Question

I am working on a program that needs to calculate the sum of 2 large integers without using the biginteger class in java. I am stuck on my for loop which calculates the sum. I am getting an extra 0 so 30 + 30 = 600.

I am pretty sure it is because I am looping through the arrays the wrong way. I need to go the opposite way (starting from the right side like you would when adding numbers) but I can’t seem to fix it without getting an out of array index error.

here is my code:

main:

import java.util.Scanner;

public class testLargeInteger
{



public static void main(String[] args)
    {
    Scanner input = new Scanner(System.in);
        String string1;
        String string2;
        int exp =0;


        System.out.print("Enter the first integer: ");
        //Store up the input string “string1” entered by the user from the keyboard.
        string1 = input.next(); 

        LargeInteger firstInt = new LargeInteger(string1);

        System.out.print("Enter the second integer: ");
        string2 = input.next(); 
        //Store up the input string “string2” entered by the user from the keyboard.
        LargeInteger secondInt = new LargeInteger(string2);

        System.out.print("Enter the exponential integer: ");
        //Store up the input integer “exp” entered by the user from the keyboard.
        exp = input.nextInt(); 


        LargeInteger sum = firstInt.add(secondInt);

        System.out.printf ("First integer: %s \n", firstInt.display());
        System.out.println("Second integer: " + secondInt.display());
        System.out.println(" Exponent: " + exp);

        System.out.printf (" Sum = %s \n", sum.display());

    }
}

Large integer:

public class LargeInteger {


    private int[] intArray;


    //convert the strings to array
    public LargeInteger(String s) { 
         intArray = new int[s.length()];
        for (int i = 0; i < s.length(); i++) {
            intArray[i] = Character.digit(s.charAt(i), 10); // in base 10
        }
    }

    public LargeInteger( int[] array ) { 
         intArray = array;
    }

    //display the strings
    public String display() {           
          String result="";

          for (int i = 0; i < intArray.length; i++) {     
            result += intArray[i];
          }
          return result.toString();
        }   

    //get first array
    public int[] getIntArray() {
           return intArray;
        }

    //ADD method to add 2 arrays together
    public LargeInteger add(LargeInteger secondInt){

        int[] otherValues = secondInt.getIntArray();

        int maxIterations = Math.min(intArray.length, otherValues.length);
        int currentResult; //to store result 
        int[] resultArray = new int[Math.max(intArray.length, otherValues.length) +1 ];

        int needToAdd = 0; //to store result should be added next step

        for(int i = 0; i < maxIterations; i++) {
            currentResult = intArray[i] + otherValues[i];
            resultArray[i] = currentResult % 10 + needToAdd; //if more than 9 its correct answer
            needToAdd = currentResult / 10; //this is what you need to add on next step
        }

        resultArray[Math.max(intArray.length, otherValues.length) ] = needToAdd;

        return new LargeInteger( resultArray );

    }

}

I have tried changing the for loop in sum to something like this:

for(int i = maxIterations; i >= 0; i--)

Answer 1

That for loop is only one of your problems.

1] you are not adding the carry properly.

2] a stack is more appropriate here than an array.

With a stack (place code inside your method):
Note: You are calling the function with number.add(num2);

public class LargeInt{
   private String number;
   public LargeInt(String num){
      this.number = num;
   }

  public String add(String num2){
    Stack<Integer> adder = toIntegerStack(this.number);//UPDATE
    Stack<Integer> addend = toIntegerStack(num2);//UPDATE
    Stack<Integer> result = new Stack<Integer>();

    int carry =0;
    int tmp = 0;

  while(!.adder.isEmpty && !addend.isEmpty()){
   tmp = adder.pop()+addend.pop()+carry;
   if(tmp > 10){
     carry = tmp/10;
     tmp%=10;
   }else{
     carry=0;
   }
   result.push(tmp);
  }//while

  while(!adder.isEmpty){
    tmp = adder.pop()+carry;
    if(tmp > 10){
     carry = tmp/10;
     tmp%=10;
   }else{
     carry=0;
   }
   result.push(tmp);
  }//while

  while(!addend.isEmpty){
    tmp = addend.pop()+carry;
    if(tmp > 10){
     carry = tmp/10;
     tmp%=10;
   }else{
     carry=0;
   }
   result.push(tmp);
 }//while

//beyond this point the result is your answer
//here convert your stack to string before returning
}
}

UPDATE TO ANSWER COMMENT:
I am also editing above to call this function to fill stacks.

private Stack<Integer> toIntegerStack(String n){
    Stack<Integer> stack = new Stack<Integer>();
    for(char c: n.toCharArray())
      stack.push(c-48);//ASCII
    return stack;
 }//toStack(String)

---

If you insist on using array, you must follow the same pattern with your array.

 int indexA=0;
 int indexB=0;
 int[] result = new int[1+A.length>B.length?A.length:B.length];
 int indexResult=result.length-1;

 while(indexA < A.length && indexB <B.length){//inside is same idea
    tmp = A[indexA++] + B[indexB++] + carry;
    //... do here as for stacks for tmp and carry
    result[indexResult--];
 }

 while(indexA < A.length){
    //do as in stack version
 }

  while(indexB < B.length){
    //do as in stack version
 }