Assign new value to double in a for loop after 6 iterations

Question

I cant figure out why x=x-z gives me the values it does. x is supposed to decrease its value by z every 6th loop, for 1000 loops, making it less probable that mrnd (math random) is greater than poa (probability of acceptance).

int i = 0; 
double y = 0.9;
double x = 1.0;
double z = 0.1;

for (int o = 0; o < 1000; o++) {
    for (i = 0; i < 6; i++) {


        Double random = Math.random();
        Double mrnd = Math.floor(random*100)/100;

        double poa = (x*y);
        System.out.println("randomkalk " + mrnd);
        System.out.println("probability" + poa);

        if (poa < mrnd ) {
            System.out.println("accept change");
        };
        if (poa > mrnd )  {
            System.out.println("deny change");
        }

    }   
    System.out.println(x-z); // This gives the right output if x=x-z is not present
    System.out.println(" nr 6 \n \n ");
    x = x-z; // Gives wrong output 
}

Possible duplicate of [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) — Fred Larson, Sep 20 '19 at 13:47
You aren't doing something "every 6th iteration". Instead, for each iteration from a 1000, you do 6 additional iterations. — M. Prokhorov, Sep 20 '19 at 13:50
Also, do you intend to have it so `x` is less than zero for most of your loop? Even if it decreased every 6th iteration from a 1000, that's means it'll be zero after 36th one, and from that point on your `poa` will always be less than `mrnd`. — M. Prokhorov, Sep 20 '19 at 13:55
"Gives wrong output" - you should tell us what you'd expect and what you get instead. Note that for `x = 1.0` and `z = 0.1` `x = x - z ` (or in short `x -= z`) would result in a negative `x` pretty quickly. Just a guess here: could it be that you actually mean to just use `x *= y` in the outer loop (thus no need for `poa`)? That would decrease `x` by 10% each time but `x` would never become negative. — Thomas, Sep 20 '19 at 13:56
Btw, why are you using `Double` here? `Math.random()` and `Math.floor()` return `double` so there's no need for all that (un-)boxing. — Thomas, Sep 20 '19 at 13:58
Added a println with o right before 6, and it does run 1000 times only, and every 6th it prints " nr 6".. — TTT, Sep 20 '19 at 14:01
The println at bottom is just for testing purposes now. What im trying to do is decrease x by z (0.1) every 6th iteration. — TTT, Sep 20 '19 at 14:04

score 0 · Accepted Answer · answered Sep 20 '19 at 13:57

As M. Prokhorov points out, you're not doing anything every 6th iteration. You're doing something 6 times every iteration. Making another loop inside your loop will only add more iterations of something; instead, try checking the counter on the outside loop.

int i = 0; 
double y = 0.9;
double x = 1.0;
double z = 0.1;

for (int o = 0; o < 1000; o++) {
    Double random = Math.random();
    Double mrnd = Math.floor(random*100)/100;

    double poa = (x*y);
    System.out.println("randomkalk " + mrnd);
    System.out.println("probability" + poa);

    if (poa < mrnd ) {
        System.out.println("accept change");
    };
    if (poa > mrnd )  {
        System.out.println("deny change");
    }

    if (o % 6 == 5) {
        x = x-z;
    }
}

Here, I use the modulus operator to check if we are on the 6th iteration

0 % 6 == 0
1 % 6 == 1
2 % 6 == 2
3 % 6 == 3
4 % 6 == 4
5 % 6 == 5 decrement
6 % 6 == 0
7 % 6 == 1
8 % 6 == 2
9 % 6 == 3
10 % 6 == 4
11 % 6 == 5 decrement
12 % 6 == 0

Thanks. I was trying to use modulus at an earlier point when i tested, but i guess i must have gotten that wrong. Works as intended now. — TTT, Sep 20 '19 at 14:09

Assign new value to double in a for loop after 6 iterations

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