Project Euler 14 Java

Question

I have been having trouble on Problem 14 on Project Euler. I don't understand why my code(Java) isn't working and any help would be appreciated.

public class Calculate {

    public static void main(String[] args){
        System.out.println(calc());
    }

    public static int calc(){
        int max = 0;
        int maxI = 0;
        for (int i = 0; i < 1000000; i++){
            if (seqLen(i) >= max){
                max = seqLen(i);
                maxI = i;
            }
        }
        return maxI;
    }

    public static int seqLen(int seed){
        if (seed <= 0) return 0;
        if (seed == 1) return 1;
        int len = 1;
        if (seed % 2 == 0){
            len += seqLen(seed/2);
        }
        else{
            len += seqLen((3*seed)+1);
        }
        return len;
    }

}

Thanks!

see this https://interviewquizandanswers.blogspot.com/2020/04/project-euler-14-longest-collatz.html — dasunse, Apr 06 '20 at 17:45

score 2 · Accepted Answer · answered Mar 05 '15 at 14:25

2

You run into an overflow with your int variables.

The maximum number appearing in this computation (when using a brute force approach) is 56991483520.

Java's int maximum value is 2^31-1 == 2147483647, which is obviously smaller.

So change your variables etc to use long. Here the max value is 2^63-1 == 9223372036854775807, which will be fitting for all values.

answered Mar 05 '15 at 14:25

Sirko

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Thank you! I forgot that it gets that big, I was only looking at the length. – monsterbananas Mar 05 '15 at 14:34

score 1 · Answer 2 · answered Mar 05 '15 at 14:25

1

You are breaking the int limit.

Using long:

public static long calc() {
    long max = 0;
    long maxI = 0;
    for (long i = 0; i < 1000000; i++) {
        long len = seqLen(i);
        if (len >= max) {
            max = len;
            maxI = i;
        }
    }
    return maxI;
}

public static long seqLen(long seed) {
    if (seed <= 0) {
        return 0;
    }
    if (seed == 1) {
        return 1;
    }
    long len = 1;
    if (seed % 2 == 0) {
        len += seqLen(seed / 2);
    } else {
        len += seqLen((3 * seed) + 1);
    }
    return len;
}

public void test() {
    System.out.println(seqLen(13));
    System.out.println(calc());
}

Gives you the correct result of 837799.

Note that there are better/more efficient algorithms than this one.

answered Mar 05 '15 at 14:25

OldCurmudgeon

64,482
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Thank you! Can you tell me where it was breaking the int limit, because the numbers didn't seem that big to me. – monsterbananas Mar 05 '15 at 14:31
I don't think, that providing the final answer for project euler is a good idea ,-) – Sirko Mar 05 '15 at 14:31
@monsterbananas - Sirko mentions it in [his answer](http://stackoverflow.com/a/28880137/823393). – OldCurmudgeon Mar 05 '15 at 14:32
@Sirko - I wouldn't call this **the** final answer - it is not even a particularly good solution. I hope OP is encouraged to improve it - there are many solutions to this problem online in many languages. – OldCurmudgeon Mar 05 '15 at 14:34
@OldCurmudgeon I was just referring to the line starting "This gives you the correct result of ..." OP should at least be able to run the code himself. – Sirko Mar 05 '15 at 14:50

score 0 · Answer 3 · answered Feb 21 '16 at 04:43

Actually, you don't have to check from 1 to 499999. You only need to check from 500000 to 999999 because the next step of any even number between 500000 and 999999 is going to be some integer from 1 to 499999. That means that an integer from 1 to 499999 cannot be the answer.

So change the for loop to the following

for (int i = 500000; i < 1000000; i++) {

}

And "i" doesn't have to be long while "seed" has to be long.

Project Euler 14 Java

3 Answers3