Question

I need a little assistance with pst1a. Here is a copy of my code.

http://dpaste.com/534798/

My thinking was, before count hits 1000, add 1 to x. If it is odd, do a for loop on it dividing x by every integer from 2 to x. If there is a remainder and x is not divided by itself then it must be prime, add to the list, add a count and break. So far its only giving me back all the odd numbers. Where did I go wrong in this logic?

Answer 1

Break only works within while/for loops

http://docs.python.org/release/2.5.2/ref/break.html

So what you are actually doing is for every odd number, creating a list starting at 2, all the way to x (line8)

(@Line 9) Then using the first number in that list, 2, dividing your odd number by it and checking to see if X is equal to 2 (Which it isn't because you are only picking odd numbers)

Obviously this will store every odd number because you are ALWAYS checking them against the integer 2.

Try rethinking your usage of range in this function. =)

Answer 2

do the opposite case if there is not a remainder and they are not equal then break since it is not prime. so we move on to the next quicker. if you look for a remainder like now you are basically appending every situation that fulfills the if condition, but you just want to append the prime.

Answer 3

I also picked 2 for my range but one could just start at 3, won't matter, but you see I took the opposite case...

# prime.py
# finds primes up to 1000


list=[]
for k in range(1001):
if ((k%2)==1):
for j in range(2,k+1):
if( ((k%j)==0) and j<k):
break
elif (not(k%j) and j==k):
list.append(k)
print list

Answer 4

I was able to get my code to work after I broke yours apart a little Beeb.

count = 0
x = 1
prime = [2]

while count < 1000:
x += 1
for i in range(2, x+1):
if x%i == 0 and x != i:
break
elif not(x%i) and i == x:
prime.append(x)
count += 1
print prime
I just have a question. I dont understand what is happening in this line

elif not(x%i) and i == x:

Can you please explain. Thanks