Finding the largest prime factor of a number using a C program

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I wrote a code to which takes a number as input and displays the largest prime factor of it. The program is working fine till 9 digit numbers. After 9 digit numbers it is behaving weirdly showing negative numbers.

#include<stdio.h>
 
int largestprimefactor(unsigned long a)
{
    int i =2 ,largeprimefactor = 2;
     
    while(a!=1)
    {
        if(a%i==0)
        {
            while(a%i==0)
            {
                a = a/i;
                 
                printf("%d ",i);
                 
                if(i>largeprimefactor)
                {
                    largeprimefactor = i;
                }
            }
        } 
         
        i++;
    }
     
    return largeprimefactor;
}
 
main()
{
    unsigned long inputnumber;
     
    printf("Enter a number : ");
    scanf("%d",&inputnumber);
     
    printf("\nThe largest prime factor of %d is %d",inputnumber,largestprimefactor(inputnumber));
}

What I have tried:

I thought it might be a data type problem so I tried using unsigned long long but it doesn't help.

Posted 27-May-18 19:19pm

Zeeking99

Updated 27-May-18 22:13pm

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2 solutions

Solution 2

Quote:
Finding the largest prime factor of a number using a C program

If you input 1, your program will say that the largest prime factor is 2. I suspect an error.
You also need to handle the result for 0 and negative numbers to be complete.

You can simplify your code

C++

while(a!=1)
{
    if(a%i==0) // you can remove the test because
    {
        while(a%i==0) // the condition is the same
        {
            a = a/i;

            printf("%d ",i);

            if(i>largeprimefactor)
            {
                largeprimefactor = i;
            }
        }
    }

    i++;
}

[Update]
You can even simplify more.

C++

while(a!=1)
{
    if(a%i==0) // you can remove the test because
    {
        while(a%i==0) // the condition is the same
        {
            a = a/i;

            printf("%d ",i);

            if(i>largeprimefactor)  // because i is never lower than largeprimefactor
            {
                largeprimefactor = i;
            }
        }
    }

    i++;
}

Posted 27-May-18 22:15pm

Patrice T

Updated 28-May-18 2:19am

v3

Comments

nv3 28-May-18 5:44am

I doubt that will work if you leave the i++ out of the loop :-)

Patrice T 28-May-18 5:55am

Read the code more carefully. :)

nv3 28-May-18 6:07am

Got it. Sorry.

Patrice T 28-May-18 6:09am

You're welcome :)

Zeeking99 28-May-18 8:34am

Sometimes i is less than largeprimefactor.

Patrice T 28-May-18 9:03am

Never. Use the debugger to see by yourself.
By just reading your code, I can ensure you that i is never less than largeprimefactor

Zeeking99 28-May-18 13:04pm

Yeah you are right. Sorry I was wrong.

Patrice T 28-May-18 13:16pm

Don't be sorry, everyone was a newbe once.
Be happy to have learned something.
If you got a try to debugger, it is the most useful thing you have learned today.

Zeeking99 18-Jun-18 11:58am

Yeah

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User 7429338 · Accepted Answer · 2018-05-27T19:44:00

While inputnumber is a long, your scanf still only expects an integer. You can accept an unsigned long like so:

unsigned long inputnumber;
scanf("%lu",&inputnumber);

Or an unsigned long long:

unsigned long long inputnumber;
scanf("%llu",&inputnumber);

For reference of the formats, see scanf - C++ Reference[^]

When showing the result, you should modify the printf format accordingly.