I want to write a program which reverses all the integers entered in an array

Question

I want to write a program which reverses all the integers entered in an array but my code displayed here isn't working properly. Here is my code:

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

class GFG {
    public static void main (String[] args) throws Exception {
        //code
          Scanner ss=new Scanner(System.in);
       int[] arr = new int[31];
       int T=ss.nextInt();
       int rem,p=0;

       for(int i=1;i<=T;i++){
           int a=ss.nextInt();
           if(a<=1000000000){
              while(a!=0){
                  rem=a%10;
                  p=p*10+rem;
                  a=a/10;                     
             }
           System.out.println(p);
         } else
           System.out.println("wrong input");
       }
    }
}

input:

2
56
78

expected output:

65
87

actual output:

65
6578

What is wrong?

Answer 1

Possible solution could be

if(a<=1000000000){
               p=0; // reinitialize p
              while(a!=0){
                  rem=a%10;
                  p=p*10+rem;
                  a=a/10;

              }

For better understanding just add this line System.out.println("P is :"+ p); in if and you will see why p=0; was required.

if(a<=1000000000){
               System.out.println("P is :"+ p);
              while(a!=0){
                  rem=a%10;
                  p=p*10+rem;
                  a=a/10;
}

OutPut:

2
65
P is :0
56
78
P is :56
5687

Answer 2

Probably you are just learning how to do things; thus you want to solve this problem "mathematically", but there is a different perspective in here.

It seems that you simply want to reverse the digits within any number; and you treat this as mathematical problem. But one can see this as string manipulation problem; which allows you to use existing functions to solve the problem:

int originalNumber = ....
String originalNumberAsString = Integer.toString(originalNumber);
String reversedNumberAsString = new StringBuilder(originalNumberAsString).reverse().toString();
int reversedNumber = Integer.parseInt(reversedNumberAsString);

Please note: there is also no need to check for the size of your input here; but you probably should check for >=0 (or do what is appropriate in your case for negative numbers).

Answer 3

There are couple of mistakes in your code (arguably), if reviewed by peer. You are trying to print p, which is not being re-initialized. Lot of unused variables in your code are removed in this answer.

• You just declared an array, but never used in your program. Not sure whether you wanted to store the user inputs in the array and once all the inputs are received, you might want to compute the reverse and display or compute the reverse and store them in your array directly. So, I just removed it
• The check a <= 1000000000 I believe is to ensure that values entered are within int range. Java provides you a constant for this in Integer wrapper class, Integer.MAX_VALUE (which is 2^31 - 1)
• The code was lacking modularity. Hence, I extracted it to the method reverse(int num) which takes care of computation

• Importing all classes from the package is not recommended, rather import only those classes that are used in your code.

  import java.util.Scanner;

  public class GFG {

  public static void main(String[] args) throws Exception {
      Scanner ss = new Scanner(System.in);
      int T = ss.nextInt();
  
      for (int i = 1; i <= T; i++) {
          int a = ss.nextInt();
  
          if (a <= Integer.MAX_VALUE) {
              a = reverse(a);
              System.out.println(a);
          } else {
              System.out.println("wrong input");
          }
      }
  }
  
  private static int reverse(int num) {
      int reverse = 0;
      while( num != 0 ) {
          reverse = reverse * 10;
          reverse = reverse + num % 10;
          num = num/10;
      }
      return reverse;
  }
  

  }

On a side note, if you look at GhostCat's answer of converting the number to String and reversing it, the approach fails for negative numbers.

Example: When user input is -51, the output would be 15-

However, it is common practice in industry to store certain long values as String. One such example is credit/debit card numbers