Taylan Ulrich Bayırlı/Kammer writes:
> Arne Babenhauserheide <arne_bab@web.de> writes:
>> The original version was in Python:
>>
>>     psi[i] - c1*(psi[i+1] - psi[i-1]) + c2*(psi[i+1] - 2.0*psi[i] + psi[i-1])
>>
>> My port to Scheme looks like this:
>>
>>     (let ((newvalue (+ (- (psir i)
>>                           (* c1 (- (psir (+ i 1)) (psir (- i 1)))))
>>                        (* c2 (+ (- (psir (+ i 1)) (* 2 (psir i)))
>>                                 (psir (- i 1)))))))
>>       (array-set! psinew newvalue i))
>
> Guile supports SRFI-105, so that could be:
>
>     {{psi[i] - {c1 * {psi[{i + 1}] - psi[{i - 1}]}}} + {c2 * {{psi[{i + 1}] - {2 * psi[i]}} + psi[{i - 1}]}}}

That’s already pretty close — I wonder why I didn’t think of the psi[i]
form.

I think a + around the equation would actually help here:

    (+ psi[i]
       (* -1 c1 {psi[{i + 1}] - psi[{i - 1}]})
       (* c2 {{psi[{i + 1}] - {2 * psi[i]}} + psi[{i - 1}]}))

Though neoteric expressions combined with curly infix make this even
easier: p{i + 1} → (p (+ i 1))

(though this did not work for me in the REPL right now — did I miss something?)

So the function psir could be used to have elegant access to elements:

    (+ psi[i]
       (* -1 c1 {psi[{i + 1}] - psi[{i - 1}]})
       (* c2 {{psi[{i + 1}] - {2 * psi[i]}} + psi[{i - 1}]}))

>     {psi[i] - c1 * {psi[i + 1] - psi[i - 1]} + c2 * {psi[i + 1] - 2 * psi[i] + psi[i - 1]}}

That looks roughly as readable as the Python version. With the + around
I think it becomes better:

    (+ psi[i]
       (* -1 c1 {psi[i + 1] - psi[i - 1]})
       (* c2 {{psi[i + 1] - {2 * psi[i]}} + psi[i - 1]}))

And I don’t think it’s worse than the Python version, because in Python
I also need to first import numpy which changes some semantics (the []
operator changes and arrays have some special properties which can
sometimes bite).

Besides: when working on the Python, I was tempted to bracket the
expression to make it easier to split the lines and make the boundary
between sub-expressions clearer:

    (psi[i]
      - c1*(psi[i+1] - psi[i-1])
      + c2*(psi[i+1] - 2.0*psi[i] + psi[i-1]))

Best wishes,
Arne

PS: CCed to David A. Wheeler who wrote SRFI-105.
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