How can I swap the first and last digits of a number in Java using for for-loops?-CodePudding

This is my code (which is not working) without using loops.

System.out.println("Enter a five digit number: ");
int number = scanner.nextInt();

int lastDigit = number % 10;            //by using the % operator, we are able to retrieve the last digit of the number

int firstDigit = number;
while(firstDigit > 9) {
    firstDigit /= 10;
}
//======================================================//
System.out.println("The first digit of the number is "   firstDigit);
System.out.println("The last digit of the number is "   lastDigit);

String string = Integer.toString(number);

char first = string.charAt(0);
char last = string.charAt(string.length() - 1);

string.replace(first, last);
string.replace(last, first);

System.out.println(string);

CodePudding user response：

You can actually skip all the loops here and use strings.

Like this:

public static int swapNr(int nr)
{
    String nrStr = String.valueOf(nr); // Turning our nr into a String

    char[] charArr = nrStr.toCharArray(); // Getting the char Array out of it

    char firstNr = charArr[0]; // Getting the first character
    char lastNr = charArr[charArr.length - 1]; // Getting the last one
    
    // Changing their places
    charArr[0] = lastNr;
    charArr[charArr.length - 1] = firstNr;
    
    return Integer.parseInt(String.valueOf(charArr)); // Turning the String into an Integer again
}

String.replace() would not be the correct way to change the place of a specific character in a String. And you must know that Strings are immutable, that means that when you call .replace(), the function returns a new String and does not update the current one.

CodePudding user response：

You can do this completely without loops, using only arithmetic:

    public static int swap(int v) {
        final int lastDigit = v % 10;
        // the number of digits - 1 (1 digit = 0, 2 digits = 1, ...)
        final int digitsMinusOne = (int) Math.floor(Math.log10(v));
        // the multiplier used for getting/setting the first digit
        // examples:
        // 1234, digitsMinusOne=3, multiplier = 10^3 = 1000
        // 827382, digitsMinusOne=5, multiplier = 10^5 = 100000
        final int multiplier = (int) Math.pow(10, digitsMinusOne);
        // the first digit can be retrieved by dividing by the multiplier
        // for example:
        // 1234 / 1000 = 1
        // 827382 / 100000 = 8
        final int firstDigit = v / multiplier;
        

        // replacing the last digit with the first digit
        v = (v - (v % 10))   firstDigit;
        // replacing the first digit with the last digit
        v = (v - (firstDigit * multiplier))   (lastDigit * multiplier);

        return v;
    }

CodePudding user response：

It's easier not to use for-loops.

static int swapFirstAndLastDigit(int n) {
    return Integer.parseInt((""   n)
        .replaceFirst("^(-)?(.)(.*)(.)$", "$1$4$3$2"));
}

public static void main(String[] args) {
    System.out.println(swapFirstAndLastDigit(12345));
    System.out.println(swapFirstAndLastDigit(-54321));
}

output:

52341
-14325