How should I increment numbers using recursion?

Question

I have a problem with numbers that I want to increment using a recursive function. The function didn't return the value as I was expecting. Upon debugging I saw the numbers being increased but when the value is returned, it appears that the adjustments to the numbers are returned in reverse order.

Here is a simple example:

private static int recursionTest(int num){
    if (num < 10){
        recursionTest(++num);
    }
    return num;
}

public static void main(String[] args) {
    System.out.println(recursionTest(1));
}

The output is always 2. When I'm using the step-through debugger I see the following:

The number increases by 1 with each iteration.
When the number reaches 10 the return statement is executed.
The debugger then highlights the "recursionTest(++num);" line but the number is decreased by 1.
The return statement is executed again.
Steps 3 and 4 is repeated until a value of 2 is finally returned.

Why is the value being decremented in the end, and how do I return the initially calculated value?

Baby · Accepted Answer · 2014-01-20 06:29:13Z

12

It should be like this:

private static int recursionTest(int num){
    if (num < 10){
       return recursionTest(++num);
    }
    else
       return num;
}

answered Jan 20, 2014 at 6:29

Baby

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5 Comments

NotMyName Over a year ago

Thanks, I didn't realise I had to use the return statement on the method-call as well.

Keerthivasan Over a year ago

@Rafa Can you explain why we need to return? I couldn't understand it. What's the difference between calling the method with and without return

Baby Over a year ago

@Octopus without return, the method will execute independently in which, the final value of num is not returned for the method. return recursionTest(++num) can also be replaced with num = recursionTest(++num). But at the end, I believe that you do understand it, maybe even better than I do :p

Keerthivasan Over a year ago

Thank you, Man. I can understand now. I learnt a new thing today ;)

Baby Over a year ago

you're welcome. glad to 'help' you understand "new thing" today :p

ggmathur · Accepted Answer · 2014-01-20 06:29:41Z

2

This is because you're still returning num instead of the result of your recursive function.

private static int recursionTest(int num){
    if (num < 10){
        return recursionTest(++num); 
    }
    return num;
}

answered Jan 20, 2014 at 6:29

ggmathur

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Comments

Ankur Shanbhag · Accepted Answer · 2014-01-20 06:30:02Z

2

Try using this:

num = recursionTest(++num);

answered Jan 20, 2014 at 6:30

Ankur Shanbhag

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1 Comment

Georgian Over a year ago

Also a valid answer, just not as clean as the two above.

Miguel Jiménez · Accepted Answer · 2014-01-20 06:30:28Z

1

Your problem is that you must return the recursive calling, not the number. If you do so you are going to obtain always your initial value + 1.

Solution:

private static int recursionTest(int num){
    if(num < 10){
        return recursionTest(num + 1);
    }

    return num;
}

answered Jan 20, 2014 at 6:30

Miguel Jiménez

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Comments

harsh · Accepted Answer · 2014-01-20 06:32:20Z

0

Try this, use end condition explicitly:

int recursionTest(int num) {
        if(num == 10){ //recursion exit condition
            return num;
        }
        return recursionTest(++num);
    }

answered Jan 20, 2014 at 6:32

harsh

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Comments

Varun · Accepted Answer · 2014-01-20 06:40:19Z

-2

private static int recursionTest(int num){
   while(num < 10){
        recursionTest(++num);
        if(num < 10){
            break;
        }
    }
    return num;
}

Above snippet will iterate while loop 10 times only so it will be fast.

answered Jan 20, 2014 at 6:40

Varun

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3 Comments

DnR Over a year ago

Then this is while loop, not recursion. And, you already specify while(num < 10) so no need to check if(num < 10) (which I assume what you mean is if(num >= 10))

Varun Over a year ago

It is not necessary to write inner IF condition, without it will also work but in that scenario if you add print statement before and after recursionTest function you will find out that it will print approx 1000 lines so I put this inner IF check so that it will print only 10 lines means when num<10 it will break the loop and will not execute further. so performance will imporve

DnR Over a year ago

oh okay I got it. but this is recursion, we don't need while loop because the recursive method itself will do the loop. :)

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