How would one go about implementing a vector or dynamic array in forth? - forth

Question

I need to a dynamic array in forth, but I don't have any idea of how I could implement it. I searched online, and couldn't find any results either. I'm very new to forth, and just starting to learn it. I think I could just use a variable to store the length, and allocate more as I go, but I don't know if this even works since I am also able to write outside of the allocated space of the array.

Answer 1

It depends on what you really need. Below is code to create dynamic arrays of cells. Quickly tested in VFX Forth and GForth. There's probably neater and better optimised versions around.
0 [IF]
dynamic array is an address stored in the dictionary pointing to
a structure stored in allocated memory
0 CELL \ data size in bytes n CELLS
1 CELL \ start of data
...
n CELL \ End of data
[THEN]
\ Take dictionary address and return addresses of the array
: array-size \ a -- n ;
# # ;
: array-data \ a -- a' ;
# CELL + ;
\ **********************************************************
\ **********************************************************
\ Expand the data structure and copy the 'old' data into it.
\ This either expands the data to size or to twice the
\ original size whichever is larger.
\ ALLOCATE THROW and FREE THROW catch & report any memory
\ errors.
\ **********************************************************
: dyn-expand \ size a-dict -- ;
DUP >R
array-size 2* MAX \ new-size = largest of the ix offset
\ or 2 * current size.
DUP CELL + ALLOCATE THROW ( new-size new-addr )
2DUP CELL + SWAP ERASE \ zero the newly allocated memory
R# array-data OVER CELL + R# array-size
( size new-addr old-data-a new-data-a old-size )
MOVE \ Shift existing data to the new addr. ( size new-addr )
R# # FREE THROW \ Free the old data's memory
TUCK ! \ Store the new size
R> ! ; \ Store the new address in the dictionary
: dynamic-array \ CREATE: count -- ; DOES> ix -- a
\ Creates a dynamic array of count cells in ALLOCATED memory.
CREATE \ count -- ;
CELLS DUP CELL + ALLOCATE THROW ( count addr )
DUP , \ Store the data address in the dictionary
2DUP ! \ Store the data size in the allocated memory
CELL + SWAP ERASE \ Zero the new data region.
DOES> \ ix -- addr-of-ix-cell ;
\ Returns the address of the ix th cell. Expanding the array if required.
( ix a )
SWAP CELLS SWAP 2DUP array-size >= IF \ ix not in allocated range
2DUP dyn-expand
THEN
( ix-cells a ) array-data + ;
: dyn-stats. \ a -- ; Prints base address and array size
." Base data address: " DUP .
." Data size in bytes: " CELL - # . ;
Quick tests and use:
16 dynamic-array ]test ok
456 0 ]test ! 4560 10 ]test ! ok
CR 0 ]test dyn-stats. ." 0th and 10th Data: " 0 ]test # . 10 ]test # . CR
Base data address: 9459864 Data size in bytes: 128 0th and 10th Data: 456 4560
******* ***
ok
1600 16 ]test ! \ This extends the array. ok
CR 0 ]test dyn-stats. ." 0th and 10th Data: " 0 ]test # . 10 ]test # . CR
Base data address: 10502952 Data size in bytes: 256 0th and 10th Data: 456 4560
ok ******** Address Changed *** & size changed unchanged!!
." 16th data: " 16 ]test # . 16th data: 1600 ok