Why does my program stop working for large values?

Question

Hi I wrote this program in response to the following question:

"What is the largest prime factor of the number 600851475143?"

The program works for "composite" equal to 13195, but it does not work if I set it equal to 600851475143.

Why does this happen?

composite = 600851475143
for m in range (2,composite):
    while composite % m == 0:
        if composite / m == 1:
            break
        else:
            composite = composite / m
print(composite)

What flavor of python do you use? Your code will not do what you expect it to do in Python 3. — DYZ
– DYZ, Commented Jun 18, 2018 at 21:48
You've got a 600-billion-iteration outer loop. That's not going to finish quickly. — user2357112
– user2357112, Commented Jun 18, 2018 at 21:52
Sorry. When I say it doesn't work I mean to say that nothing happens. The program doesn't run for a sufficiently large value of "composite." — Sir know-a-ton
– Sir know-a-ton, Commented Jun 18, 2018 at 21:52

Zev · Accepted Answer · 2018-06-18 21:58:21Z

2

The issue is the combination this line:

for m in range (2,composite):

and this line:

    composite = composite / m

Range won't recalculate while you are looping. See this question for more details: Changing the number of iterations in a for loop

Instead, your code should be the following (to fix that issue):

composite = 600851475143

m = 2
while m < composite:
    while composite % m == 0:
        print(composite, m)
        if composite / m == 1:
            break
        else:
            composite = composite / m
    m += 1
~

answered Jun 18, 2018 at 21:58

Zev

3,5111 gold badge23 silver badges43 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Why does my program stop working for large values?

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related