Converting C++ for loop python

Question

Tried searching around for a similar question, but was unable to solve this. Not sure how to properly convert some of the features in C++ loops. Especially the count < 20 condition in main().

Original question: https://rosettacode.org/wiki/Anti-primes

#include <iostream>

int countDivisors(int n) {
    if (n < 2) return 1;
    int count = 2; // 1 and n
    for (int i = 2; i <= n/2; ++i) {
        if (n%i == 0) ++count;
    }
    return count;
}

int main() {
    int maxDiv = 0, count = 0;
    std::cout << "The first 20 anti-primes are:" << std::endl;
    for (int n = 1; count < 20; ++n) {
        int d = countDivisors(n);
        if (d > maxDiv) {
            std::cout << n << " ";
            maxDiv = d;
            count++;
        }
    }
    std::cout << std::endl;
    return 0;
}

My attempted solution:

def countDivisors(n):
    if (n < 2):
        return 1
    count = 2

    for i in range(2, int(n/2)-1, 1):
        print(i)    
        if(n%i == 0):
            count = count + 1
    return count

def main():
    print("The first 20 anti-primes are: ")
    n = 1
    while count < 20:    
        d = countDivisors(n)
        if(d > maxDiv):
            print(n)
            maxDiv = d
            count += 1
            n += 1
    return 0

Required answer:

1 2 4 6 12 24 36 48 60 120 180 240 360 720 840 1260 1680 2520 5040 7560

You reference maxDiv too early. What is initial value for maxDiv? — Andrew Hamel
– Andrew Hamel, Commented Dec 16, 2019 at 19:00
0. i actually initialized maxDiv and count to 0, but i removed it before i posted here. — Storm Claw
– Storm Claw, Commented Dec 16, 2019 at 19:04
Do your best to avoid thinking in C++ when writing Python code. Logic may be logic, but how best to implement logic in the two languages is very, very different. — user4581301
– user4581301, Commented Dec 16, 2019 at 19:33

tevemadar · Accepted Answer · 2019-12-16 19:35:01Z

1

Strictly speaking about the two for loops:

for (int i = 2; i <= n/2; ++i)

would become

for i in range(2,int(n/2)+1):

The int(n/2)-1 in your code would be i < n/2-1 in C.

The 20 one,

int maxDiv = 0, count = 0;    // <-- !
for (int n = 1; count < 20; ++n) {
    int d = countDivisors(n);
    if (d > maxDiv) {
        std::cout << n << " ";
        maxDiv = d;
        count++;
    }

is almost there, just you do not have maxDiv and count initialized in the Python one.

maxDiv = 0  # <-- !
count = 0   # <-- !
n = 1
while count < 20:    
    d = countDivisors(n)
    if(d > maxDiv):
        print(n)
        maxDiv = d
        count += 1
    n += 1

edited Dec 16, 2019 at 19:35

answered Dec 16, 2019 at 19:05

tevemadar

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3 Comments

Martin Bonner supports Monica Over a year ago

Your translation of the second loop is wrong. The original one in C++ will loop more than 20 times, and break whencount, not i has reached 20.

tevemadar Over a year ago

@MartinBonnersupportsMonica true, I was focusing on the other loop.

tevemadar Over a year ago

@StormClaw: I do not want to edit it again, but the n += 1 part was an "autocorrect" without commenting: so in the question it was "too indented" and thus part of the if, not the while. Then it works, see ideone.com/48bWNu

Andrew Hamel · Accepted Answer · 2019-12-16 19:09:18Z

0

You could try this. Does this yield the result you're looking for?

def countDivisors(n):
    if (n < 2):
        return 1
    count = 2

    for i in range(2, int(n/2)-1, 1):
        print(i)    
        if(n%i == 0):
            count = count + 1
    return count

def main():
        print("The first 20 anti-primes are: ")
        n,maxDiv = 1,0
        for d in range(1,20):    
            d = countDivisors(n)
            print(d)
            if(d > maxDiv):
                maxDiv = d
            n+=1
        return 0
if __name__ == "__main__":
    main()

edited Dec 16, 2019 at 19:09

answered Dec 16, 2019 at 19:04

Andrew Hamel

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2 Comments

Storm Claw Over a year ago

tried it, didnt work out of the box. I'll take a closer look later to see if any small changes could possibly fix it.

Storm Claw Over a year ago

added it in question, along with original question

Martin Bonner supports Monica · Accepted Answer · 2019-12-16 19:27:08Z

0

Fundamentally the C for loop and the Python for loop are different. You have two choices.

The first choice is to do as you did, and use a Python while loop to replicate the test-and-stop part of the C for loop. If you do this, you have to write out the iteration yourself.

The second choice is to use a Python for loop to replicate the iteration, and write out the test-and-stop part of the code.

import itertools
.
.
.
        for n in itertools.count(1):    
            d = countDivisors(n)

            if(d > maxDiv):
                print(n)
                maxDiv = d
                count += 1
                if count >= 20:
                    break

I think I prefer this form, but it is very much a matter of taste.

answered Dec 16, 2019 at 19:27

Martin Bonner supports Monica

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1 Comment

Storm Claw Over a year ago

I'm trying to avoid using libraries preferably hence why i'm using the while method. It's getting late here, so i'm gonna take another crack at this tomorrow. Mabye someone will come up with a solution by then.

Michael Chatiskatzi · Accepted Answer · 2019-12-16 21:16:02Z

0

I use a map function within countDivisors.

def countDivisors(n):
    if n < 2:
        return 1
    return 2 + sum(map(lambda x: 0 if n % x else 1, range(2, int(n / 2) + 1)))


def main():
    max_div = 0
    count = 0
    print("The first 20 anti-primes are:")
    x = 1
    while count < 20:
        d = countDivisors(x)
        if d > max_div:
            print(x, end=' ')
            max_div = d
            count += 1
        x += 1
    return 0


if __name__ == '__main__':
    main()

Try it online!

Output:

The first 20 anti-primes are:
1 2 4 6 12 24 36 48 60 120 180 240 360 720 840 1260 1680 2520 5040 7560

edited Dec 16, 2019 at 21:16

answered Dec 16, 2019 at 20:51

Michael Chatiskatzi

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