在假布尔值上使用not语句

Question

I'm always assumed that if a statement was declared false, such as boolean valid = false , then writing !valid would in turn reverse valid to be true. Yet, was this an entirely wrong misconception? For instance, in the code below;

for (int check=3; check <= primeLimit; check += 2) {
    boolean divisorFound=false;
    for (int div=3; div<=Math.sqrt(check) && !divisorFound; div += 2) {
        if (check%div==0)
            divisorFound=true;
    }

Is the second for loop searching for divisorFound to be true of false? Because if it was implying that the divisor is false, wouldn't it just say :

div<=Math.sqrt(check) && divisorFound;

Answer 1

I'm assuming you didn't write this code and are trying to understand what it's purpose is. The inner loop should continue until a divisor is found. So it repeatedly checks to see if !divisorFound evaluates to true . In other words, "has a divisor not been found yet?".

Once a divisor is found, divisorFound is set to true and the expression !divisorFound evaluates to false , so the loop will stop.

If the condition check that you suggested was used:

div<=Math.sqrt(check) && divisorFound

The loop body would never be reached. divisorFound is false to start with so the continue condition would immediately fail and the loop would terminate.

Answer 2

The second loop is saying to continue looping when div is less than Math.sqrt(check) and no divisor has been found. When divisorFound has been set to true, then the inner loop will fail its condition.

I think you must have misread the code as this is a pretty straight forward boolean condition.