Multiplying array elements by n, and storing them in a new array

Question

I have an array, in this case double [] v, which will take 4 values (0, 1, 2, 3).

I need to multiply each element of the array by "c" and store the result of each one back in a new array. I have tried it but i always end up getting the error message that double cannot be converted to double []. I can't figure out why and the examples here in SO have not helped.

public class q7_2014 {
    public static double [] multiply ( double [] v, double c){
        double [] newArray = new double [4];
        double multiply = 0;

        for (int i = 0; i < v.length; i++){
            multiply = v[i]*c;



            System.out.println(multiply);

        }



        return newArray;

    }

}

The code works fine and prints what is supposed to go into the array, but I just can't figure out how.

what am I missing? (the print statement was just to check if the code was working as expected in terms of results). Thanks

I don't believe this is a duplicate, since i'm not asking how to initialize an array since you can see in my code that I do that on the second line. I'm simply asking how to store the multiplication of each element of one array into another one. Thank you anway — Miguel, May 27 '18 at 01:33

StaticBeagle · Answer 1 · 2018-05-27T01:24:59.123

1

You need to set the result of the multiplication back into newArray

public class q7_2014 {
    public static double [] multiply (double[] v, double c){
        double[] newArray = new double[v.length];
        // You don't need multiply since
        // you need to return newArray
        //double multiply = 0;

        for (int i = 0; i < v.length; i++){
            newArray[i] = v[i] * c;
            // uncomment if you need to print the current value
            // System.out.println(newArray[i]);
        }
        return newArray;
    }
}

edited May 27 '18 at 01:24

answered May 27 '18 at 01:18

StaticBeagle

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@Downvoter: Care to give a reason why? – StaticBeagle May 27 '18 at 01:31

score 1 · Answer 2 · answered May 27 '18 at 01:19

1

Try this code:

public class q7_2014 {
public static double [] multiply ( double [] v, double c){
    double [] newArray = new double [4];
    for (int i = 0; i < v.length; i++){
        newArray[i] = v[i]*c;
        }

    return newArray;
   }
}

answered May 27 '18 at 01:19

Nirel

1,734
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score 1 · Accepted Answer · answered May 27 '18 at 01:22

You should store the results of your multiplication in newArray. Also, you should size newArray by the passed in array v length. Like,

public static double[] multiply(double[] v, double c) {
    double[] newArray = new double[v.length];
    for (int i = 0; i < v.length; i++) {
        newArray[i] = v[i] * c;
    }
    return newArray;
}

And in Java 8+, you could produce the same result with a single line of code using a DoubleStream and a map operation. Like,

public static double[] multiply(double[] v, double c) {
    return DoubleStream.of(v).map(d -> d * c).toArray();
}

Thanks for your suggestion. In fact i was just missing newArray "[i]" . Im sure your single line of code would also work, but since im still new to Java i'm not familiar with this syntax. — Miguel, May 27 '18 at 01:29

Multiplying array elements by n, and storing them in a new array

3 Answers3