retroforth/doc/book/techniques/arrays

126 lines
2.4 KiB
Text
Raw Normal View History

# Working With Arrays
RETRO offers a number of words for operating on statically sized
arrays.
## Namespace
The words operating on arrays are kept in an `a:` namespace.
## Creating Arrays
The easiest way to create an array is to wrap the values in a
`{` and `}` pair:
```
{ #1 #2 #3 #4 }
{ 'this 'is 'an 'array 'of 'strings }
{ 'this 'is 'a 'mixed 'array #1 #2 #3 }
```
You can also make an array from a quotation which returns
values and the number of values to store in the a:
```
[ #1 #2 #3 #3 ] a:counted-results
[ #1 #2 #3 #3 ] a:make
```
## Accessing Elements
You can access a specific value with `a:th` and `fetch` or
`store`:
```
{ #1 #2 #3 #4 } #3 a:th fetch
```
## Find The Length
Use `a:length` to find the size of the array.
```
{ #1 #2 #3 #4 } a:length
```
## Duplicate
Use `a:dup` to make a copy of an a:
```
{ #1 #2 #3 #4 } a:dup
```
## Filtering
RETRO provides `a:filter` which extracts matching values
from an array. This is used like:
```
{ #1 #2 #3 #4 #5 #6 #7 #8 } [ n:even? ] a:filter
```
The quote will be passed each value in the array and should
return TRUE or FALSE. Values that lead to TRUE will be collected
into a new array.
## Mapping
`a:map` applies a quotation to each item in an array and
constructs a new array from the returned values.
Example:
```
{ #1 #2 #3 } [ #10 * ] a:map
```
## Reduce
`a:reduce` takes an array, a starting value, and a quote. It
executes the quote once for each item in the array, passing the
item and the value to the quote. The quote should consume both
and return a new value.
```
{ #1 #2 #3 } #0 [ + ] a:reduce
```
## Search
RETRO provides `a:contains?` and `a:contains/string?`
to search an array for a value (either a number or string) and
return either TRUE or FALSE.
```
#100 { #1 #2 #3 } a:contains?
'test { 'abc 'def 'test 'ghi } a:contains/string?
```
## Implementation
In memory, an array is a count followed by the values. As an
example, if you have an array:
{ #10 #20 #30 }
In memory this would be setup as:
| Offset | Value |
| ------ | ----- |
| 000 | 3 |
| 001 | 10 |
| 002 | 20 |
| 003 | 30 |
You can construct one on the fly by keeping a pointer to
`here` and using `,` to place the values. E.g.,
here [ #3 , #10 , #20 , #30 , ] dip
An example of this can be seen in this excerpt from an example
(*example/Primes.forth*):
:create-set (-a)
here #3000 , #2 #3002 [ dup , n:inc ] times drop ;