How do I initialize an array of arrays in BCPL?

The let keyword is used only for creating new local variables (also functions and possibly other things, but that's not really relevant for your question). So, the statement:

let stringArr = newvec(12)

is valid in creating the new variable stringArr or, more precisely:

• a 12-cell anonymous vector somewhere; and
• the stringArr variable holding the address of that vector.

However:

let stringArr!i = newvec(5)

is not valid, because stringArr!i isn't actually a new variable. It's simply the memory contents of cell number i in the already existing stringArr vector.

In other words, the statement:

let stringArr = newvec(12)

creates both the initial pointer cell and the second layer of pointers, the latter of which will not point to anywhere useful yet:

+-----------+    +-------------+
| stringArr | -> | stringArr!0 | -> ?
+-----------+    +-------------+
                 | stringArr!1 | -> ?
                 +-------------+
                 :      :      :
                 +-------------+
                 | stringArr!N | -> ?
                 +-------------+

And, since those second-layer pointers already exist, you shouldn't be using let to set them^(a). The right way to do what you're trying to achieve is:

let stringArr = newvec(12)   // Create new vector AND variable,
                             //   set variable to point at vector.
stringArr!i := newvec(5)     // Create new vector, set existing
                                  cell to point at it.

^(a) It's similar in C in that you wouldn't write:

int xyzzy[10];      // Make ten-element array.
int xyzzy[0] = 42;  // Set first element of array.

In that case, the second line isn't supposed to be defining a new variable, rather its intent is simply to set one of the existing elements to a given value. It should instead be:

int xyzzy[10];  // Make ten-element array.
xyzzy[0] = 42;  // Set first element of array.