Parsing double value from String in Java

Question

I am supposed to extract the double values from the following string:

r ( 4.5, 1.0 ) 10.5 7 ;

However my code deletes everything but the digits and that's not what I want. My code:

import java.io.File;
import java.util.Scanner;


public class mine {

    public static void main(String[] args) throws Exception
    {

      String str;
      String numbers;

      Scanner SC=new Scanner(System.in);

      System.out.print("Enter string that contains numbers: ");
      str=SC.nextLine();

      //extracting string
      numbers=str.replaceAll("[^0-9]", "");

      System.out.println("Numbers are: " + numbers);
    }
}

Output of my code:

Enter string that contains numbers: r ( 4.5, 1.0 ) 10.5 7 
Numbers are: 45101057

I want to find a way to assign the numbers to variable. So for example using the following string: r ( 4.5, 1.0 ) 10.5 7 (Assuming a,b,c,d are already declared) I want this:

a = 4.5
b = 1.0
c = 10.5 
d = 7

This question is somewhat ill-formed. How exactly will your input look like? Is `(( 2.0, 42, 7), 13) 36` a valid input? is `.3` valid? is some String, representing a number unrepresentable in `float` or `double` a valid input? How should this unrepresentable number be preseneted (flooring, ceiling, nearest value)?... — Turing85, Apr 05 '18 at 18:50

score 1 · Answer 1 · answered Apr 05 '18 at 18:45

If you would like to keep floating-point numbers and separators, you must include dot character and space, in addition to decimal digits, into the replacement regex:

numbers = str.replaceAll("[^0-9. ]", "");

Demo.

Once you have the string, separating out the individual numbers can be done using the technique discussed in this Q&A: Convert a string of numbers into an array.

score 0 · Answer 2 · answered Apr 05 '18 at 18:42

Having variables in that fashion is only going to be useful if you're guaranteed that you have four values in your file. For flexibility's sake, it's better to place this in a list instead.

List<Double> numbers = new ArrayList<>();
// assuming str already has the numbers...
for(String s : str.split(" ") {
    numbers.add(Double.parseDouble(s));
}

Declaring these as individual variables uses 99% of the same approach above, just swapping out a list for the individual values. I leave that as an exercise for the reader.

score 0 · Answer 3 · answered Apr 05 '18 at 18:52

Try this (it uses a second scanner to try to parse each token as a Double; if it's not a double, the parsing will fail):

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);

    System.out.print("Enter string that contains numbers: ");
    String str = sc.nextLine();

    Scanner token = new Scanner(str);
    while token.hasNext() {
         String tokenStr = token.next();
         try {
             double d = Double.parseDouble(tokenStr);
             // YOUR CODE: do something with d here, such as appending
             // it to an array, assigning it to a var, or printing it out.

         } catch (NumberFormatException e) {
             // no op
         }
    }
}

It works except for example if the input is : "r ( 4.5, 1.0 ) 10.5 7" ; It doesn't read the 4.5 because of the comma. — Pape Traore, Apr 05 '18 at 18:58
Well, I'd argue that "4.5," isn't a number and shouldn't be recognized as one. How do you want to handle things such as "4x5" (should this give 2 numbers, or 0)? Are the characters to ignore only on the ends of each token? — kmell96, Apr 05 '18 at 19:02

Parsing double value from String in Java

3 Answers3