# Python code for generating prime# list and evaluting a number

• Comp Sci

## Homework Statement

I am attempting to allow a user to enter a number and check to see if it is in a list of already generated prime numbers

Here's my attempt

Where can I put the code to allow the number to be checked, the prime list generator works fine alone

Code:
def buildPrimeList ():

primeList = [1, 2]
possiblePrime = 3
print "To calculate a list of prime values less than or equal a number..."

x=int(raw_input("Enter a number higher then 3   "))

while (possiblePrime <= x):  # changed input method
divisor = 2
isPrime = True
y= int(raw_input("To see if a number is in the list enter that number enter -1 to exit: "))
while (divisor < possiblePrime ): #change condition to only less than possible prime
if ((possiblePrime % divisor) == 0):
isPrime = False
divisor = divisor + 1

if (isPrime):
primeList.append(possiblePrime)
print primeList

if y== -1:
print "Goodbye"
break
if y in primeList:
print y, "Is in the list"
if y not in primeList:
print y, "Is not in the list"

possiblePrime = possiblePrime + 2

return primeList

buildPrimeList()

Last edited:

## Answers and Replies

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Dick
Homework Helper
Why are you asking them to enter a number to check if it's in the list before you are done building the list? Why don't you move the y stuff outside of the loop that builds the list?

The user is supposed to be repeatedly prompted, that is why the y=raw_input is in the loop

Dick
Homework Helper
It's still in the wrong place. Make a loop outside of buildPrimeList and put your input there.

Thanks,

I did this and it works well enough, any general suggestions on the code

Code:
def buildPrimeList ():

primeList = [1, 2]
possiblePrime = 3
print "To calculate a list of prime values less than or equal a number..."

x=int(raw_input("Enter a number higher then 3   "))

while  (possiblePrime <= x):  # for personal preference changed range to raw_input, so that the range can be arbitrary, though at a certain point a large range will consume a-lot of memory
divisor = 2
isPrime = True

while (divisor < possiblePrime ):
if ((possiblePrime % divisor) == 0):
isPrime = False
divisor = divisor + 1
if (isPrime):
primeList.append(possiblePrime)

possiblePrime = possiblePrime + 2
while (primeList==primeList):
y= int(raw_input("To see if a number is in the list enter that number enter -1 to exit: "))
if y== -1:
print "Goodbye"
break
if y in primeList:
print y, "Is in the list"
if y not in primeList:
print y, "Is not in the list"

return primeList