First Program Bug (Newbie)

[Benjamin Serrato]

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]

 

[Mensanator]

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
#
print
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

 

[Daniel Fetchinson]

P.S. What is the chance I'll get spam for using my real email address?

Exactly 1.

I currently don't get any so...

.... you will get now.

Sorry to disappoint you, but it's better to be prepared in advance
than be shocked later

Cheers,
Daniel

 

[Gabriel Genellina]

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.

You may look at the fact.py demo script (somewhere in your python
installation)

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
base = 2 # added

Note that you can increment candidate by 2 (starting at 3, all primes
are odd numbers). And after testing 2, you can increment base by 2
also.
And you can exit somewhat earlier: if base*base>candidate there is no
point in keep trying (now you`re exiting when base*2>candidate)

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.

return only makes sense inside a function. Wait until the next
lesson...

 

[Jeff Schwab]

P.S. What is the chance I'll get spam for using my real email address?

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[oog1e]

Hey, big thanks to you and Gabriel for replying. I couldn't quite follow
what you did yet. What is this 'gimpy' thing and where can I read about
it? In response:

First: Heh, I'm a little embarrassed I didn't notice this. I thought 'I
only need to check up to half' but it didn't occur that base**2 was more
than 1/2 candidate.

Second: I don't quite follow this.

Third: Yeah, I found that bug just before, and because of, posting.

Fourth: I used the incrementing command and incremented candidates by
two to check only odds and now the program is a lot faster.

Thanks a lot for yall's replies. Three more things before I go. What is
the square root function in python? I couldn't find the fact.py demo
script. I didn't mean return I meant 'continue'.

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
#
print
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

 

[Mensanator]

Hey, big thanks to you and Gabriel for replying. I couldn't quite follow
what you did yet. What is this 'gimpy' thing

gmpy. It's the GMP (Gnu Multi-Precision) C-library in
a Python wrapper. It supports arbitrary precision integers,
floats and rationals. Although Python has built in Big
Integers, the gmpy versions can sometimes run rings around
Python's. Also, lots of functions unavailable elswhere.

and where can I read about it?
http://code.google.com/p/gmpy

In response:

First: Heh, I'm a little embarrassed I didn't notice this. I thought 'I
only need to check up to half' but it didn't occur that base**2 was more
than 1/2 candidate.

Second: I don't quite follow this.

Actually, I'm not following it anymore either. For a while my
test program was claiming 27 was prime. All these false primes
had 0 as the remainder of candidate/2. I put this in to reject
them, but I don't see the problem anymore.

Third: Yeah, I found that bug just before, and because of, posting.

Fourth: I used the incrementing command and incremented candidates by
two to check only odds and now the program is a lot faster.

Thanks a lot for yall's replies. Three more things before I go. What is
the square root function in python?

There's one in the math module. Use the one in gmpy if you have it.

I couldn't find the fact.py demo
script. I didn't mean return I meant 'continue'.

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
        #
        print
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

 

[Andreas Tawn]

[snip]

There's one in the math module. Use the one in gmpy if you have it.

Or raise to the power 1/power
3.0

Also works for cube root, quad root etc.

3.0

3.0

3.0

Cheers,

Drea