Lil java help for a noob please

[gam3r]

Hey everyone.

I'm trying to write a program which converts decimal into binary using recursion and vice versa.

So far i have been able to write the method for decimal to binary and it compiles fine and runs ok, apart from the fact that it displays the binary answer backwards.

Here is my code:

public static String decTobin (int n){
	if (n==0){
		return "0";
	}
	if (n%2==0){
		return "0" + decTobin(n/2);
	}
	else {
		return "1" + decTobin(n/2);
		}
 
 
}

For example, if i put in 5 to convert to binary, instead of getting 0101 as the answer i get 1010.

How can i reverse the output? I have been trying so hard...

Thanks

 

[lumination]

Well think about it, run through on a piece of paper what happens if n = 5.

PS: Your answer isn't backwards - think of 5 as 101, not 0101.

Another thing, the only way for your method to exit is if n==0 which then results in 0 always being the last digit (on the right hand side) - does every binary number in the world have a 0 as the furthest digit on the RHS?

 

[gam3r]

Thank you very much for the reply..

I ran through it on a piece of paper with 5, and i get 1010 as well? which is reads left to right to make 5 instead of right to left like it should when reading binary.

Am i doing something wrong?

 

[benjo plz.]

change it to return dectobin(n/2) + "1"?

Another thing, the only way for your method to exit is if n==0 which then results in 0 always being the last digit (on the right hand side) - does every binary number in the world have a 0 as the furthest digit on the RHS?

edit, gotcha

 

[lumination]

Look at the last sentence of my previous post.

Then try running your code going through all the numbers from 1 to 10. You'll notice the last digit of all your answers will be 0. This is incorrect.

 

[benjo plz.]

yeah, would be easier to do this iteratively.

 

[lumination]

change it to return dectobin(n/2) + "1"?

I believe this is correct.

Which I guess means I was wrong when I said your answer wasn't backwards.

 

[benjo plz.]

I believe this is correct.

Which I guess means I was wrong when I said your answer wasn't backwards.

but it won't work

 

[benjo plz.]

yeah, would be easier to do this iteratively.

like this...

public static String decTobi(int n) {
        String bin = "";
        if (n == 0) {
            return "0";
        }
        while (n > 0) {
            if (n % 2 == 0) {
                bin = "0" + bin;
                n = n / 2;
            } else {
                bin = "1" + bin;
                n = n / 2;
            }

        }
        return bin;

 

[lumination]

but it won't work

Why not?

 

[benjo plz.]

Why not?

because it calls itself, so it won't add the number.

 

[gam3r]

Hey guys, thanks for the replies.

Im just trying to work through an exercise and it says i have to try and implement the method recursively and not use iteration. Just slightly puzzled now, i can understand what you guys are saying. If only there was a way to reverse the output, that way the zero on the rsh would be irrelevant in binary if you were reading from right to left.

Hope you understand what im trying to say.

 

[benjo plz.]

If only there was a way to reverse the output, that way the zero on the rsh would be irrelevant in binary if you were reading from right to left.

Hope you understand what im trying to say.

Actually, binary numbers starting with 0 are negative .

and maybe new StringBuffer(decTobin(int n)).reverse()) would work .

 

[gam3r]

Errr aren't binary numbers starting with 1 negative? At least thats what i was taught...:S

 

[lumination]

You don't need to reverse the output, this will work fine

public static String dec (int n){
if (n==0){
return "0";
}
if (n%2==0){
return dec(n/2) + "0";
}
else {
return dec(n/2) + "1";
}


}

 

[daven1986]

Errr aren't binary numbers starting with 1 negative? At least thats what i was taught...:S

the man is correct. the MSB is negative.

 

[lumination]

A binary number starting with 1 is negative but only if you chose to include an extra bit representing the sign.

 

[benjo plz.]

Errr aren't binary numbers starting with 1 negative? At least thats what i was taught...:S

yep, now I remember why I stopped doing that stuff

You don't need to reverse the output, this will work fine

I'd screwed up my syntax

 

[gam3r]

It worked ...i could have sworn i tried that, i remember getting some compilation error about not being able to get to the bit after the method is called recursively..

Thanks guys

Now i need to try and implement binary to decimal...lol i'll get back to you most likely as i will probably get stuck with that too..

 

[lumination]

Binary to decimal is easier.