Prime numbers

[Gordon]

Someone tested this listing with RB and LB
cls:print time$()
for priem=999999999901 to 999999999999 step 2
for deler=3 to sqr(priem) step 2
if priem mod deler = 0 then exit for
next deler
if priem mod deler = 0 then [geenPriem]
print priem
[geenPriem]
next priem
print time$()
end

LB works slowly but flawless
RB runs fast but makes two misstakes

Gordon

[Gordon]

Are there some differences between the
sqr function or the mod function of RB and LB?

Gordon

This listing gives two wrong answers in RB

[Carl Gundel - admin]

Sorry, I'll try to figure this out. Right now I don't have an answer.

-Carl

[Carl Gundel - admin]

Okay, this is a very strange bug in the FOR/NEXT loop. For some reason your posted program it is not stepping by 2. The following works as expected but the code you provided acts as though there is no step 2:

for priem=999999999901 to 999999999999 step 2
print priem
next priem
Here is a reworked program that works correctly:

cls:print time$()
priem=999999999901
while priem <= 999999999999
for deler=3 to sqr(priem) step 2
if priem mod deler = 0 then exit for
next deler
if priem mod deler = 0 then [geenPriem]
print priem
[geenPriem]
priem = priem + 2
wend
print time$()
end
What's encouraging about this is that the reworked program is even faster than the version you posted.

I will get to the bottom of this. THANKS for posting this bug.

-Carl