Why does Math.sqrt return 0 instead of the length of a vector?

Question

public static double[] creeVec(double x, double y, double z){

    //cree et retourne un tableau
    double[] tab = new double[3];
    tab[0]=x;
    tab[1]=y;
    tab[2]=z;
    return tab;
}

public static double longueur(double[] tab){

    //calcule la longueur
    tab = new double[3];
    double carre = ((tab[0]*tab[0])+(tab[1]*tab[1])+(tab[2]*tab[2]));
    return Math.sqrt(carre);
}

And then when I called them to get my result of Math.sqrt() I get 0

    double[] v1 = Vec3DTab.creeVec(10, 20, 30);
    double[] v2 = Vec3DTab.creeVec(4, 5, 6);

    //affiche la longueur de v1
    double LongueurV1 = Vec3DTab.longueur(v1);
    System.out.println(LongueurV1);

score 4 · Answer 1 · answered Jan 29 '16 at 04:10

4

The code tab = new double[3]; in longueur(double[] tab) method creates a new double array object and initializes with default value of 0.

Hence it returns 0.

Please remove this line and you will get proper value.

answered Jan 29 '16 at 04:10

Harisudan Kuppusami

303
1
6

score 1 · Answer 2 · answered Jan 29 '16 at 04:08

1

Doing:

double[] v1 = Vec3DTab.creeVec(10, 20, 30);
double[] v2 = Vec3DTab.creeVec(4, 5, 6);

is bad form, do:

double[] v1 = Vec3DTab.creeVec(10.0, 20.0, 30.0);
double[] v2 = Vec3DTab.creeVec(4.0, 5.0, 6.0);

but it seems like you are going to a lot of rigmarole to get

double[] v1 = {10.0, 20.0, 30.0};
double[] v2 = {4.0, 5.0, 6.0};

but your real problem is here:

public static double longueur(double[] tab){

//calcule la longueur
tab = new double[3];     // <== this is hiding the tab above which has the values
                         // this just has zeros.

answered Jan 29 '16 at 04:08

John Hascall

9,176
6
48
72

Disagree on it being bad form to omit the `.0`, though totally agree on the apparent roundabout way here of defining a simple 3-array. I find `{10, 20, 30}` easier on the eyes. Or `{10d, 20d, 30d}` if you really want to be explicit about types. – Riaz Jan 29 '16 at 04:27
I see beginners do stuff like `double ftoc = 9/5;` a lot; so IMO it's best to get people in the practice of always making doubles look like doubles. I know experienced programmer love shortcuts (like how C allows `if (intvar)` but Java makes you say `if (intvar != 0)`, but they are to me an easily avoided common source of errors. – John Hascall Jan 29 '16 at 13:10

score 1 · Answer 3 · answered Jan 29 '16 at 04:12

Reason: you are clearing the value you passed in to the method., therefore all the array elements are set to zero.

//calcule la longueur
 tab = new double[3];     //  <-- here are you clearing the vector

Solution: Remove the new initialization of the tab array

public static double longueur(double[] tab){
    //calcule la longueur
    double carre = ((tab[0]*tab[0])+(tab[1]*tab[1])+(tab[2]*tab[2]));
    return Math.sqrt(carre);
}

Why does Math.sqrt return 0 instead of the length of a vector?

3 Answers3