Java ArrayList stores different values when using for loop

Question

I am trying to generate the x and y values of a circle with radius one, for x=rcos(a) and y=rsin(a), but when I use the three different methods below I only get the correct values only by adding them manually, but I would like to add them in a loop.

 ArrayList<Double> cCoords = new ArrayList<Double>();
 double pi = Math.PI;
 double pi_over_two = Math.PI/2.0;
 double pi_over_four = Math.PI/4.0;
 double pi_over_180 = Math.PI / 180.0;
 double piVals = 0;


 cCoords.add(Math.cos(0));
 cCoords.add(Math.sin(0));
 cCoords.add(Long.valueOf(Math.round(Math.cos(pi_over_two))).doubleValue());
 cCoords.add(Math.sin(pi_over_two));
 cCoords.add(Math.sin(pi_over_two+pi_over_two+pi_over_two));

////////Below values are printed correctly

    cCoords.add(Math.cos(pi_over_four) );
    cCoords.add(Math.sin(pi_over_four));

    cCoords.add(Math.cos(pi_over_four+pi_over_four));
    cCoords.add((Math.sin( pi_over_four+pi_over_four)) );

    cCoords.add( Math.cos(pi_over_four+pi_over_four+pi_over_four) );
    cCoords.add(Math.sin(pi_over_four+pi_over_four+pi_over_four));

    cCoords.add(Math.cos(pi_over_four +pi_over_four+pi_over_four+pi_over_four) );
    cCoords.add(Math.sin(pi_over_four +pi_over_four+pi_over_four+pi_over_four));

    cCoords.add(Math.cos(pi_over_four+pi_over_four + pi_over_four+pi_over_four+pi_over_four));
    cCoords.add(Math.sin( pi_over_four+pi_over_four +pi_over_four+pi_over_four+pi_over_four) );

    cCoords.add( Math.cos(pi_over_four+pi_over_four+pi_over_four +pi_over_four+pi_over_four+pi_over_four) );
    cCoords.add(Math.sin(pi_over_four+pi_over_four+pi_over_four + pi_over_four+pi_over_four+pi_over_four));


//////////////Above values are printed correctly


    for(int i = 0; i < 6; i++) { // Wrong values

        cCoords.add(i, Math.cos(pi_over_four) );
        cCoords.add(i+1, Math.sin(pi_over_four));

        pi_over_four+=pi_over_four;

    }
/////


    for(int i = 0; i < 6; i++) { // x=1 coordinate gets repeated with different y values

        cCoords.add( Math.cos(pi_over_four) );
        cCoords.add( Math.sin(pi_over_four));

        pi_over_four+=pi_over_four;

    }

////

    for(int i = 0; i < cCoords.size()-1; i+=2) {
        System.out.print("(" + cCoords.get(i) + ",");
        System.out.println(cCoords.get(i+1) + ")\n");
    }

ninja.coder · Accepted Answer · 2016-04-02 05:30:43Z

2

You are getting the wrong values because you are inserting the incorrect values in the loop after changing the value of variable pi_over_four itself.

For instance, in the below code snippet which you say is printing the value correctly, you have doubled the value for the next iteration i.e. 2 * pi_over_four from just pi_over_four:

cCoords.add(Math.cos(pi_over_four));
cCoords.add(Math.sin(pi_over_four));

cCoords.add(Math.cos(pi_over_four + pi_over_four));
cCoords.add(Math.sin(pi_over_four + pi_over_four));

In the next iteration it becomes 3 * pi_over_four:

cCoords.add(Math.cos(pi_over_four + pi_over_four + pi_over_four));
cCoords.add(Math.sin(pi_over_four + pi_over_four + pi_over_four));

Note that here you are not changing the value of variable pi_over_four. You are just doing n * pi_over_four and then storing it back into the list.

But if you'll see your loop carefully, you are changing the value of pi_over_four itself which is wrong. What you should be doing instead is something like this:

for(int i = 0; i < 6; i++) {
    cCoords.add(i, Math.cos((i + 1) * pi_over_four));
    cCoords.add(i+1, Math.sin((i + 1) * pi_over_four));
}

answered Apr 2, 2016 at 5:30

ninja.coder

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2 Comments

user1420482 Over a year ago

Thank you for the help, found out the loop should be similar to your suggestion : int j=0; for(int i = 0; i < 6; i+=2) { cCoords.add(i, Math.cos( (j ) * pi_over_four) ); cCoords.add(i+1, Math.sin( (j ) * pi_over_four) ); j+=0.5 )

ninja.coder Over a year ago

@user1420482 Your Welcome! :)

Natecat · Accepted Answer · 2016-04-02 05:23:36Z

1

pi_over_four is changing each time you loop, so in reality, instead of multiplying it by 2 each time you are multiplying it by 2^x where x is i+1. To imagine this, on the first loop, pi_over_four is equal to pi/4, on the second it's equal to pi/2, but then you add pi/2 to that instead of pi/4, making the 3rd loop have it be equal to pi. Store your new pi_over_four in a different variable to fix this.

answered Apr 2, 2016 at 5:23

Natecat

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