output 不正确（交换数字的第一位和最后一位）

Question

the output is almost correct except the first digit is not being removed. the code should swap the first and last digit of the number for example - if the input is 756 it should give 657 as output right now the code is showing 7657 the first digit is not being removed. –

package questionsOnLoops;
import java.lang.Math; 
import java.util.Scanner;

public class OSJIDS {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        
        Scanner srv = new Scanner(System.in);
        
        System.out.println("Enter any number: ");
        int n = srv.nextInt();
        
        int temp = n; //input number
        int c = 0;
        int f =n; //first digit
        int l; //last digit
        int result;
        
        while(temp>0) {
            temp = temp/10;
            c = c+1;
        }
        while(f>=10) {
            f = f/10;
        }
        g = n%10;
        result = (n/10)*10+f;
        result = (int) ((result%Math.pow(10,c))+(g*Math.pow(10,c)));
        System.out.println(result);
    }

}

Answer 1

Your code makes it quite hard to see what is exectly happening.

There are a few problems I see in your code (next to readability). You were able to detect the first and the last number, but getting to the result seems to contain some mistakes

• result = (input/10)*10+f; here you take the input (506), you divide it by 10, multiply it by 10 to abuse the the int so that your fractions are removed and restored to the original number. Then you append the first number again.

• With the calculation you did above, you removed the last number, and added the first number as the last number. You are still missing the step to add the first number. It would be easier and better to just create a new number based on your calculations

Calculating the new number

You already have most of the ingredients to calculate the new number. You already have the firstDigit and the last digit calculated. To get to the result we take the last number, and multiply it with 10 until it's in the correct position to be first. Next we append the first number, so that it's last in our new number finally, we add the inbetween numbers again. (you can calculate the inbetween numbers by removing the first and last number from the original input)

I've updated your code example with the above mentioned changes. I've also gave the variables a bit more clear names to indicate what they contain

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

    System.out.println("Enter any number: ");
    int input = srv.nextInt();        
    int temp = input;
    int length = 0;
    int firstDigit =input;
    int lastDigit;
    int inbetweenNumbers;

    while(temp>0) {
        temp = temp/10;
        length = length+1;
    }
    while(firstDigit>=10) {
        firstDigit = firstDigit/10;
    }
    lastDigit = input%10;

    inbetweenNumbers = input - lastDigit;
    inbetweenNumbers -= firstDigit * (int)Math.pow(10,length-1);

    //calculating the result
    int result = 0;
    result += lastDigit * (int)Math.pow(10,length-1);
    //Add last number (first digit of the input)
    result += firstDigit;
    //add the other numbers back
    result+= inbetweenNumbers;
    
    System.out.println(result+" "+length);
}

Answer 2

Let's use your example input, 756.

This gets rid of the last digit, 6, and instead puts the first digit there:

        result = (n/10)*10+f;

So now result is 757. Next you try to put the 6 where the first 7 is:

        k = (int) ((result%Math.pow(10,c))+(g*Math.pow(10,c)));

This isn't quite correct. c is 3, the number of digits. So Math.pow(10,c) equals 1000.0. But you needed to use 100 in those two places, first to get rid of the 7 and then to add the 6 there. Try subtracting 1 from c in the expression.

My suggestions for more readable code

• Don't declare a variable until you use it; inirialize each in the declaration.
• Use better variable names. For example:
  • input rather than n
  • length or numberOfDigits instead of c
  • firstDigit instead of f
  • lastDigit instead of g
  • lastDigitReplaced instead of result
  • finalResult instead of k

Answer 3

I do recommend making your variables a bit more descriptive, but that is an easy fix. I am guessing from your title that you want to swap the first and last digits? This method converts the numbers to strings, making appends and insertions relatively easier in this case.

import java.util.*;
import java.io.*;

class Main {
    public static void main(String[] args) {
        Scanner srv = new Scanner(System.in);
    
        System.out.println("Enter any number: ");
        int num = srv.nextInt();
        int lastDig = num;
        int firstDig = num%10;

        int len = 0;
        while(lastDig>9) {
            lastDig/=10;
            len++;
        }
    
        String ans = Integer.toString(num).substring(1, len);
        ans = firstDig+ans+lastDig;
        System.out.println(ans);
            
    }
}