Prime Factor Program

[Evan Keenlyside]

Ok, I have a problem.

I'm trying to make a program that prints all the prime factors of a given number and the 29 numbers one less than it. These numbers iare 432731 -> 432702.

So here is my program:

public class evan5ccc
{
/* the method tests whether a number is prime*/

public static boolean Primes(int input)
{
for (int x=2; x < input; x++)
if (input % x == 0)
return false;
return true;
}

/*
computes the smallest prime factor of the input number,
returns that number divided by its smallest prime factor
*/

public static int factor (int input)
{
if (input == 1)
return input;
else
{
for (int x=2; x <= input; x++)
if (input % x == 0)
{
System.out.print(" " + x );
return (input / x);
}
}
return 0;
}

public static void main(String args[])
{
int number;// number to be decomposed

for (number=432731; number > 432701; number--)

System.out.println("Prime factors of " + number + ":");


while (number != 1)
{
number = factor(number);
}
}
}

And here is the output:

Prime factors of 123456760:
Prime factors of 123456761:
Prime factors of 123456762:
Prime factors of 123456763:
Prime factors of 123456764:
Prime factors of 123456765:
Prime factors of 123456766:
Prime factors of 123456767:
Prime factors of 123456768:
Prime factors of 123456769:
Prime factors of 123456770:
Prime factors of 123456771:
Prime factors of 123456772:
Prime factors of 123456773:
Prime factors of 123456774:
Prime factors of 123456775:
Prime factors of 123456776:
Prime factors of 123456777:
Prime factors of 123456778:
Prime factors of 123456779:
Prime factors of 123456780:
Prime factors of 123456781:
Prime factors of 123456782:
Prime factors of 123456783:
Prime factors of 123456784:
Prime factors of 123456785:
Prime factors of 123456786:
Prime factors of 123456787:
Prime factors of 123456788:
Prime factors of 123456789:
3 3 3607 3803

This is copied and pasted from the sample output, so the numbers are wrong, but it basically prints just the last number's prime factors, instead of all 30. Also, it prints it on the next line, because i used println, but if I use print then it all goes into a big mess.

The output is supposed to look like this:

Prime factors of 123456760: 2 2 2 5 7 271 1627
Prime factors of 123456761: 123456761
Prime factors of 123456762: 2 3 3 11 13 47963
Prime factors of 123456763: 4021 30703
Prime factors of 123456764: 2 2 617 50023
Prime factors of 123456765: 3 5 523 15737
Prime factors of 123456766: 2 1051 58733
Prime factors of 123456767: 7 17636681
Prime factors of 123456768: 2 2 2 2 2 2 2 2 3 160751
Prime factors of 123456769: 53 283 8231
Prime factors of 123456770: 2 5 29 425713
Prime factors of 123456771: 3 3 3 17 268969
Prime factors of 123456772: 2 2 30864193
Prime factors of 123456773: 11 83 135221
Prime factors of 123456774: 2 3 7 7 419921
Prime factors of 123456775: 5 5 13 19 19993
Prime factors of 123456776: 2 2 2 79 195343
Prime factors of 123456777: 3 5779 7121
Prime factors of 123456778: 2 23 1531 1753
Prime factors of 123456779: 109 173 6547
Prime factors of 123456780: 2 2 3 3 5 47 14593
Prime factors of 123456781: 7 41 149 2887
Prime factors of 123456782: 2 61728391
Prime factors of 123456783: 3 2797 14713
Prime factors of 123456784: 2 2 2 2 11 11 43 1483
Prime factors of 123456785: 5 24691357
Prime factors of 123456786: 2 3 20576131
Prime factors of 123456787: 31 31 128467
Prime factors of 123456788: 2 2 7 13 17 71 281
Prime factors of 123456789: 3 3 3607 3803

So how do I make all 30 prime factorizations happen? I've been staring at this code for some time now and I can't figure it out.

Can anybody help?

