'Get n-th element from stack in Forth
Is there a way to access an element of the stack by its index in Forth, without popping all the elements above it?
For example, if I have the numbers 1 to 1000 pushed to the stack, how can I get the 500th element?
Solution 1:[1]
500 PICK
...will copy the element 500 levels down the stack to the top of the stack in Forth79.
More relevant: PICK is in the core extension wordset in ISO93 Forth, the base of the current standard. The definition of PICK in this standard is 0-based, e.g. '0 PICK' is equivalent to 'DUP'. See section 6.2.2030
Solution 2:[2]
And if the Forth you're using don't have PICK, you could define it as
: PICK ?DUP IF SWAP >R 1- RECURSE R> SWAP EXIT THEN DUP ;
(Of course, an iterative version would also be possible.)
Sources
This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.
Source: Stack Overflow
| Solution | Source |
|---|---|
| Solution 1 | Albert van der Horst |
| Solution 2 | Lars Brinkhoff |