Checking each character for a number

Question

I am trying to loop through a string and check each character if one of the characters is a number. If it is a number, I want to return it as true. I have a string \"crash\"

Answer 1

I just ran that code and I get false, as expected. Please double-check that you’re running it correctly.

Here’s a simpler way to express that function, by the way:

public boolean isNumber(String string) {
  for (int i = 0; i < string.length(); i++) {
    if (!Character.isDigit(string.charAt(i))) {
      return false;
    }
  }

  return true;
}

Answer 2

Your code should work correctly, although I would probably use this instead:

public boolean isNumber(String newString)
{
    for (int i=0; i != newString.length(); i++)
    {
        if (!Character.isDigit(newString.charAt(i))) 
        {
            return false;
        }
    }
    return true;
}

// a regex equivalent
public boolean isNumberRegex(String newString)
{
    return newString.match("\\d+");
}

The method above checks if all characters are digits.

If I misunderstood your question and you want to check if any of the characters is a digit:

public boolean hasNumber(String newString)
{
    for (int i=0; i != newString.length(); i++)
    {
        if (Character.isDigit(newString.charAt(i))) 
        {
            return true;
        }
    }
    return false;
}

// regex equivalent
public boolean hasNumberRegex(String newString)
{
    return newString.match(".*\\d.*");
}

Answer 3

I think this code should work, but to my mind, setting a variable and then breaking just to return it is ugly. (I know other coders like this; IMHO they are wrong.) I also dislike introducing unnecessary test variables, like NullUserException's solution. I would just return directly.

[EDIT: This code is the same as Brockman's]

public boolean isNumber() /* Note: returns true for empty string */
{
String newString = "crash";
for (int i=0; i<newString.length(); i++)
{
   if (!Character.isDigit(newString.charAt(i)))
   {
    return false; /* non-digit detected */
   }
 }
return true; /* all characters were digits */
}

Answer 4

Well you can use Integer.parseInt("string") and catch the exception.

try {
     int num = Integer.parseInt("string");
     return true;
} catch (NumberFormatException nfe) {
     return false;
}

Or another way with regEx:

if ("string".replaceAll("\\d+","").length() > 0) { 
    //false 
} else { 
    //true
}

Answer 5

Maybe I didn't understand you correctly... but since you're using the same variable "isNumber", and continuing when you get a positive match... the result you'll return will always be of the last character of the String, except when you get a non numeric character, in which case, you exit right away.

Do you want to check if the whole String is a number? Or if it contains a number?

Answer 6

public static boolean isNumber(String str)
{
    int len = str.length();
    boolean isNumber = false;

    for(int i = 0; i < len; i++)
    {
        if(Character.isDigit(str.charAt(i)))
            return true;
    }

    return isNumber;
}