## Re-using complex data structures

Sometimes you want your functions to behave in a ‘functional’ way, i.e. return fresh results without side effects, sometimes you want them to re-use and modify existing data in a destructive way - consider the difference between `append`

and `nconc`

for an example.

Well, you can have your cake and eat it too, by using optional (or keyword) parameters. Here’s an example: Let’s assume you’re writing a function `complex-matrix-stuff`

which takes two matrices `m1`

and `m2`

as its arguments and computes and returns a resulting matrix the size of which depends on `m1`

and `m2`

, i.e. for a fresh result you’ll need an empty matrix which’ll be created by, say, `(make-appropriate-result-matrix-for m1 m2)`

.

The classical textbook way to implement this function will more or less look like this:

```
(defun complex-matrix-stuff (m1 m2)
(let ((result (make-appropriate-result-matrix-for m1 m2)))
;; ... compute storing the results in RESULT
result))
```

And you’ll use it like this:

```
(setq some-matrix (complex-matrix-stuff A B))
```

But why not write it like so:

```
(defun complex-matrix-stuff (m1 m2
&optional
(result
(make-appropriate-result-matrix-for m1 m2)))
;; ... compute storing the results in RESULT
result)
```

Now you have it both ways. You can still “make up results” on the fly as in:

```
(setq some-matrix (complex-matrix-stuff A B))
```

But you can also (destructively) re-use previously allocated matrices:

```
(complex-matrix-stuff A B some-appropriate-matrix-I-built-before)
```

Or use your function like this:

```
(setq some-other-matrix
(complex-matrix-stuff A B some-appropriate-matrix-I-built-before))
```

in which case you’ll end up with:

```
* (eq some-other-matrix some-appropriate-matrix-I-built-before)
T
```

## Using `ADJUST-ARRAY`

instead of consing up new sequences with `SUBSEQ`

Most CL functions operating on sequences will accept `start`

and `end`

keywords so you can make them operate on a sub-sequence without actually creating it, i.e. instead of

```
(count #\a (subseq long-string from to))
```

you should of course use

```
(count #\a long-string :start from :end to)
```

which’ll yield the same result but not create an unnecessary intermediate sub-sequence.

However, sometimes it looks like you can’t avoid creating new data. Consider a hash table the keys of which are strings. If the key you’re looking for is a sub-string of another string you’ll most likely end up with

```
(gethash (subseq original-string from to)
has-table)
```

But you don’t have to. You can create *one* displaced string and reuse it multiple times with `adjust-array`

:

```
(let ((substring (make-array 0
:element-type 'character
:displaced-to ""
:displaced-index-offset 0)))
;; more code
(gethash
(adjust-array substring (- to from)
:displaced-to original-string
:displaced-index-offset from)
hash-table)
;; even more code
)
```