String cannot be converted to double error in a function

Question

I am trying to output NaN in string format when x is NaN. However, the console throws up String cannot be converted to double error when I am trying to return string of "NaN" when Double.isNaN is true. I tried parsing string r as a double using Double.parseDouble() but to no avail.

Here is my code:

public static double heaviside(double x) {
    String r = "NaN";
    r = Double.parseDouble(r);
    double result;
    if (Double.isNaN(x)) return r;
    else if (x < 0.0) return result = 0.0;
    else if (x == 0.0) return result = 0.5;
    else return result = 1.0;
}

Console output

ActivationFunction.java:6: error: incompatible types: double cannot be converted to String r = Double.parseDouble(r); ^ ActivationFunction.java:8: error: incompatible types: String cannot be converted to double if (Double.isNaN(x)) return r; ^

I suggest you using an IDE like IntelliJ or Eclipse. They are very helpful when learning a new language. — Fabio Filippi
– Fabio Filippi, Commented May 27, 2020 at 7:37

Amongalen · Accepted Answer · 2020-05-27 07:40:20Z

3

You problem comes from the fact, that you try to assign double to String variable. r is defines as String and Double.parseDouble(r) returns double. It will work if you assing it to result instead, like so: double result = Double.parseDouble(r);

However, there is no need to parse new double from String in the first place. You want to return NaN when x == NaN. You can just return x in that case because, well, x is NaN. No need for another variable, parsing and all of that.

answered May 27, 2020 at 7:40

Amongalen

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Comments

Pranav P · Accepted Answer · 2020-05-27 07:32:36Z

1

You are trying to convert String into double and storing its value in r which is a type of String.

You should create another variable of double type and can store that value in that like this:

public static double heaviside(double x) {
    String r = "NaN";
    double d = Double.parseDouble(r);
    double result;
    if (Double.isNaN(x)) return d;
    else if (x < 0.0) return result = 0.0;
    else if (x == 0.0) return result = 0.5;
    else return result = 1.0;
}

I hope it will help you. Happy coding..!

answered May 27, 2020 at 7:32

Pranav P

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3 Comments

Amongalen Over a year ago

The question is, what is the point of all of this? "If x is NaN, create a new double representing NaN and return that new value". Why don't just return x in the first place?

Pranav P Over a year ago

You are right. I give this much explanation because @palerabbit can understand that he was trying to store double into String and that is an actual error that he mentioned into question.

HC Tang Over a year ago

Thanks for the input. I was trying to get "NaN" in my output instead of infinity but yes what you have here makes alot of sense! cheers @Amongalen

Eric Gutiérrez · Accepted Answer · 2020-05-27 07:23:39Z

0

"NaN" is not a double, you can't convert it to double and not return x because Java returns an exception on line 2 of your code and does not continue to run the rest of the code

answered May 27, 2020 at 7:23

Eric Gutiérrez

1551 bronze badge

4 Comments

Amongalen Over a year ago

Funny enough, I've just tried double test = Double.parseDouble("NaN"); and... it runs just fine.

Fabio Filippi Over a year ago

The error in that line is not that "NaN" is not a double. But that parseDouble returns a double and not a String, so you cannot assign it to a String.

Pranav P Over a year ago

I agree with you @Amongalen, I run that too and it works fine not sure why. But I think, I tried to print in 1System.out.println(Double.parseDouble("NaN))` which converts double into String again.

user207421 Over a year ago

Compilation errors are not exceptions, and neither compilation errors nor exceptions are returned. Don't misuse standard terminology.

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