Efficient implementation for: "Python For Else Loop" in Java

Question

In Python there is an efficient for .. else loop implementation described here

Example code:

for x in range(2, n):
    if n % x == 0:
        print n, 'equals', x, '*', n/x
        break
else:
    # loop fell through without finding a factor
    print n, 'is a prime number'

In Java I need to write more code to achieve the same behavior:

finishedForLoop = true;
for (int x : rangeListOfIntegers){
    if (n % x == 0)
    {
        //syso: Some printing here
        finishedForLoop = false
        break;
    }
}
if (finishedForLoop == true){
    //syso: Some printing here
}

Is there any better implementation similar to Python for .. else loop in Java?

You can replace if (finishedForLoop == true) with if (finishedForLoop). No need to test for true, the if statement does that implicitly :) — fredoverflow
– fredoverflow, Commented Nov 7, 2013 at 8:21

Dog · Accepted Answer · 2013-06-24 17:51:53Z

82

It's done like this:

class A {
    public static void main(String[] args) {
        int n = 13;
        found: {
            for (int x : new int[]{2,3,4,5,6,7,8,9,10,11,12})
                if (n % x == 0) {
                    System.out.println("" + n + " equals " + x + "*" + (n/x));
                    break found;
                }
            System.out.println("" + n + " is a prime number");
        }
    }
}

$ javac A.java && java A
13 is a prime number

answered Jun 24, 2013 at 17:51

Dog

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3 Comments

fredoverflow Over a year ago

Why the labeled break? There is only one loop to break out of.

Eric Over a year ago

@FredOverflow: it's not breaking out of the loop, but the enclosing block.

Zecc Over a year ago

@FredOverflow To elaborate on Eric's comment: an unlabeled break would continue on the statement that follows the loop and still flow through the println statement.

Jon Skeet · Accepted Answer · 2017-07-21 20:00:18Z

15

When I need to do something like this, if no extra information is needed, I typically try to break it out into a separate method - which can then return true/false or alternatively either the value found, or null if it's not found. It doesn't always work - it's very context-specific - but it's something worth trying.

Then you can just write:

for (...) {
    if (...) {
       return separateMethod();
    }
}
return null; // Or false, or whatever

edited Jul 21, 2017 at 20:00

answered Oct 25, 2012 at 13:15

Jon Skeet

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1 Comment

tucuxi Over a year ago

for cases when a different method is not ideal, you can always "break out" of the loop using a labelled break... but it is very rare to see this in practice.

Joe · Accepted Answer · 2012-10-25 13:14:38Z

2

No. That's the simplest. It's not that complicated, it's just syntax.

answered Oct 25, 2012 at 13:14

Joe

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3 Comments

tucuxi Over a year ago

also, even if you find something "simpler", it will also be "smart". And "smart" is difficult-to-read

Michael Over a year ago

+1, Yes you are right, i am still interested in more "smart" solutions:-)

Joe Over a year ago

I don't see what's not smart about using the minimum syntax. To answer your question straight, no there is nothing simpler and there is nothing smarter.

Mathias Begert · Accepted Answer · 2016-11-11 13:07:20Z

2

Since java8 there is a way to write this with "nearly no" code:

if(IntStream.range(2, n).noneMatch(x -> n % x == 0)) {
   System.out.println(n + " is a prime number");
}

BUT: this would be less efficient than the classical looping-with-break-and-flag method.

answered Nov 11, 2016 at 13:07

Mathias Begert

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Comments

Minion91 · Accepted Answer · 2012-10-25 13:14:59Z

-1

No, there is no mechanism like this in Java

answered Oct 25, 2012 at 13:14

Minion91

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