Recursion - incrementing a number and returning

Question

Ok, this must be something really stupid....my statement is returning 2:

When I print out the values they are all correct
The return statement is wrong somehow

Code:

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

public static int countToTen(int last_answer){
    last_answer++;
    if(last_answer != 10){
        countToTen(last_answer);
    }
    return  last_answer;
}

Hint: you're not doing anything with the return value of countToTen when you call it recursively... — Jon Skeet
– Jon Skeet, Commented May 15, 2014 at 21:02
thank you Mark, so why must i do it that way? Please post this answer so i can give you credit — Petro
– Petro, Commented May 15, 2014 at 21:03
Well, try either debugging through the code or just think about what it's going to return... — Jon Skeet
– Jon Skeet, Commented May 15, 2014 at 21:04
You are not passing the argument by reference, it is a copy. You are doing a modification in last_answer in the context of method, but this change is not propagate outside, because in the end you return last_answer++ — carlosvin
– carlosvin, Commented May 15, 2014 at 21:11

Matt · Accepted Answer · 2014-05-15 21:08:13Z

4

Try replacing your if statement with:

if(last_answer != 10){
    return countToTen(last_answer);
}

Without the return statement, the recursive calls do get executed, but the calculated result is never returned.

The order of calls with your broken code looks like:

countToTen(1)
 -> countToTen(2)
 -->  countToTen(3)
 ---> more calls to countToTen()
 --- ... --> countToTen(10) // do nothing, and return to the top-level method call
 -> return 2 // 2 because you incremented it using lastAnswer++

edited May 15, 2014 at 21:08

answered May 15, 2014 at 21:03

Matt

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2 Comments

Petro Over a year ago

So why is "return" neccessary, and could you please post it inside your answer for others! Thanks :)

Petro Over a year ago

Very good, please up this question for future viewers. I marked it as the answer, thanks again!

Audrius Meškauskas · Accepted Answer · 2014-05-15 21:06:11Z

2

Your function returns the value from the first call. It is the initial value (1) incremented once by the ++ statement, so the function returns 2.

Integers are passed by value in Java, incrementing the passed value inside the function does not change the value outside:

int x = 0;
v(x);
// x still 0 here.

void v(int x) {
  x = 100;
}

answered May 15, 2014 at 21:06

Audrius Meškauskas

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1 Comment

Petro Over a year ago

Thank you, this is the explanation I was looking for!

aProperFox · Accepted Answer · 2014-05-15 21:04:28Z

0

Try this:

public static int countToTen(int last_answer){

  last_answer++;

  if(last_answer != 10){
    return countToTen(last_answer);
  }
  else {
    return last_answer;
  }
}

answered May 15, 2014 at 21:04

aProperFox

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2 Comments

Lee Fogg Over a year ago

Thats the same thing except now it wont compile now because there isn't a mandatory return.

Brian Over a year ago

It will compile. All code paths return a value (there is no code path which bypasses both of Tyler's return statements). Some people prefer if(condition){return value;} else {return othervalue;} over if(condition){return value;} return othervalue;, as it makes it more obvious that return othervalue; only happens if condition is false.

Sara S · Accepted Answer · 2014-05-15 21:07:30Z

0

If you want to print out 1,2,3,4 ... 10, you need to print out the answer in every phase separately

public static void main (String [] args){
    countToTen(1);
}

public static void countToTen(int last_answer){

    System.out.println(last_answer);
    last_answer++;

    if(last_answer <= 10){
    countToTen(last_answer);
    }
}

answered May 15, 2014 at 21:07

Sara S

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1 Comment

Petro Over a year ago

Hi Sarah, no i was just wondering why my ending return was 2 and not 10

carlosvin · Accepted Answer · 2014-05-15 21:16:34Z

0

My proposal

public static int countToTen(int last_answer){

   last_answer++;

    if(last_answer < 10){
        return countToTen(last_answer);
    } else {
        return last_answer;
    }
}

You are not passing the argument by reference, it is a copy. You are doing a modification in last_answer in the context of method, but this change is not propagate outside, because in the end you return last_answer++

answered May 15, 2014 at 21:16

carlosvin

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Comments

Pragmateek · Accepted Answer · 2014-05-15 21:38:00Z

0

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

public static int countToTen(int last_answer)
{    
    last_answer++;

    if(last_answer != 10)
    {
        return countToTen(last_answer); //Here was the error
    }

    return  last_answer;
}

edited May 15, 2014 at 21:38

Pragmateek

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answered May 15, 2014 at 21:03

jspurim

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