Question

I’ve created a prime number generator and wanted the user to specify the number of prime numbers generated. I was wondering how I could change “1000” to a raw_input without the program throwing errors back at me as it has been doing. Also how do I save this list generated to a .txt file? Thanks in advance

Code:

no_of_primes = 1
candidate = 2
start = 2
list_of_primes = []

while no_of_primes <= 5000:
    result = candidate % start

    if result > 0:
        start +=1

    elif result == 0:
        if start == candidate:
            list_of_primes.append(candidate)
            candidate +=1
            no_of_primes +=1
            start =2
        else:
            candidate +=1
            start = 2

print list_of_primes

Answer 1

First, replace the 5000 with a variable (like, say, primes_to_generate). Then before the while loop, set that variable by calling int on the result of raw_input. The int will convert it from a string to a number, fixing your error:

primes_to_generate = int(raw_input("How many to generate? "))

You can also use a try block to catch errors for if someone, say, types in “ninety” rather than “90”:

try:
    primes_to_generate = int(raw_input("How many to generate? "))
except ValueError:
    print "Whoa! That's not a number I know!"
    # You'd probably then want to let the user retry
    # by enclosing this whole thing in a loop or something.