[Layne Lund]

Look at this bit of code:

public static void main(String args[])
{
  int number; // number to be decomposed
 
  for (number=432731; number > 432701; number--)
    System.out.println("Prime factors of " + number + ":");
 
  while (number != 1)
  {
    number = factor(number);
  }
}

With the above formatting, can you see why only the last number is factored? If not, here's an explanation:

The for loop repeats only the first statement immediately following it. In this case, the println() statement is repeated for each value of number. Finally, teh while loop repeats and factors the current value of number (which happens to be the last one you want.

So to start fixing this problem, you need to make sure the for loop repeats the factoring alogrithm for each number. Something like this will get you started in the right direction:

for (number=432731; number > 432701; number--) {
  System.out.println("Prime factors of " + number + ":");
 
  while (number != 1)
  {
    number = factor(number);
  }
}

This will get the repetition started. However, you have another problem: the variable number changes in the while loop. I'll let you play with this to see if you can come up with a solution to this problem.

Good luck!

Layne

[Evan Keenlyside]

When I put brackets around the for loop contents it just prints out one statement.

And how do I fix this part:

number = factor(number);

That has to be wrong. I have a feeling that is what is screwing it up.

[Evan Keenlyside]

Anybody? Please, I really need help on this one. It's driving me crazy. I don't know how to resolve the variable problem. This is really frustrating. Everything I try and do either sends the program into a horrible infinite loop or makes the output just one line.

[Fisher Daniel]

Hi Evan,
I want to help you...
I have changed your code like this...

  
public class evan5ccc
{
/* the method tests whether a number is prime*/
 
public static boolean Primes(int input)
{
for (int x=2; x < input; x++)
if (input % x == 0)
return false;
return true;
}
 
/*
computes the smallest prime factor of the input number,
returns that number divided by its smallest prime factor
*/
 
public static int factor (int input)
{
if (input == 1)
return input;
else {
for (int x=2; x <= input; x++) {
if (input % x == 0)
{
System.out.print(" " + x );
return (input / x);
} else {
continue;
}
}
}
return 0;
}
 
public static void main(String args[])
{
int number; // number to be decomposed
 
for (number=432731; number > 432701; number--) {
System.out.print("Prime factors of " + number + ":");
int value = number;
while (value != 1)
{
value = factor(value);
}
System.out.println("");
 
}
}
 

}
 

And I have the result like this

  
D:\TestFolder>java evan5ccc
Prime factors of 432731: 13 33287
Prime factors of 432730: 2 5 109 397
Prime factors of 432729: 3 3 3 11 31 47
Prime factors of 432728: 2 2 2 54091
Prime factors of 432727: 432727
Prime factors of 432726: 2 3 7 10303
Prime factors of 432725: 5 5 19 911
Prime factors of 432724: 2 2 251 431
Prime factors of 432723: 3 144241
Prime factors of 432722: 2 23 23 409
Prime factors of 432721: 432721
Prime factors of 432720: 2 2 2 2 3 3 5 601
Prime factors of 432719: 7 7 8831
Prime factors of 432718: 2 11 13 17 89
Prime factors of 432717: 3 97 1487
Prime factors of 432716: 2 2 108179
Prime factors of 432715: 5 37 2339
Prime factors of 432714: 2 3 41 1759
Prime factors of 432713: 432713
Prime factors of 432712: 2 2 2 7 7727
Prime factors of 432711: 3 3 48079
Prime factors of 432710: 2 5 43271
Prime factors of 432709: 29 43 347
Prime factors of 432708: 2 2 3 107 337
Prime factors of 432707: 11 139 283
Prime factors of 432706: 2 19 59 193
Prime factors of 432705: 3 5 7 13 317
Prime factors of 432704: 2 2 2 2 2 2 6761
Prime factors of 432703: 103 4201
Prime factors of 432702: 2 3 3 3 3 2671

I dont know whether the result is what you want or not.
But I just remind you that you should use bracked for conditional logic.
It will help us to know the behavior of our code inside that conditional.

correct me if I am wrong...
Help this helps...
thanks
daniel

[Evan Keenlyside]

WOW.

Thank you. I REALLY appreciate it.

I knew that stupid while loop was the problem. I'm just not good enough at java to fix the problem.

Thanks again.