On python, how can i print up to a certain amount of random numbers from a list of generated numbers?

Question

I have it set so the user enters a max input and then the program figures out which numbers are prime and then prints it all to a file. But at the moment it only prints the closest prime number to the upper input. I need it to print 20 random numbers and put them on a file. Also with print (num) 20 times it can print 20 different outputs, but if I add print (num) 20 times it prints the same number 20 times

import random
#Always start at 2
lower = 2
#Take input from the user
upper = int(input("Enter maximum size for prime number to be: "))
for num in range(lower,upper + 1):
  if num > 1:
    #Prime numbers are greater than 1
    for i in range(2,num):
        if (num % i) == 0:
            break
        #End program if number is 0
        #Print results
    else:
        randomprimes =(num)
        primes =(random.choice(randomprimes))
        import sys
        former, sys.stdout = sys.stdout, open('Prime Number Generator Output.txt', 'w')
        print (primes)
        results, sys.stdout = sys.stdout, former
        results.close()

Please don't like to an image of your code. Include your code in your question. — BrenBarn, Jan 25 '15 at 23:50

L3viathan · Answer 1 · 2015-01-26T00:14:11.257

2

random.choice from the random module returns a random item from a given iterable.

If you want 20 random items from a given list without repeating, use random.sample. (Thanks, Hugh Bothwell)

edit: Wouldn't normally do this, but here's a solution:

import random, math

upper = int(input("Enter maximum size for prime number to be: "))
primes = [x for x in range(2, upper) if x not in [j for i in range(2, math.ceil(math.sqrt(upper))) for j in range(i*2, upper, i)]]
with open("Prime Number Generator Output.txt", "w") as f:
    for prime in random.sample(primes, min(len(primes),20)):
        f.write(str(prime) + "\n")

edited Jan 26 '15 at 00:14

answered Jan 25 '15 at 23:50

L3viathan

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... for 20 items, use `random.sample()` – Hugh Bothwell Jan 25 '15 at 23:51
so how would i put that in my code – Zarnab Qureshi Jan 26 '15 at 00:01
I put a solution to what I think the problem is in my answer. – L3viathan Jan 26 '15 at 00:14
i added random choice and i keep getting this error from it. Traceback (most recent call last): File "C:\Users\USER\Desktop\Python\Prime Number test TASK 3.py", line 19, in primes =(random.choice(randomprimes)) File "C:\Python34\lib\random.py", line 253, in choice i = self._randbelow(len(seq)) TypeError: object of type 'int' has no len() – Zarnab Qureshi Jan 26 '15 at 00:17
Can't really say much without seeing the code that caused this, but it's likely `randomprimes` isn't a list/sequence, but rather a single integer. – L3viathan Jan 26 '15 at 00:20
edited the code in the original question also tried randomprime =(random.choice(num)) where i have print (primes). same error random.samples gave same error but the error was coloured like code instead of red – Zarnab Qureshi Jan 26 '15 at 00:23
yes, `num` is a single int. I'm not gonna solve your homework for you. – L3viathan Jan 26 '15 at 00:27
i dont understand why num being a single integer would affect it – Zarnab Qureshi Jan 26 '15 at 00:28
Then you haven't read my answer thoroughly. Also look up the python docs for random.choice. – L3viathan Jan 26 '15 at 00:28
dont worry now. i retried your solution and i works now. i retract my last comment – Zarnab Qureshi Jan 26 '15 at 00:35

On python, how can i print up to a certain amount of random numbers from a list of generated numbers?

1 Answers1