Question

I am trying to solve this question " Create a program to count by prime numbers. Ask the user to input a number, then print each prime number up to that number." And I wrote the following code:
number = int (raw_input ("Enter a number and the program will give you a list of prime numbers upto that number: "))
for item in range (2, number):
x = 1 or item
y = int (y)
y != x
if item%y == 0:
break
else:
print item


It says y is not defined. Actually, I want to set y as "any other integer than 1 or the item in the range (2, number) ". Please help!

Answer 1

x=1 or item
what are you trying to do here?
and if I get you right i don't think you can do such thing with y

I understood you algorithm, what you trying to do but i don't think it's possible, you should thing another way of calculating primes

Answer 2

thank you for replying. Prime numbers aren't divisible by any other numbers than '1' or 'themselves'. So before writing the argument item%y, I wanted to assign y as an integer which is neither '1' nor the item being divided. To do this I set x = 1 or item (i.e. the item being dealt with in every loop in the range (2, 100)). Then, I set y != x. But this gave me an error saying y is not defined. Everything else seems to be ok though. I think "y = int y" or something like that works in java. All i want to do is set y as an integer which is neither 1 nor the item(being dealt with) in the range.

Answer 3

Try initializing y outside of the for loop

Answer 4

As a simple exercise to get started, consider writing an odd number generator and then add to that. e.g.

>>> for i in range(3,30):
... if i%2 != 0:
... oddlist.append(i)
...
>>> print oddlist
[3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29]

Also, you don't need to worry about setting data types.

Answer 5

or use a second loop for divisors

print 1, 2, 3
for number in the range of five to user input:
for divisor in the range of two to number:
if number mod divisor equals zero:
number is not prime
stop testing number
if number is prime:
print number

Answer 6

Thankyou all for replying!
@abishop: I tried y= int(y) outside the loop. Still it says y is not defined.
@mkorangestripe: For odd numbers it is quite easy, but things get a bit trickier for prime nos.
@bwCA: I wrote the following code as per your instructions:
print 1, 2, 3
input = int (raw_input ("Give a number: "))
for number in range (5, input):
for divisor in range (2, number):
if number%divisor == 0:
break
else:
print number
(hope i wrote it correctly)
But i get numbers like 9, 15, 21.... along with prime numbers in the result.

Answer 7

if you just want the answer, here it is
http://dpaste.com/663809/

Answer 8

thank you so much! it works :)

Answer 9

how can you do the same in C syntax