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Thread: retrieving percentages
 04192011, 02:54 PM #1Senior Member
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retrieving percentages
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.
Java Code:percents = percentages(votes, total, index, percents);
Java Code: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; }
 04192011, 03:07 PM #2
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.How to Ask Questions the Smart Way
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 04192011, 03:11 PM #3
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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.
Java Code:int i = 10; int j = 15 j / i == 1; (double)j / i == 1.5; (double)j / (double)i == 1.5;
 04192011, 03:13 PM #4
Also, if you're going to be checking for equality, I'd give this a readthrough: What Every Computer Scientist Should Know About FloatingPoint Arithmetic
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 04192011, 03:17 PM #5
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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.
 04192011, 03:27 PM #6
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:
Java Code:double s=0; for (int i=0; i<26; i++) s += 0.1; System.out.println(s);
I'm not sure I follow. Single '=' is assignment. Double "==" checks for equality.Last edited by KevinWorkman; 04192011 at 03:35 PM.
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 04192011, 03:38 PM #7
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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.
 04192011, 03:57 PM #8
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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:
Java Code:public class T { public static void main(String[] args) { for (double x= 0.1; x != 1; x+= 0.1) System.out.println(x); } }
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 04192011, 08:12 PM #9Senior Member
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here is the data file that i read:
Java Code:Smith 80,000 Jones 100,000 Scott 75,000 Washington 110,000 Duffy 125,000 Jacobs 67,000
 04192011, 08:20 PM #10
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. Use [code][/code] tags when posting code. That way people don't want to stab their eyes out when trying to help you.
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 04202011, 01:12 PM #11Senior Member
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so i updated my method to doubles, and here is what i have:
Java Code: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; }
 04202011, 01:18 PM #12
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 04202011, 01:49 PM #13
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My first post explains the problem, and shows you the change to make. Just in case, you should be casting to double.
 04202011, 03:12 PM #14Senior Member
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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?
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 04202011, 03:24 PM #16Senior Member
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got it: System.out.printf("%,.0f", total);
 04202011, 03:26 PM #17
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