package primecdesc;

import java.util.Scanner;

public class Primecdesc {


    public static void main(String[] args) {


Scanner myScanner = new Scanner (System.in);
       int a,b=0,ctr=0, ktr=1;
       System.out.print("Enter a number: ");
       a = myScanner.nextInt();
       while(b>=1001)
       {
           if (a%b==0)
           b++;
           ctr--;
       }
       /*for(b=1;b<=1001;b++)
       {
           if(a%b==0)
               ctr--;
       }*/
       if(ctr==1)
       {
           System.out.println(a + " is a prime number!!!");
           System.out.print("Factor of " + a + " is itself only!!!");
           System.out.println(" ");
           for (a=1; ktr<11; ktr++)
           {
               if(a%2==1 && a%3!=0 && a%25!=0 || a==1 || a==2 || a==3){
                   System.out.print(a + " ");
           }
               a--;
               if (a>1) {break;}
            }
       }
            /*while(ktr<11){
                   if(a%2==1 && a%3!=0 && a%25!=0 || a==1 || a==2 || a==3){
                   System.out.print(a + " ");
                   ktr++;
               }*/
               
       
       else if(ctr>=3)
       {
           System.out.println(a + " is a composite number!!!");
           System.out.print("Factor of " + a + " are ");
           while (b<=a)
           {
             if(a%b==0)
                   if(b==1)
                       System.out.print(b + " ");
                   else
                       System.out.print(b + " ");
             b++;
           }
           /*for(b=1;b<=a;b++)
           {
               if(a%b==0)
                   if(b==1)
                       System.out.print(b + " ");
                   else
                       System.out.print(b + " ");
           }*/
           System.out.print("!!!");
           System.out.println(" ");
           for (a=1; ktr<11; ktr++)
           {
               if(a%2==0 || a%3==0 || a%5==0)
               {
                   System.out.print(a + " ");
                   ktr++;
               }
               a--;
               if (a>4) {break;}
           }
       }
           /*while(ktr<11){
               if(a%2==0 || a%3==0 || a%5==0){
                   System.out.print(a + " ");
                   ktr++;
               }
               a--;
               if (a>4) {break;}
        }*/
       
        else
           {
               System.out.println(a + " is a prime number!!!");
               System.out.print("Factors of " + a + " are 1 and itself only!!!");
               System.out.println(" ");
               [B]for (a=1; ktr<11; ktr++)
                {
                   if(a%3!=0 && a%5!=0 && a%4!=0 && a%2!=0 && a%7!=0 || a==2 || a==3 || a==5)
                   {
                   System.out.print(a + " "); 
                   }
               
               a--;
               if (a<1){break;}
                } 
           }[/B]
               /*while(ktr<11){
                   if(a%3!=0 && a%5!=0 && a%4!=0 && a%2!=0 && a%7!=0 || a==2 || a==3 || a==5){
                   System.out.print(a + " ");
                   ktr++;
               }
               a--;
               if (a<1){break;}
           }
           }*/
           
           System.out.print("!!!");
           System.out.println(" ");
           System.out.println("Have a nice day!!!");
        
    }
 }

On the bold part is my problem. When my loop is in while form, it runs perfectly, but when i try to change it to "for loop", it displays a wrong output.

Your FOR loops are completely wrong

for (a=1; ktr<11; ktr++)            // you start A at 1 but use KTR as the loop index
    {
        if(a%2==0 || a%3==0 || a%5==0)
        {
            System.out.print(a + " ");
            ktr++;                      // you already do this in the FOR loop
        }
        a--;                            // you decrement A and use it to exit 
        if (a>4) {break;}               //    the loop but that's what the KTR<11 
                                        //    is supposed to do in the FOR loop
                                        //    Also, you start A at 1 and decrement, 
                                        //    so how does it get to 4?
     }

The other loop (the bolded one)

for (a=1; ktr<11; ktr++)    // you start A at 1 but use KTR as the loop index
{                           // You never use initialize nor use KTR
    if(a%3!=0 && a%5!=0 && a%4!=0 && a%2!=0 && a%7!=0 || a==2 || a==3 || a==5)
    {
        System.out.print(a + " "); 
    }
               
    a--;                // you decrement A and use it to exit 
    if (a<1){break;}    //    the loop but that's what the KTR<11 
}                       //    is supposed to do in the FOR loop
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