# Calculating the sum of a *lot* of primes

Geoff Canyon gcanyon at gmail.com
Wed Nov 11 09:29:49 EST 2015

```Ha, caught me being lazy. For loops like

repeat with i = 1 to fld 1

there is (almost) no difference -- the field gets referenced one time at
the start and that's it.

But since I was lazy about how a loop with "step 2" in it would exit, I was
slower.

Thanks!

gc

On Wed, Nov 11, 2015 at 7:34 AM, Peter M. Brigham <pmbrig at gmail.com> wrote:

> There may be other factors more important for speed, but one thing is to
> avoid having to access the contents of a field 1.5 billion times, by
> putting fld 1 into a variable, say "maxNbr", and then doing
>
> repeat until i > maxNbr…
>
> -- Peter
>
> Peter M. Brigham
> pmbrig at gmail.com
> http://home.comcast.net/~pmbrig
>
>
> On Nov 11, 2015, at 2:45 AM, Geoff Canyon wrote:
>
> > I'm playing with a challenge to calculate the sum of the primes less than
> > 3,000,000,000
> >
> > So far I have this, which gets the job done for the primes up to about a
> > billion, but then dies (the whole app) without even a dialog. It's also
> not
> > very fast. It would probably take an hour or two to finish.
> >
> > Any suggestions for efficiency or non-crashed-ness?
> >
> > function bAdd x,y
> > -- adds two positive numbers of arbitrary length
> >   put 0 into c
> >   repeat with i = 14 to max(length(x),length(y))
> >      add char -i to 13 - i of x + char -i to 13 - i of y to c
> >      put char -14 to -1 of c before r
> >      delete char -14 to -1 of c
> >   end repeat
> >   return c & r
> > end bAdd
> >
> > on mouseUp
> >   put 2 into T
> >   put 0 into S
> >   put 3 into i
> >   repeat until i > fld 1 -- fld 1 contains 3000000000
> >      if P[i] is empty then
> >         put bAdd(i,T) into T
> >         put i & comma after P[i*3]
> >      else
> >         repeat for each item x in P[i]
> >            put x & comma after P[i + x + x]
> >         end repeat
> >         delete P[i]
> >      end if
> >      if the seconds > S then
> >         put the seconds into S
> >         put i && T into fld 2
> >         unlock screen
> >         lock screen
> >      end if
> >      add 2 to i
> >   end repeat
> >   put i && T into fld 2
> > end mouseUp
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```