Java variable may not have been initialized

Question

I'm working on Project Euler Problem 9, which states:

A Pythagorean triplet is a set of three natural numbers, a < b < c, for which,

a^2 + b^2 = c^2

For example, 3^2 + 4^2 = 9 + 16 = 25 = 52.

There exists exactly one Pythagorean triplet for which a + b + c = 1000. Find the product abc.

Here's what I've done so far:

class Project_euler9 {

    public static boolean determineIfPythagoreanTriple(int a, int b, int c) {
        return (a * a + b * b == c * c);
    }   

    public static void main(String[] args) {
        boolean answerFound = false;
        int a, b, c;
        while (!answerFound) {
            for (a = 1; a <= 1000; a++) {
                for (b = a + 1; b <= 1000; b++) {
                    c = 1000 - a - b;
                    answerFound = determineIfPythagoreanTriple(a, b, c);
                }
            }
        }
        System.out.println("(" + a + ", " + b + ", " + c + ")");
    }
}

When I run my code, I get this error:

Project_euler9.java:32: error: variable a might not have been initialized
        System.out.println("The Pythagorean triplet we're looking for is (" + a + ", " + b + ", " + c + ")");

Note: I get this for each of my variables (a, b, and c) just with different line numbers.

I thought that when I declared a, b, and c as integers, the default value was 0 if left unassigned.

Even if this weren't the case, it looks to me like they all do get assigned, so I'm a bit confused about the error.

Why is this happening?

Answer 1

As others have pointed out your variables a, b, c may not have been initialized by the time you get to System.out.println. There are several ways to avoid this.

Initialize variables explicitly, so:

int a = 0;

Use a do...while loop, which ensures your loop is run at least once, so:

do {
    [...]
} while (!answerFound);

Use loop-scoped variables, which avoid local-scoped variables hanging around, so:

for (int a = 1; a <= 1000; a++) {
    [...]
}

Answer 2

The Java compiler has to be convinced that when we get to the line HERE, it can prove that a, b, and c have been set to something:

   boolean answerFound = false;
   int a, b, c;
   while (!answerFound) {
       for (a = 1; a <= 1000; a++) {
           for (b = a + 1; b <= 1000; b++) {
               c = 1000 - a - b;
               answerFound = determineIfPythagoreanTriple(a, b, c);
           }
       }
   }
   // HERE

From the compiler's point of view, if the loop is executed zero times, then it will get to HERE without the variables being initialized; therefore, you can't use them. We know that this is impossible, because we know that answerFound is initialized to false and therefore the loop will be executed at least once, and also that each for loop will be executed at least once. But the language has to have consistent rules to determine which programs are legal and which ones aren't; in order to keep things from being overly complex, the compiler isn't required to make the kinds of deductions that would be necessary to prove that a, b, and c are always initialized. I haven't looked at the actual "definite assignment" rules in detail, but I think they're already fairly complex.

So even though you know that the variables will be initialized, you can't convince the compiler of that. So just initialize them when they're declared.

int a = 0, b = 0, c = 0;

By the way, since the while is outside the for loops, this code will not exit right away when answerFound becomes true. It still executes the for loops until they're done, and only then does it test to see whether to leave the while loop. You'll need to solve that.

P.S. If you want to cheat, note that every Pythagorean triple has the form a = m² - n², b = 2mn, c = m² + n², for some m and n. You could actually get the answer without a computer, using algebra. But go ahead and finish the program you started--it's great practice.

Answer 3

Instance variables (in your case, they would be integers) are assigned to 0 be default. Local variables not. (From Java Docs)

If the loop is not entered, then your variables won't be initialized, that's the reason of the error.

What you can do is initialize them when declaring:

int a=0, b=0, c=0;

Answer 4

You're right and wrong in your thinking:

Where you're right:

• Yes, uninitialized variables may be assigned a value by the compiler, but a) it is considered bad style, b) You shouldn't depend on it, c) That does not apply to local variables (as declared in a method)

Where you are wrong:

• Local variables are not assigned a default value, only instance variables.

For reference, take a look at The docs:

Default Values

It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or null, depending on the data type. Relying on such default values, however, is generally considered bad programming style.

The following chart summarizes the default values for the above data types.

Data Type Default Value (for fields)
byte 0
short 0
int 0
long 0L
float 0.0f
double 0.0d
char '\u0000'
String (or any object) null
boolean false

Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.

Answer 5

When you declare local variables in a method, you have to assign them values before use them.

Answer 6

Your problem is this line:

System.out.println("(" + a + ", " + b + ", " + c + ")");

which is after the while (!answerFound) {...} loop. The compiler thinks that there may be a case where one or more of the variables a, b or c isn't initialised.

Use this line:

int a=0, b=0, c=0;

when declaring the variables, so that they are initialised when declared, and the error should go away.

Answer 7

Do this, at the beginning of the method after the local variables are declared:

a = b = c = 0;

The error is basically stating that Java can't be sure that the variables have a value assigned when they reach the System.out.println(). Remember: in Java only attributes have default values, all local variables must be explicitly initialized at some point.