CODEWITHSUNDEEP
javainteger
Semicolon
Semicolon

Reputation: 33

sum of first and last digit in the integer

The method written in the below code needs to take integer and result sum of 1st digit and last digit in the integer.

NOTE: The reason i am asking this question though i got to know the correct solution is i need to understand why my code is not working also as that makes me a better programmer please help.

public class FirstLastDigitSum {
    public static int sumFirstAndLastDigit(int number)//to add first and last 
//digits of a given interger
    {
        int firstdigit=0;
        int lastdigit=0;

        if(number>=0 && number<=9)
        {
            return number+number;
        }

        else if(number>9)
        {
            lastdigit=number%10;
            while(number>0)
            {
                number/=10;
                if(number<=9 & number>=0){
                    firstdigit=number;
                }

            }
            return firstdigit+lastdigit;
        }
        else
        return -1;
    }
    public static void main(String[] args)
    {
        System.out.println(sumFirstAndLastDigit(121));
    }
}

In the above code if i keep number/=10 after if block like below

if(number<=9 & number>=0){
                        firstdigit=number;
                    }
number/=10;

then my code is giving proper results. like if i input 121 to the method as first digit is 1 and second digit is 1 it is summing both and giving me result 2. which is absolutely correct

But if keep number/=10 above the if block like below

 number/=10;
    if(number<=9 & number>=0){
                            firstdigit=number;
                        }

Then my code is not giving proper result it is giving only last number which is 1.

I am not at all understanding why this is happening can any one explain please.

Upvotes: 1

Views: 6917

Answers (4)

Ramil Mammadov
Ramil Mammadov

Reputation: 532

If the number is less than 0 then return -1. Otherwise the last number can be found by modulus 10, and the first number found by dividing by 10 until that number is less than 10.

public static int sumFirstAndLastDigit(int number) {
    if (number < 0) {
        return -1;
    }

    int last = number % 10;
    int first = number;
    
    while (first >= 10) {
        first = first / 10;
    }

    return first + last;
}

Upvotes: 2

elbraulio
elbraulio

Reputation: 994

In this case:

while(number>0)
{
    number/=10;
    if(number<=9 & number>=0){
        firstdigit=number;
    }
}

Note that you change number after you check number > 0. That means that number can be 0 when reaches the if statement and if it is 0 it will be accepted as first digit because it satisfies the condition number<=9 & number>=0. As an example, when you test it with 121 you have this values for number 121, 12, 1 and 0. The last one satisfies the if statement and set first digit to 0.

In this case:

while(number>0)
{
    if(number<=9 & number>=0){
        firstdigit=number;
    }
    number/=10;
}

You change number before you check number > 0. That means that the if statement will never have a number = 0 and firstdigit will never be 0. As an example, when you test it with 121 you have this values for number 121, 12, and 1. The last one satisfies the if statement and set first digit to 1. After that you divide number by zero and it becomes 0 and it stop the while loop.

Upvotes: 0

flakes
flakes

Reputation: 23584

Lets break this up into two parts. Get the last digit of the number, and getting the first digit of the number.

The first part of the problem is easy. Its exactly what you already have; just take modulo 10 of the number. 

int lastdigit = number % 10;

The second part is a little trickier, but not too much. While the number is greater than 10 divide it by 10 (using integers you will have truncation for the remainder). One problem I see in your solution is that you keep checking the value even after a digit is discovered. That means if the value was 1234, you correctly find 1, but then overwrite it to 0 with an extra loop iteration 

int firstdigit = number;
while (firstdigit >= 10) {
    firstdigit /= 10;
}

And that's it, you are done. Just return the value.

return firstdigit + lastdigit;

Upvotes: 3

Tim Biegeleisen
Tim Biegeleisen

Reputation: 520878

This is not a direct solution to your problem, but we can fairly easily handle this using regex and the base methods:

int number = 12345678;
String val = String.valueOf(number);
val = val.replaceAll("(?<=\\d)\\d+(?=\\d)", "");
number = Integer.parseInt(val);
int sum = number % 10 + number / 10;
System.out.println(sum);

Upvotes: -1

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