java: how for loop work in the case of BigInteger

You use these syntax instead:

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:

    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 BigInteger as a loop index is highly atypical. long is usually enough for this purpose.

API links

• java.math.BigInteger
  • int compareTo(BigInteger val) from interface Comparable
  • BigInteger subtract(BigInteger val)
  • BigInteger add(BigInteger val)
  • static BigInteger valueOf(long val)

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The compareTo idiom

From the documentation:

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)0), where  is one of the six comparison operators.

In other words, given BigInteger x, y, these are the comparison idioms:

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 in general.

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Note on immutability

BigInteger, like String, is an immutable object. Beginners tend to make the following mistake:

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:

s = s.trim();
bi = bi.add(BigInteger.ONE);

http://stackoverflow.com/questions/3024186/java-how-for-loop-work-in-the-case-of-biginteger