a Python tutorial.

2

In other words, lists form a semigroup. If you want subtraction you have to go through the Grothendiek construction....

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Asked: ** 2012-02-03 17:21:13 +0200 **

Seen: **4,114 times**

Last updated: **Feb 03 '12**

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