Question

I was trying to solve Project Euler problem 10 using python, but my program gave a wrong result. Since I am a complete noob to python and I could not find any fault in my (apparently brute-force) logic, I wrote a program in java (almost translated it), and it gave a different result, which turned out to be right.

Here is the python code:

from math import *

limit = 2000000

def isPrime(number):
    if number == 2: return 1
    elif number % 2 == 0: return 0
    elif number == 3: return 1
    elif number == 5: return 1
    elif number == 7: return 1
    else:
        rootOfNumber = sqrt(number)
        tag = 3
        while tag < rootOfNumber:
            if number % tag != 0:
                tag += 2
            else:
                break           ###   
        if tag >= rootOfNumber: ###EDIT: it should by only tag > rootOfNumber here
            return 1            ###       Thats what the problem was.
        else:
            return 0

sum = 2 # 2 is an even prime, something we are not iterating for
for i in range(3, limit, 2):
    if isPrime(i) == 1:
        sum += i

print(sum)
print('done...')

The equivalent java code is:

public class Prob10{
static int limit = 2000000;
static long sum = 2L; // 2 is an even prime, something we are not iterating for

public static void main (String[] args) {
    for(int i = 3; i < limit; i+=2) {
    if( isPrime(i) ) 
        sum += i;
    }
    System.out.println(sum);
}

private static boolean isPrime (int number) {
    if (number == 2) return true;
    else if (number == 3 || number == 5 || number == 7) return true;
    else {
        double rootOfNumber = Math.sqrt(number);
        int tag = 3;
        while (tag < rootOfNumber) {
            if (number % tag != 0) 
                tag +=2;
            else
                break;
        }
        if (tag > rootOfNumber)
            return true;
        else
            return false;
    }
}

}

I think I am doing some silly mistake or missing some subtle point.

p.s. I know my isPrime implementation is not too good. I am not printing the outputs because it may spoil the problem for others.

Any comments about (bad) style in the python program are welcome.

Answer 1

Try running with your code for example isPrime(49). You should figure out your problem from there. You have replaced a > with a >= in if (tag > rootOfNumber)
.Also as some coding style, you could just replace the first lines with:

if i in (2, 3, 5, 7): return 1
elif number % 2 == 0: return 0
else:
    ......