Re: First Program Bug (Newbie)
- From: Mensanator <mensanator@xxxxxxx>
- Date: Mon, 17 Mar 2008 18:20:14 -0700 (PDT)
On Mar 17, 7:03 pm, Benjamin Serrato <benjamin.serr...@xxxxxxxxx>
wrote:
I Found It!! The following was a post asking for help finding a bug. I
thought I needed help with my syntax, but just before sending I found
the bug on line 13. Line 13 should read: "base = 2". I would still
appreciate any comments on my program. Maybe there is a better way to do
it that I didn't think about. I know it's not that interesting but it's
my first one.
[post]
I'm almost halfway through an online tutorial I found on the python.org
site and decided to stop and write a program for myself. I got it in my
head to write a program to print all the primes; the idea came from
another tutorial. The program prints some non-prime numbers. In a
comparison to lists of primes the program prints eight non-primes for
numbers less than 150. Those numbers are [27,35,87,95,119,123,143,147].
Here is the program.
base = 2
candidate = 3
while True:
while candidate % base != 0:
base = base + 1
if base > (candidate / 2):
print candidate
candidate = candidate + 1
base = 2
else:
candidate = candidate + 1
The following is a rundown of the program.
candidate: A possible prime number.
base: Start point for divisibility testing.
outer while loop: Causes the program to loop.
inner while loop: Obviously, checks if 'candidate' mod 'base' equals
zero. If not base is incremented by one then 'if loop'.
if loop: After base has been incremented this checks whether all the
possible divisors have been used up. If they have then 'candidate' must
be prime. So, candidate is printed, a new candidate is chosen and the
base is reset.
else loop: If 'candidate' mod 'base' equals zero, then 'candidate' is
not prime and a new candidate is chosen.
I realize yall probably didn't need all that.
At first I tried to use 'return' on the 'else' block to cause the
program to loop, but I don't understand 'return' yet and that didn't
work. So, I put the rest into another while loop and was really happy to
find it worked but the program prints some non-prime numbers.
Thanks, Benjamin Serrato
P.S. What is the chance I'll get spam for using my real email address? I
currently don't get any so...
[/post]
Several items.
First, you don't need to check base larger than sqrt(candidate).
Second, see comments following dc.
Third, you need to add base = 2 to end of program, otherwise next
candidate starts base where previous candidate left off.
Fourth, use +=1 to increment.
Finally, throw this away and use gmpy. :-)
import gmpy # for Quality Assurance
base = 2
candidate = 3
p = 0 # prime count
while p<100: # limit the loop to 100 primes
while candidate % base != 0:
base += 1
#if base > (candidate / 2):
if base > (gmpy.sqrt(candidate)): # only need to test to
sqrt(candidate)
dc = divmod(candidate,2) # remainder==0 gives false
primes
if dc[1]==1:
#
# the gmpy functions are for QA, verifies that what you call a
prime
# really is one and the next_prime makes sure you don't skip
any
# these can be removed once working
#
candidate,gmpy.is_prime(candidate)>0,gmpy.next_prime(candidate)
p += 1
candidate += 1
base = 2
else:
candidate += 1 # false prime, reset
base = 2
else:
candidate += 1
base = 2 # failure to reset causes false
primes
## 3 True 5
## 5 True 7
## 7 True 11
## 11 True 13
## 13 True 17
## 17 True 19
## 19 True 23
## 23 True 29
## 29 True 31
## 31 True 37
## 37 True 41
## 41 True 43
## 43 True 47
## 47 True 53
## 53 True 59
## 59 True 61
## 61 True 67
## 67 True 71
## 71 True 73
## 73 True 79
## 79 True 83
## 83 True 89
## 89 True 97
## 97 True 101
## 101 True 103
## 103 True 107
## 107 True 109
## 109 True 113
## 113 True 127
## 127 True 131
## 131 True 137
## 137 True 139
## 139 True 149
## 149 True 151
## 151 True 157
## 157 True 163
## 163 True 167
## 167 True 173
## 173 True 179
## 179 True 181
## 181 True 191
## 191 True 193
## 193 True 197
## 197 True 199
## 199 True 211
## 211 True 223
## 223 True 227
## 227 True 229
## 229 True 233
## 233 True 239
## 239 True 241
## 241 True 251
## 251 True 257
## 257 True 263
## 263 True 269
## 269 True 271
## 271 True 277
## 277 True 281
## 281 True 283
## 283 True 293
## 293 True 307
## 307 True 311
## 311 True 313
## 313 True 317
## 317 True 331
## 331 True 337
## 337 True 347
## 347 True 349
## 349 True 353
## 353 True 359
## 359 True 367
## 367 True 373
## 373 True 379
## 379 True 383
## 383 True 389
## 389 True 397
## 397 True 401
## 401 True 409
## 409 True 419
## 419 True 421
## 421 True 431
## 431 True 433
## 433 True 439
## 439 True 443
## 443 True 449
## 449 True 457
## 457 True 461
## 461 True 463
## 463 True 467
## 467 True 479
## 479 True 487
## 487 True 491
## 491 True 499
## 499 True 503
## 503 True 509
## 509 True 521
## 521 True 523
## 523 True 541
## 541 True 547
## 547 True 557
.
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