當我遇到一個有趣的堆棧溢出異常時,我在做Project Euler上的問題14(注意:我沒有在尋找Project Euler問題的解決方案)。概率計算中的堆棧溢出異常
我的非概率方法工作得很好,但是當我用概率方法嘗試同樣的問題時,我遇到了堆棧溢出異常。有趣的是,例外只發生在約17%的時間。一千個運行產生了166個例外。
我知道我的概率邏輯是有缺陷的,但我更關心異常的原因以及如何防止它們發生。我是否需要做一些內存管理,也許在使用它們後將一些變量設置爲null?如果是的話,關鍵點在哪裏呢?
的代碼如下:
public class Problem14_LongestCollatzSequence {
private static final int STARTING_CHAIN_LENGTH = 1;
private static final int PROBABLY_RIGHT = 100000;
/**
* Calculate and return the Collatz sequence of a given number.
*
* @param number The number for which the Collatz sequence is to be
* calculated.
* @param chainlength The length of the chain for the number. This should
* start with an initial value of 1.
* @return The Length of the Collatz sequence.
*/
private static int getChainLength(long number, int chainlength) {
// All chains should end with 1.
if (number != 1) {
// If the number is even, halve the number, otherwise multiply it by 3 and add 1.
if (number % 2 == 0) {
number = number/2;
} else {
number = number * 3 + 1;
}
// Call this function again.
return getChainLength(number, ++chainlength);
}
// Return the length of the chain.
return chainlength;
}
/**
* Determine and return the number below a maximum value that will result in
* the longest Collatz chain.
*
* @param maxStartingNumber The maximum value (exclusive) of the numbers
* that will be tested.
* @return The number that will produce the longest Collatz sequence in the
* given range.
*/
private static int calculateLongestChain(int maxStartingNumber) {
Random random = new Random();
int probabilityCounter = 0;
int currentChainNumber = 0;
int longestChainNumber = 0;
int currentChainLength = 0;
int longestChainLength = 0;
// Get the chain length of random numbers until a certain number of unsuccsessful attempts have been made.
while (probabilityCounter <= PROBABLY_RIGHT) {
currentChainNumber = random.nextInt(maxStartingNumber);
currentChainLength = getChainLength(currentChainNumber, STARTING_CHAIN_LENGTH);
// If the current chain-length is bigger than the previously calculated one, reset the counter and update the chain number, otherwise increase the counter.
if (currentChainLength > longestChainLength) {
probabilityCounter = 0;
longestChainLength = currentChainLength;
longestChainNumber = currentChainNumber;
} else {
++probabilityCounter;
}
}
return longestChainNumber;
}
private static int calculateLongestChainNP(int maxStartingNumber) {
// Non-probabilistic way to calculate the longest Collatz sequence.
int currentChainLength = 0;
int longestChainLength = 0;
int longestChainNumber = 0;
// Simply loop through all the numbers in the range to calculate the one resulting in the longest sequence.
for (int i = 1; i < maxStartingNumber; i++) {
currentChainLength = getChainLength(i, STARTING_CHAIN_LENGTH);
if (currentChainLength > longestChainLength) {
longestChainLength = currentChainLength;
longestChainNumber = i;
}
}
return longestChainNumber;
}
public static void main(String[] args) {
int exceptionCount = 0;
for (int count = 0; count < 1000; count++) {
try {
int testNumber = 1000000;
System.out.println("Probabilistic answer: " + calculateLongestChain(testNumber));
System.out.println("Non-probabilistic answer: " + calculateLongestChainNP(testNumber) + "\n");
} catch (java.lang.StackOverflowError soe) {
exceptionCount++;
System.err.println(soe + "\n");
}
}
System.out.println("Exception count: " + exceptionCount);
}
}
我想提供完整的輸出爲好,但使我在字符的限制。
使用while循環而不是遞歸在'getChainLength()' - 這應該加快了一點東西和免費頗多堆棧。 – tucuxi