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Thread: Why does my code always print out true.

  1. #1
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    Default Why does my code always print out true.

    Hello!

    Consider this code

     
    import java.util.Arrays;
    public class test {
     
    	static boolean isDigit(char ch) {
    		if( '0' <= ch  && ch <='9') {
    			Out.println("true");
    			return true;
    		} else {
    			Out.println("false");
    			return false;
    		}
    	}
     
     
     
    	public static void main (String[] args) {
    		Out.println("Put in a digit");
    		char ch = In.readChar();
    		isDigit(ch);
     
     
    	}
     
     
    }

    Now the goal is to print out true whenever we input a number between 0 and 9,and for every other number we want to print out false.But for some reason it is always printing out true,no matter what number I put in.Worth mentioning is that the numbers are not int but char.Why does it print out true even for values of 120 ?

  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Why does my code always print out true.

    What is the value in the variable: ch?
    Print it to see. In case the value is not a printable character cast ch to int:
       System.out.println("ch=" + (int)ch + " " + ch);  // print ch to see value
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Why does my code always print out true.

    At what point in the code do I want to see the value of ch? When the user inputs the value?

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    Super Moderator Norm's Avatar
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    Default Re: Why does my code always print out true.

    Since it is never changed, any place would work.
    Probably immediately after it is assigned a value would be good.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Why does my code always print out true.

    Okay,very very interesting results;

    for 12;

    12
    ch=49 1
    true

    for 8

    8
    ch=56 8
    true

    for 222

    222
    ch=50 2
    true


    The code

     
    import java.util.Arrays;
    public class test {
     
    	static boolean isDigit(char ch) {
    		if( '0' <= ch  && ch <='9') {
    			Out.println("true");
    			return true;
    		} else {
    			Out.println("false");
    			return false;
    		}
    	}
     
     
     
    	public static void main (String[] args) {
    		Out.println("Put in a digit");
    		char ch = In.readChar();
    		System.out.println("ch=" + (int)ch + " " + ch);
    		isDigit(ch);
     
     
    	}
    }

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    Super Moderator Norm's Avatar
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    Default Re: Why does my code always print out true.

    Do you see what the value of ch is? It is a single character. Only the first char of a group that is entered is returned by the readChar method.

    I didn't know what value readChar would return so I cast the ch to int to see its value.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Why does my code always print out true.

    Yea I see it as well, the values of the char are single digit that is why its always printing true.Which means my logic wasnt at fault but the readChar() method.Thank you for your help,I'll adjust this with the method.

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