problem while comparing char with relational operators

Question

import java.util.Scanner;
public class ValidString {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter the String: ");
        //String s = scan.nextLine();
        StringBuilder str = new StringBuilder(scan.nextLine());
        System.out.println(isValidString(str));
        scan.close();
    }

    public static boolean isValidString(StringBuilder str) {
        for(int i = 0;i<str.length();i++) {
        int s = (int)newStr.charAt(i);
            if(s>=97 && s<=122 || s>=65 && s<=90){
                
                continue;
            }else {
                return false;
            }
        }
        return true;
    }

}

I was expecting the output as true if string contains only Alphabets else false value. Can anyone trace out what's the problem with this code.

condition str.charAt(i)<='A' && str.charAt(i)>='Z' && str.charAt(i)<='a' && str.charAt(i)>='z') is never satisfied. — krzydyn
– krzydyn, Commented Mar 17, 2023 at 18:13
To put it more simply - imagine you were dealing with integers. Your condition would be something like if (x <= 5 && x >= 10 && x <= 25 && x >= 30). What value would you expect to satisfy all those parts? (I'd also suggest that if you call str.charAt(i) once and store the result in a local variable to use in your condition multiple times, your condition is likely to end up being more readable.) — Jon Skeet
– Jon Skeet, Commented Mar 17, 2023 at 18:14
Yes, I realise that - but I suggest you look really carefully at what I've suggested (both parts). What value of x is both "less than or equal to 5" and "greater than or equal to 10"? Then apply that same thinking to character comparisons. — Jon Skeet
– Jon Skeet, Commented Mar 17, 2023 at 18:19
@SURAJMAHAKAL you're missing the point of the comments. Try a sample input string for which isValidString(StringBuilder str) is supposed to return true. Pretend you are the computer, and follow the logic of that method. Use pen and paper for your computer memory. Create truth tables with columns such as c <='A', c>='Z', and so on. — Old Dog Programmer
– Old Dog Programmer, Commented Mar 17, 2023 at 18:20
... where c, in my previous comment, represents the current value pointed to by str.charAt(i). — Old Dog Programmer
– Old Dog Programmer, Commented Mar 17, 2023 at 18:29

Arvind Rachuri · Accepted Answer · 2023-03-17 18:23:37Z

1

Simply put, you will never have a character that is <= A and >=Z because the range A-Z is contiguous and ascending. The same is true for the range a-z. So instead of
str.charAt(i)<='A' && str.charAt(i)>='Z' && str.charAt(i)<='a' && str.charAt(i)>='z'
you need this -
!(str.charAt(i)>='A' && str.charAt(i)<='Z') && !(str.charAt(i)>='a' && str.charAt(i)<='z')

answered Mar 17, 2023 at 18:23

Arvind Rachuri

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