Now here’s a task that cries out for a simple solution: put a border round a matrix. My matrix is boolean and represents a QR code (yes, we’ll come to that) but it’s the same problem as, say, putting a 1px border on an image.

So let’s examine it as wrapping a char matrix in spaces. We’ll use `3 4#"ABCDEGHIJKL"`

.

## Amend At

Our first strategy is to manipulate indexes, which is often an efficient approach in q. We make a larger blank matrix for the result and write the original matrix in the right place.

Start with the shape of the matrix; that is, count the rows and columns. (*Shape* is a concept that q did not inherit from its ancestor APL, but is easy enough to calculate.) We use the Zen monks for a point-free expression.

```
q)show M:3 4#"ABCDEFGHIJKL"
"ABCD"
"EFGH"
"IJKL"
q)count each 1 first\M / shape of M
3 4
```

So the result shape is 5 6, and here is our blank template:

```
q){n:2+count each 1 first\x; n#" "}M
" "
" "
" "
" "
" "
```

For the last move we could use Amend Each `.'`

to map each item of `M`

to a row-column pair in the result. But it should be more efficient to raze `M`

and use Amend At `@`

to map *all* its items to the vector `prd[n]#" "`

and *then* reshape it. Something like

```
q){n:2+s:count each 1 first\x; n#@[prd[n]#" "; ??? ;:;raze x]}M
" "
" ABCD "
" EFGH "
" IJKL "
" "
```

Above, `???`

is some expression that returns the target indices for the items of `M`

. Let’s start with an easy expression – wrong, but easy. We’ll write the items of `M`

into the first positions of the result.

```
q){n:2+s:count each 1 first\x; n#@[prd[n]#" ";til prd s;:;raze x]}M
"ABCDEF"
"GHIJKL"
" "
" "
" "
```

Next we come to an often-overlooked overload of `vs`

and `sv`

: they encode and decode different (and variable) arithmetic bases. English pounds have 100 pennies (once known as New Pence) but once had 240, of which 12 made a shilling; and 20 shillings a pound.

```
q)240*4.50 / £4.50 in old pence
1080f
q)100 20 12 vs 240*4.50 / £4.50 was £4 10s 0d.
4 10 0f
q)%[;240]100 20 12 sv 4 10 0 / £4/10/- in decimal coinage
4.5
q)%[;240]100 20 12 sv 4 17 6 / not every sterling amount has an exact equivalent
4.875
```

We can use `vs`

and `sv`

to convert between row-col pairs and equivalent vector indices.

```
q){n:2+s:count each 1 first\x; n#@[prd[n]#" ";n sv flip 1 1+/:s vs/:til prd s;:;raze x]}M
" "
" ABCD "
" EFGH "
" IJKL "
" "
```

The above has a certain elegance in that it is probably efficient for a large matrix, but it does seem a lot of code for a simple task. If our matrices are small, perhaps we can see a simpler way?

## Join

Join `,`

looks like an obvious candidate. (And it will lead us to something about `flip`

we might not have known; but we’ll come to that.) We have to apply it to each of four sides, but we have decided we don’t necessarily need the fastest expression for this.

Looks straightforward: Join for top and bottom, Join Each for the sides.

```
q),[;" "] " ",'" ",M,'" "
" "
" ABCD "
" EFGH "
" IJKL "
" "
```

Ah, not *quite* that straightforward. Joining an atom doesn’t use scalar extension the same way Join Each does. We could count the first row…

```
q){row:enlist(count first x)#" ";" ",'(row,x,row),'" "}M
" "
" ABCD "
" EFGH "
" IJKL "
" "
```

Better, but the refactoring itch remains.

The simplest operation is the Join Each, which exploits scalar extension.

When I flour an unbaked loaf, I don’t daub flour over it, I *roll* it in the flour.

```
q)reverse flip ,'[" "] M
"DHL"
"CGK"
"BFJ"
"AEI"
" "
q)reverse flip ,'[" "] reverse flip ,'[" "] M
"LKJI "
"HGFE "
"DCBA "
" "
q)4{reverse flip ,'[" "] x}/M
" "
" ABCD "
" EFGH "
" IJKL "
" "
```

I don’t need the (admittedly tiny) overhead of a lambda to apply a series of unaries. I can use a composition.

```
q)4(reverse flip ,'[" "]@)/M
" "
" ABCD "
" EFGH "
" IJKL "
" "
```

Now here’s a surprise: we don’t need the Each.

```
q)4(reverse flip ,[" "]@)/M
" "
" ABCD "
" EFGH "
" IJKL "
" "
```

How does *that* work? It turns out that `flip`

uses scalar extension. The items of its argument must conform; that is, they must be same-length lists – or atoms. But the result *will* have same-length lists.

```
q)flip M
"AEI"
"BFJ"
"CGK"
"DHL"
q)flip M,enlist "XYZ" / must conform!
'length
[0] flip M,enlist "XYZ"
^
q)flip M,"X"
"AEIX"
"BFJX"
"CGKX"
"DHLX"
q)
```

Loaf floured! And the QR code? Watch this space.