Thread: For loop-ing decimal format

Hi all, I'm trying to find a way to use a for-loop to determine the number of decimal places that my program will output to. Specifically I want the the decimal place to be determined at run time by the user from a Scanner input following a prompt for a number. So if the user inputs 3 the program only outputs to "0.###".

So i guess i need to somehow make a DecimalFormat that adds a "#" to the back of it itself until it reaches the user's input (that's where the for-loop comes in)? I need some help with the specifics of writing though...

Many thanks in advance.

need to somehow make a DecimalFormat that adds a "#" to the back of it itself until it reaches the user's input (that's where the for-loop comes in)

Yes that sounds like you could use a loop to concatenate #s to a String until you have what the user requested.
Do you know how to write a loop?
Do you know how to concatenate a String onto another String?

You dont have to actually loop to do this. DecimalFormat has a method called setMaximumFractionDigits(int). So, we can simply use this to set the number of places. Here is a snippet of code I made and tested.
WHERE:
d is our double Variable we are rounding
places is the number of decimal places
df is our DecimalFormat

double d = 9.987656;
    	int places = 3;
    	DecimalFormat df = new DecimalFormat();
    	df.setMaximumFractionDigits(places);
    	System.out.println(df.format(d));

Originally Posted by Norm

Yes that sounds like you could use a loop to concatenate #s to a String until you have what the user requested.
Do you know how to write a loop?
Do you know how to concatenate a String onto another String?

I'm pretty new to java, i can write a loop but i don't know how to concatenate a string.

wouldn't i have to change the value of int places in the coding to change the number of decimal places with this method though?

Thanks a lot, code works like a charm your way.

I tried to code it as a for-loop as well just for practice, and I got that to work kind of. It rounds to desired decimal places, but I can't find an easy way to get rid of the null on the end of those...

String pattern = "0.";
String decis = null;

//now we're setting up the Scanner to read user inputs

Scanner input = new Scanner(System.in);

/*the written out prompts for program's ease of use and setting
input results as usable variables
*/

System.out.println ("please input a number to square.");

number = input.nextDouble();

System.out.println ("to how many places?");

places = input.nextInt();

//the math operation to be carried out by the program,
//could have been more complex if desired

square = (number * number);

//for-loop to concatenate the string "decis" "place" # of times to itself


for (decimal = 1; decimal <= places; decimal++ ){
decis += ("#");
}

//DecimalFormat made by concatenating 2 strings
DecimalFormat fmt = new DecimalFormat (pattern + decis);

System.out.println ("the square of " + (number) + " is " + fmt.format(square));
}
}

there's definitely something more elegant than writing a line to delete "null" I'm sure.

Give an example of the output. I'm having trouble visualizing where the null is.

formerly i would get outputs like so:

please input a number to square.
564.2212
to how many places?
1
the square of 564.2212 is 318345.6null

just changed

String decis = null;

to

String decis = "";

runs look a lot better now:

please input a number to square.
3.2839276
to how many places?
3
the square of 3.2839276 is 10.784

That is interesting. You'd think when you set decis to null, when you tried to add to it you would get a compiler error or something, but it seems to just concatinate to it as if null was the String set to it. Hmm.

I thought setting it to null was the same as not giving it a value? that way only the #s would appear later.

either way I'm done with this, your original version solves all so thanks again.