For loop - like Python range function

Question

I was wondering if in Java there is a function like the python range function.

range(4)

and it would return

[0,1,2,3]


        

Answer 1

What you can do to substitute the range in python in java can be done with the following code. NOTE: I am not going off of your code, I am just writing a small piece of code and showing it to you.

in python you would do.. . .

if -2 <= x <= 10:
     print(x) 

in java you would substitute this range and do. . ..

if(x >= -2 && x <= 10){
System.out.println("x")
}

By doing the above code in java, you don't need a range, but you have the -2 <= x <=10 range and split it into x >= -2 and x <= 10. It means the same thing, but the one I explained in java may take the compiler a longer time to read. So if you are using python go with the former's code format, and if you are using java, use the latter's code format.

Answer 2

If you really, really want to obtain an equivalent result in Java, you'll have to do some more work:

public int[] range(int start, int end, int step) {
    int n = (int) Math.ceil((end-start)/(double)step);
    int[] arange = new int[n];
    for (int i = 0; i < n; i++)
        arange[i] = i*step+start;
    return arange;
}

Now range(0, 4, 1) will return the expected value, just like Python: [0, 1, 2, 3]. Sadly there isn't a simpler way in Java, it's not a very expressive language, like Python.

Answer 3

As far as I know, there's not an equivalent function in java. But you can write it yourself:

public static int[] range(int n) {
    int[] ans = new int[n];
    int i;
    for(i = 0; i < n; i++) {
        ans[i] = i;
    }
    return ans;
}

Answer 4

Java 8 (2014) has added IntStream (similar to apache commons IntRange), so you don't need external lib now.

import java.util.stream.IntStream; 

IntStream.range(0, 3).forEachOrdered(n -> {
    System.out.println(n);
});

forEach can be used in place of forEachOrdered too if order is not important.

IntStream.range(0, 3).parallel() can be used for loops to run in parallel

Answer 5

Use Apache Commons Lang:

new IntRange(0, 3).toArray();

I wouldn't normally advocate introducing external libraries for something so simple, but Apache Commons are so widely used that you probably already have it in your project!

Edit: I know its not necessarily as simple or fast as a for loop, but its a nice bit of syntactic sugar that makes the intent clear.

Edit: See @zengr's answer using IntStream in Java 8 .

Answer 6

Without an external library, you can do the following. It will consume significantly less memory for big ranges than the current accepted answer, as there is no array created.

Have a class like this:

class Range implements Iterable<Integer> {

    private int limit;

    public Range(int limit) {
        this.limit = limit;
    }

    @Override
    public Iterator<Integer> iterator() {
        final int max = limit;
        return new Iterator<Integer>() {

            private int current = 0;

            @Override
            public boolean hasNext() {
                return current < max;
            }

            @Override
            public Integer next() {
                if (hasNext()) {
                    return current++;   
                } else {
                    throw new NoSuchElementException("Range reached the end");
                }
            }

            @Override
            public void remove() {
                throw new UnsupportedOperationException("Can't remove values from a Range");
            }
        };
    }
}

and you can simply use it like this:

    for (int i : new Range(5)) {
        System.out.println(i);
    }

you can even reuse it:

    Range range5 = new Range(5);

    for (int i : range5) {
        System.out.println(i);
    }
    for (int i : range5) {
        System.out.println(i);
    }

---

As Henry Keiter pointed out in the comment below, we could add following method to the Range class (or anywhere else):

public static Range range(int max) {
    return new Range(max);
}

and then, in the other classes we can

import static package.name.Range.range;

and simply call

for (int i : range(5)) {
    System.out.println(i);
}