what do these mean

[Ian Kelly]

On Sun, May 20, 2012 at 10:52 AM, e-mail mgbg25171
<mgbg25171 at blueyonder.co.uk> wrote:
> There's a little forth program written in python here
> #http://openbookproject.net/py4fun/forth/forth.py
> I'm struggling to understand what these lines mean.
>
> def rJnz (cod,p) : return (cod[p],p+1)[ds.pop()]
> def rJz  (cod,p) : return (p+1,cod[p])[ds.pop()==0]

rJz is the jump-if-zero instruction used by the If and Until
constructs.  It creates a tuple containing the next instruction and
the instruction to jump to.  Which element of the tuple to return is
determined by the expression "ds.pop()==0", which is True (i.e. 1) if
the top of the stack is 0, or False (i.e. 0) if it is not.

rJnz appears to be a complementary jump-if-nonzero instruction that is
never actually used by the program and appears to be buggy in any
case.  It has the order of the tuple backwards, and the "ds.pop()"
expression makes the false assumption that the item at the top of the
stack is already 0 or 1 and not something else.