java long變量表現得像int變量

Question

我認為“長”類型變量的行為類似於“ int”類型變量。 我認為是這種情況，因為隨着變量變得越來越大，當它試圖超過2147483647（這是“ int”類型的最大值）時，它實際上會翻轉為負數。 顯然，“長”類型的最大值要寬容得多。

我想我應該在這里粘貼我的代碼。 請不要苛刻代碼，它根本沒有重構。 另外，您會看到許多la腳的嘗試，迫使代碼將我所討論的變量視為“長整型”類型，但是這些努力的總體效果並不理想。

注意，此代碼處理“ N選擇X”或組合。 輸入的是組合數和N。代碼循環遍歷X的可能值，直到找到匹配項或直到超過匹配的可能性為止（或直到​​計算出的組合“為負”為止）。

先謝謝您的幫助。

public class primativeLongPractice {

    private static long fctrl (long num) {
          long ans = 1L;
          for (long i=num; i>0; i--) ans = ans * i;
          return ans;
        }

    private static long nchoosex (long n, long x) {
          long y = n - x;
          if (y>x) {
            long temp = y;
            y=x;
            x=temp;
          }
          long ans = 1L;
          for (long i=n; i>x; i--) ans = ans * i;
          return ans/fctrl(y);
        }

    public static long checkchoose(long m, int n) {
          long N = (long)n;
          long combos = 0L;
          long x = 1L; // starting out at 1 and going up
            // compute "n choose x" call it combos
            combos = nchoosex(N,x);
            System.out.println(n + " choose " + x + " equals " + combos + "; m equals " + m);
            if (combos==m) return x;
            while ((combos>1)&&(combos<m)) {
              x = x + 1;
              combos = nchoosex(N,x);
              System.out.println(n + " choose " + x + " equals " + combos + "; m equals " + m);
              if (combos==m) return x;
            }
          System.out.println("Didn't find anything");
          return -1L;
        }

    public static void main(String[] args) {
        long p = 155117520L;
        int q = 30;
        long r = checkchoose(p,q);
        System.out.println("For inputs " + q + " and " + p + " the function returned " + r);
    }
}

Answer 1

我很快調試了它。

對於n = 30, x = 14您的值ans 長時間溢出並產生值

ans = -5769043765476591616

和

fctrl(y) = 87178291200

這使得結果似乎由於整數溢出而滾動。

Answer 2

long int確實比int更具兼容性，但它仍然可能溢出。

我在您的代碼中添加了一些額外的打印以顯示問題：

private static long fctrl(long num) {
    long ans = 1L;
    for (long i = num; i > 0; i--) {
        // Are we going tyo overflow?
        if (ans * i < 0) {
            System.out.println("fctrl overflow!!! " + ans + " at " + i);
        }
        ans = ans * i;
    }
    return ans;
}

private static long nchoosex(long n, long x) {
    long y = n - x;
    if (y > x) {
        long temp = y;
        y = x;
        x = temp;
    }
    long ans = 1L;
    for (long i = n; i > x; i--) {
        if (ans * i < 0) {
            System.out.println("nchoosex overflow!!! " + ans + " at " + i);
        }
        ans = ans * i;
    }
    return ans / fctrl(y);
}

public static long checkchoose(long m, int n) {
    long ln = (long) n;
    long combos = 0L;
    long x = 1L; // starting out at 1 and going up
    // compute "n choose x" call it combos
    combos = nchoosex(ln, x);
    System.out.println(n + " choose " + x + " equals " + combos + "; m equals " + m);
    if (combos == m) {
        return x;
    }
    while ((combos > 1) && (combos < m)) {
        x = x + 1;
        combos = nchoosex(ln, x);
        System.out.println(n + " choose " + x + " equals " + combos + "; m equals " + m);
        if (combos == m) {
            return x;
        }
    }
    System.out.println("Didn't find anything");
    return -1L;
}

public void test() {
    System.out.println("Hello");
    long p = 155117520L;
    int q = 30;
    long r = checkchoose(p, q);
    System.out.println("For inputs " + q + " and " + p + " the function returned " + r);
}

它打印

nchoosex溢出！！！ 745747076954880000在17