Summing array values java

Question

I'm feeding in an array of length 8 say if trials is 100 it might be of the form 93 5 2 0 0 0 0 0, but whatever the values I have in the array I only get 0.6 back. If anyone can see if I'm making a stupid error that would be great. I've tried it with a for loop but keep getting 0.6.

static void getMetric(int[]a, int trials){
    double metric = 0;
    int i =0;
    while(i<8){
        if(i==0){
            double x = (a[0] / trials) - (2 / 15);
            metric += Math.abs(x);
            i++;
        }
        else if(i>0 && i<7){
            double x = (a[i] / trials) - 0.1;
            metric += Math.abs(x);
            i++;
        }
        else{
            double x = (a[7] / trials) - (2 / 15);
            metric += Math.abs(x);
            System.out.println(""+metric);
            i++;
        }
    }
}

Does this method return 0.6 all the time? It doesn't seem to have a return value, so where are you seeing 0.6? — aldrin
– aldrin, Commented Jul 3, 2011 at 11:06

KarlP · Accepted Answer · 2011-07-07 11:01:05Z

3

You use integer division ( 5 / 3 = 1; 2 / 15 = 0).

So instead of a[0] / trials, you should have a[0] / (double) trials;

Instead of 2 / 15 you should have 2 / 15.0 etc.

edited Jul 7, 2011 at 11:01

KarlP

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answered Jul 3, 2011 at 11:03

Petar Ivanov

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3 Comments

user528676 Over a year ago

double x = (double)((a[7] / trials) - (2 / 15)) ?

KarlP Over a year ago

Isn't it : a[0] / (double) trials ?

Petar Ivanov Over a year ago

no, this doesn't work: (double)(2 / 15) == 0.0, because it first does the integer division and then convert to double

Johannes Weiss · Accepted Answer · 2011-07-03 11:17:27Z

It looks like you need double-division and not int-division. Remember:

int a = 96;
int b = 100;
double c = a / b; //will be 0.0!

so the following program should do the same, but more correct, I think (and shorter):

static void getMetric(int[] a, int trials){
    double metric = Math.abs((((double)a[0]) / trials) - (2 / 15));

    for (int i = 1; i < 7; i++) {
        metric += Math.abs((((double)a[i]) / trials) - 0.1);
    }

    metric += Math.abs((((double)a[7]) / trials) - (2 / 15));

    System.out.println(""+metric);
}

and that one is even more reable and robust:

static void getMetric(int[] a, int trials){
    double metric = calcMetricDiff(a[0], trials, 2.0 / 15.0);

    for (int i = 1; i < a.length - 1; i++) {
        metric += calcMetricDiff(a[i], trials, 0.1);
    }   

    metric += calcMetricDiff(a[a.length-1], trials, 2.0 / 15.0);

    System.out.println(""+metric);
}   

private static double calcMetricDiff(double val, int trials, double diff) {
    return Math.abs((val / trials) - diff);
}

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