java - 如何与Java中的BigInt进行比较以确定哪个是更大的BigInt

Question

我正在尝试比较两个手动创建且未使用内置 BigInt 类的 BigInt。现在我正试图确定如何找到 2 中较大的数字。例如，如果我想找到 123 和 134 之间哪个数字更大，并且我传递了两个我想返回的 BigInts如果 123 是第一个传递的数字，则返回 false；如果传递第二个数字，则返回 True。请看下面的代码:

private boolean thisBigIntGreaterThanOrEqualToOther(BigInt b1, BigInt b2){
      boolean value = true;
      if(b1.bigInt.size() >= b2.bigInt.size()){
          for(int i = 0; i < b2.bigInt.size(); i++){
              if(b1.bigInt.get(i) >= b2.bigInt.get(i)){
                  value = true;
              }
              else{
                 value = false;
              }
          }
      }
      else{
          value = false;
      }
      return value;
  }

正如您在我的代码中看到的，我考虑过尝试比较每个数字，但是当每个数字都为 1 时，我遇到了一个问题，它将值设置为 true。

下面的 BigInt 类:

    public class BigInt {

       //Instance variables for the class, a boolean for the sign and an ArrayList to store the input String

       private boolean pos = true;
       private ArrayList<Integer> bigInt = new ArrayList<Integer>();

       //No argument constructor, creates a big integer of value zero

       public BigInt () {
           this.pos = true;



       }

     //Constructor for big integers input as int

       public BigInt (int newBigInt) {
          String inputInt = Integer.toString(newBigInt);
           inputInt = handleSign(inputInt);
           inputInt = checkNumber(inputInt);
           for(int i = inputInt.length() - 1; i >=0; i--) {
               bigInt.add(Integer.parseInt(inputInt.substring(i, i+1)));
           }
       }

       //Constructor for big integers input as strings

       public BigInt (String newBigInt) {

           newBigInt = handleSign(newBigInt);
           newBigInt = checkNumber(newBigInt);
           for(int i = newBigInt.length() - 1; i >=0; i--) {
               bigInt.add(Integer.parseInt(newBigInt.substring(i, i+1)));
           }
       }
private String handleSign(String num) {

       if(num.charAt(0) == '+' || num.charAt(0) == '-') {
           if(num.length() == 1) {
               throw new ErrorMessage("Invalid value: sign only, no integer.");
           }

           if(num.charAt(0) == '-') {
               this.pos = false;
           }

           else {
               this.pos = true;
           }
           num = num.substring(1);
       }

       return num;
   }
   // Private method to remove leading zeros from add/subtract methods

   private BigInt removeZeros(BigInt result){

       for(int i = 0; i < result.bigInt.size(); i++){
           if(result.bigInt.get(i) == 0){
               result.bigInt.remove(i);
           }
       }
       return result;
   }

   //Private method to check the number; remove leading zeros and check for leading spaces (throw error message)

   private String checkNumber(String num) {

       if(num.charAt(0) == ' ') {
           throw new ErrorMessage("Invalid value: leading blank space.");
       }
       if(num.charAt(0) == '0'){
           while(num.length() > 1 && num.charAt(0) == '0') {
               num = num.substring(1);
           }
       }
       return num;
   }

   //toString method

public String toString() {

       String answer = "";
       for(int i = bigInt.size() - 1; i >=0; i--) {
           answer = answer + bigInt.get(i);
       }
       if(this.pos == false){
           return "-" + answer;
       }
    return answer;

Answer 1

比较两个的方法BigInt s 以多种方式被破坏:

<强>1。您迭代的方向错误:

for(int i = 0; i < b2.bigInt.size(); i++)

从最低有效数字开始，这意味着 20将被视为小于 11 。将其更改为

for(int i = b2.bigInt.size() - 1; i >= 0 ; i--)

<强>2。您覆盖比较结果

如果您的代码达到了设置 value = false; 的程度它不会返回或退出循环。这意味着在下一次迭代中该值将被覆盖。这意味着突然13和23被认为是平等的。

BigInt c = new BigInt("13");
BigInt d = new BigInt("23");
System.out.println(BigInt.thisBigIntGreaterThanOrEqualToOther(c, d));
System.out.println(BigInt.thisBigIntGreaterThanOrEqualToOther(d, c));

输出为

true
true

更改value = false;至return false;

<强>3。您不检查是否 b1.bigInt.size() > b2.bigInt.size()

这会导致您的方法返回 131小于 23 .

按以下方式更改代码:

if(b1.bigInt.size() > b2.bigInt.size()){
  return true;
} else if(b1.bigInt.size() < b2.bigInt.size()){
  return false;
} else {
  // the other comparison code
}

一些最后的评论:

实现 Comparable interface 是一个很好的设计因为它允许您在类中使用许多库方法。

编辑:代码现在不再使用库函数

public class BigInt implements Comparable<BigInt> {

    ...

    @Override
    public int compareTo(BigInt other) {
        int c = this.bigInt.size() - other.bigInt.size();
        if (c != 0) {
            return c;
        } else {
            for (int i = this.bigInt.size() - 1; i >= 0; i--) {
                c = this.bigInt.get(i) - other.bigInt.get(i);
                if (c != 0) {
                    return c;
                }
            }
            return 0;
        }
    }
}