java：在 BigInteger 的情况下 for 循环如何工作
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java: how for loop work in the case of BigInteger
提问by Sanjeev
I want to take Input from the user as Big-Integer and manipulate it into a For loop
我想将用户输入作为 Big-Integer 并将其操作为 For 循环
BigInteger i;
for(BigInteger i=0; i<=100000; i++) {
System.out.println(i);
}
But it won't work
但它不会工作
can any body help me.
有谁能够帮助我。
采纳答案by polygenelubricants
You use these syntax instead:
您可以改用这些语法：
BigInteger i = BigInteger.valueOf(100000L); // long i = 100000L;
i.compareTo(BigInteger.ONE) > 0 // i > 1
i = i.subtract(BigInteger.ONE) // i = i - 1
So here's an example of putting it together:
所以这里有一个把它放在一起的例子：
for (BigInteger bi = BigInteger.valueOf(5);
bi.compareTo(BigInteger.ZERO) > 0;
bi = bi.subtract(BigInteger.ONE)) {
System.out.println(bi);
}
// prints "5", "4", "3", "2", "1"
Note that using BigIntegeras a loop index is highly atypical. longis usually enough for this purpose.
请注意，BigInteger用作循环索引是非常不典型的。long通常足以达到这个目的。
API links
接口链接
The compareToidiom
该compareTo成语
From the documentation:
从文档：
This method is provided in preference to individual methods for each of the six boolean comparison operators (
<,==,>,>=,!=,<=). The suggested idiom for performing these comparisons is: (x.compareTo(y)<op>0), where<op>is one of the six comparison operators.
对于六个布尔比较运算符（
<、==、>、>=、!=、<=）中的每一个，此方法优先于单独的方法提供。执行这些比较的建议习语是：( )，其中是六个比较运算符之一。x.compareTo(y)<op>0<op>
In other words, given BigInteger x, y, these are the comparison idioms:
换句话说，给定BigInteger x, y，这些是比较习语：
x.compareTo(y) < 0 // x < y
x.compareTo(y) <= 0 // x <= y
x.compareTo(y) != 0 // x != y
x.compareTo(y) == 0 // x == y
x.compareTo(y) > 0 // x > y
x.compareTo(y) >= 0 // x >= y
This is not specific to BigInteger; this is applicable to any Comparable<T>in general.
这不是特定于BigInteger; 这适用于任何Comparable<T>一般情况。
Note on immutability
关于不变性的注意事项
BigInteger, like String, is an immutable object. Beginners tend to make the following mistake:
BigInteger，就像String，是一个不可变的对象。初学者容易犯以下错误：
String s = " hello ";
s.trim(); // doesn't "work"!!!
BigInteger bi = BigInteger.valueOf(5);
bi.add(BigInteger.ONE); // doesn't "work"!!!
Since they're immutable, these methods don't mutate the objects they're invoked on, but instead return new objects, the results of those operations. Thus, the correct usage is something like:
由于它们是不可变的，这些方法不会改变调用它们的对象，而是返回新对象，即这些操作的结果。因此，正确的用法类似于：
s = s.trim();
bi = bi.add(BigInteger.ONE);
回答by vodkhang
I think this code should work
我认为这段代码应该有效
public static void main(String[] args) {
BigInteger bigI = new BigInteger("10000000");
BigInteger one = new BigInteger("1");
for (; bigI.compareTo(one) == 0; bigI.subtract(one)) {
bigI = bigI.add(one);
}
}
回答by Justin K
Well, first of all, you have two variables called "i".
好吧，首先，您有两个变量，称为“i”。
Second, where's the user input?
其次，用户输入在哪里？
Third, i=i+i unboxes i into a primitive value, possibly overflowing it, and boxes the result in a new object (that is, if the statement even compiles, which I haven't checked).
第三， i=i+i 将 i 拆箱成一个原始值，可能会溢出它，并将结果装箱到一个新对象中（也就是说，如果语句甚至编译，我还没有检查过）。
Fourth, i=i+i can be written as i = i.multiply(BigInteger.valueof(2)).
第四，i=i+i 可以写成 i = i.multiply(BigInteger.valueof(2))。
Fifth, the loop is never run, because 100000 <= 1 is false.
第五，循环永远不会运行，因为 100000 <= 1 是假的。
