CODEWITHSUNDEEP
javabit-manipulation
limp_chimp
limp_chimp

Reputation: 15133

Trying to do bitwise AND with two ints in Java, but the compiler tries to use them as bools

Here's a function I wrote in C++ (which works) and am now trying to convert to Java:

static String parseAInstruction(String line) {
    int n = getValue(line);
    if (n>=0) {
        String inst = "0";
        for (int i=14; i>=0; --i) {
            if (Math.pow(2, i) & n)
                inst += "1";
            else
                inst += "0";
        }
        return inst;
    } else {
        return "error";
    }
}

This compiles just fine in C++, but when trying to compile in Java, I get the following error:

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    Type mismatch: cannot convert from int to boolean

    at parser.Parser.parseAInstruction(Parser.java:38)
    at parser.Parser.main(Parser.java:50)

Looking up bitwise AND in Java, it seems that it should be the same. It shouldn't have anything to do with booleans at all. Can anyone tell me the problem?

Upvotes: 0

Views: 1236

Answers (5)

user555045
user555045

Reputation: 64904

Math.pow in Java is calculated with a logarithm and an exponentiation (unlike C++, where the compiler often optimizes calls to pow), both very slow, approximate, floating point operations.

It doesn't even make sense to use here - you're working with bits, so work with bits:

(n & (1 << i)) != 0 is what you're looking for.

Upvotes: 1

maerics
maerics

Reputation: 156364

In Java you can only use boolean values in if clauses. So the result of a bitwise AND of two numbers is not valid in an if statement.

Try this instead: if ((Math.pow(2, i) & n) != 0) {.

[Edit] However, as commenter @jlordo points out, next you'll have to resolve the problem of doing a bitwise AND between a double and an integer.

Upvotes: 4

Andrew Cooper
Andrew Cooper

Reputation: 32576

The problem is that you can't just use an integer in the conditon expression for if. you need something like:

        if (((int)Math.pow(2, i) & n) != 0)

Upvotes: 3

user207421
user207421

Reputation: 310840

Trying to do bitwise AND with two ints in Java, but the compiler tries to use them as bools

No it doesn't. The result of the bitwise AND is of type int. Java requires the expression in an if statement to be of type boolean, and yours isn't. You need to convert the expression to boolean.

Upvotes: 1

jlordo
jlordo

Reputation: 37813

Your problem is here:

if (Math.pow(2, i) & n)

In Java, you need to have a boolean value as a condition. You probably want:

if (((int)(Math.pow(2, i)) & n) != 0)

The cast to int is necessary because Math.pow() will return a double and the & operator is only defined for two int operands.

Upvotes: 2

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