retrieving percentages

[droidus]

i am trying to get the percentage of votes for each candidate for my program.
votes is an array (with # of votes for each candidate, total is the total number of votes, index shows me how many candidates there are (so i only run the loop "i<index" times), and percents is the initialized array.

percents = percentages(votes, total, index, percents);

this is my method to get each of the percents:

public static int[] percentages(int votes[], int total, int index, int percents[]) {
	
		int i = 0;
	
		for (i=0; i<index; i++) {
			percents[i] = (votes[i]/total);
		}
	
	return percents;
	
	}

after printing out the percents, i get 0 for each one.

[KevinWorkman]

Dividing an int by and int returns an int. I assume that votes[i]/total gets you a number between 0 and 1. If you store that as an int, it's going to truncate and store zero.

You probably want to use a double instead.

[sunde887]

Integer division seems to be the culprit here. Since the calculated percent will be a number >=0 && < 1, the decimal will be truncated. To get around integer division you will have to cast one, or both values to a double. You will also have to return a double array.

int i = 10;
int j = 15
j / i == 1;
(double)j / i == 1.5;
(double)j / (double)i == 1.5;

edit: too slow :(

[KevinWorkman]

Integer division seems to be the culprit here. Since the calculated percent will be a number >=0 && < 1, the decimal will be truncated. To get around integer division you will have to cast one, or both values to a double. You will also have to return a double array.

int i = 10;
int j = 15
j / i == 1;
(double)j / i == 1.5;
(double)j / (double)i == 1.5;

edit: too slow :(

Also, if you're going to be checking for equality, I'd give this a read-through: What Every Computer Scientist Should Know About Floating-Point Arithmetic

[sunde887]

Also, if you're going to be checking for equality, I'd give this a read-through: What Every Computer Scientist Should Know About Floating-Point Arithmetic

That looks like a lengthy read. I'll be sure to be checking that out throughout the next few days.

When explaining stuff to people I usually use == to convey equality instead of =, I know = would do fine, but I figure, in case they copy/paste I already used the correct version.

[KevinWorkman]

That looks like a lengthy read. I'll be sure to be checking that out throughout the next few days.

Well, here's the gist:

Floating point arithmetic is rarely exact. While some numbers, such as 0.5, can be exactly represented as a binary (base 2) decimal (since 0.5 equals 2^-1), other numbers, such as 0.1, cannot be. As a result, floating point operations may result in rounding errors, yielding a result that is close to -- but not equal to -- the result you might expect. For example, the simple calculation below results in 2.600000000000001, rather than 2.6:

  double s=0;

  for (int i=0; i<26; i++)
    s += 0.1;
  System.out.println(s);

Source: Java performance tuning tips

When explaining stuff to people I usually use == to convey equality instead of =, I know = would do fine, but I figure, in case they copy/paste I already used the correct version.

I'm not sure I follow. Single '=' is assignment. Double "==" checks for equality.

[sunde887]

Sorry about the confusion, it just seems natural to use a single = when generally typing, but to programmers you use == so when I'm saying something equals another I just tend to stick to == always now instead. I'm just being confusing. Thanks for the summary of the thread. I hope the op finds all this helpful as well.

[JosAH]

Sorry about the confusion, it just seems natural to use a single = when generally typing, but to programmers you use == so when I'm saying something equals another I just tend to stick to == always now instead. I'm just being confusing. Thanks for the summary of the thread. I hope the op finds all this helpful as well.

Better read that article; chances are higher that'll understand that, say, the following doesn't do what you naively might expect it to do:

public class T {
	public static void main(String[] args) {
		for (double x= 0.1; x != 1; x+= 0.1)
			System.out.println(x);
	}
}

kind regards,

Jos

[droidus]

here is the data file that i read:

Smith  80,000
Jones   100,000
Scott   75,000
Washington   110,000
Duffy   125,000
Jacobs   67,000

i get the total of all the numbers, then take each number, and divide by the total to get my percentage. i am getting 0 for some reason. i guess my array is empty?

[Dark]

A percentage will result in a number between 0 and 1. It's been explained that you should use a double instead of an int. Also don't forget that double values must have a 1.0 to produce the decimal.

Also the article posted is worth the read so your code does not have miscalculations.

[droidus]

so i updated my method to doubles, and here is what i have:

public static double[] percentages(int votes[], int total, int index, double percents[]) {
	
		int i = 0;
	
		for (i=0; i<index; i++) {
			percents[i] = (votes[i]/total);
		}
	
	return percents;
	
	}

i still get 0's though.

[JosAH]

so i updated my method to doubles, and here is what i have:

public static double[] percentages(int votes[], int total, int index, double percents[]) {
	
		int i = 0;
	
		for (i=0; i<index; i++) {
			percents[i] = (votes[i]/total);
		}
	
	return percents;
	
	}

i still get 0's though.

votes[ i ] and total are still int typed values so the division will be an int divistion. You did read your text book did you?

kind regards,

Jos

[sunde887]

My first post explains the problem, and shows you the change to make. Just in case, you should be casting to double.

[droidus]

ok. so i have a bunch of 0's after the decimal when i print out the values now. how do i get rid of these? can i use printf?

[KevinWorkman]

ok. so i have a bunch of 0's after the decimal when i print out the values now. how do i get rid of these? can i use printf?

What happened when you tried? Also look into DecimalFormat.

[droidus]

got it: System.out.printf("%,.0f", total);

[sunde887]

Glad to have helped. Please mark your thread solved with the thread tools at the top.