What's the best way to join two lists?

Question

I have two lists that contain some data (numeric data or/and strings)? How do I join these two lists, assuming that the lists do not contain sublists? Which choice is preferred and why? `set first [concat $first $second]` `lappend first $second` `append first " $second"`

Accepted Answer

It is fine to use `concat` and that is even highly efficient in some cases (it is the recommended technique in 8.4 and before, and not too bad in later versions). However, your second option with `lappend` will not work at all, and the version with `append` will work, but will also be horribly inefficient.

Other versions that will work:

```
# Strongly recommended from 8.6.1 on
set first [list {*}$first {*}$second]
```

```
lappend first {*}$second
```

The reason why the first of those is recommended from 8.6.1 onwards is that the compiler is able to optimise it to a direct "list-concatenate" operation.

What's the best way to join two lists?

Solution

Problem