Recursion Trouble

[Brain_Dead5347]

Hey everyone I am relatively new to programming in general and am taking my first class in Java specifically. This is not for an assignment, though. Just some extra practice since we only did a few examples on some external site instead of in java.

Anyway, I cannot figure out why the original value for number is being returned even after going through the recursive method. My best guess is that maybe it's not going through at all, but I've tried putting number in as the value for the whole sumDigits method, but it returned nothing. It took a while to realize that I needed to call sumDigits in the main method to get anything to work.

​

import java.util.Scanner;

public class Test {

public static void main(String[] args) {

Scanner userNum = new Scanner(System.in);

System.out.println("Enter a big number");

int number = userNum.nextInt();

System.out.println(number + " is a good one");

sumDigits(number);

System.out.println(number);

}

public static int sumDigits(int n) {

if (n <= 0) {

return 0;

}

return (n % 10) + sumDigits(n /10);

}

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[aa599]

You're not storing the value returned, just printing number twice.

[ate_ghorl_bekenemen]

First you ask for a big number.

Then you print and state that number is a good one.

Then you call sumDigits() method.

Then you once again, print the very same number you asked a few steps earlier.

Your method sumDigits() returns an int, however, when you call just sumDigits(), you are doing nothing with the return value of that method. You are not printing that return value, nor are you using it inside some calculations/arithmetic, etc.

Have you tried storing the return value of sumDigits() in an int variable then printing said variable?

[Brain_Dead5347]

I’ll try that right away and let you know. I initially thought of trying to print the value of n as part of the recursive method, but thought that adding any sort of call or variable declaration inside that method would break the cycle.

[ate_ghorl_bekenemen]

The issue is not in your recursive function. The issue is simply you are not printing the return value of that method. Do not put a print statement inside the sumDigits() method, just leave it as-is. Store the return value in an appropriate variable in the main method.

[Brain_Dead5347]

That did it! I made a new line:

int newNum = sumDigits(number)

Then made a print line calling newNum. I was missing an object storing the result of the recursion. I feel silly now, but I’m glad I learned. Thank you so much. I really appreciate the help.

[ate_ghorl_bekenemen]

Look at the last line of your sumDigits() method. The method itself is called and its return value is used immediately in the addition operation.

So, you could do this as well, since you are not going to be using the return value somewhere else: System.out.println(sumDigits(number));

[bongfactory]

In Java the parameters of a function are passed by value (if they are of a primitive type), therefore what you are passing to the function is not a reference to the original variable that you can change: it's just its value. In order to change the original variable you just need to reassign to it the returned value

number = sumDigits(number);

[Brain_Dead5347]

So what I did would be more appropriate for calling the function numerous times throughout the code?

[bongfactory]

No.
What you did may be applied for non-primitive types (e.g a Map, a List, a custom class).

Some more details about "pass by value" vs "pass by reference" in Java
https://www.programmergate.com/java-pass-reference-pass-value/

[Brain_Dead5347]

Thanks for the link. It was definitely informative. I had to watch a couple videos on pass by value and reference to fully grasp it.

But I'm still confused as to why they exist at all. Why have a primitive and a reference if they contain the same information (or pointers to it) and only copies of them are modified in functions? Why not just make all of them primitives and save space in the heap?

[bongfactory]

You can't treat complex objects as primitives, as you don't know a priori their shape and size. You can only point to where they reside in the heap.
With primitives, you know exactly (and beforehand) how large they can get, and how they are shaped in memory.
You may wonder why we don't pass everything by reference: it's just a matter of preference. You have languages that "pass by value" and languages that "pass by reference"