if..else if..else loop not executing properly

Question

I'm writing a program that asks the user to input a three digit number then adds each number by 6 with a modulus of 10.

For example if I enter 619, the output I shall receive is 275.

My only problem is when I enter 100, I receive 1360, instead of 766 like I'm supposed to.

Here is what I have so far:

int main()
{
    //declaring variables
    int numbers, newNumbers, input;
    int newInput = 0;

    //User input
    printf("Enter a three-digit number: ");

    //reads input (%d) and stores it as "input" (&input)
    scanf("%d", &input);

    //check if input is 3 digits. If not, print message
    if(input < 100 || input > 999)
    {
        printf("Invalid input, the number must be between 100 and 999 \n");
        return 0;
    }

    //loops program until input is 0
    while(input > 0)
    {
        //modulus 10
        numbers = input % 10;
        //adds 6 to input. Modulus 10
        newNumbers = (numbers + 6) % 10;

        //if input > 10, calculates new input
        if(input > 100)
            newInput = newNumbers;
        else if(input > 10)
            newInput = (newNumbers * 10) + newInput;
        else
            newInput = (newNumbers * 100) + newInput;

        input /= 10;
    }
//prints result
printf("Output: %d \n", newInput);
return 0;

}

Answer 1

In your code, by saying

if(input > 100)
        newInput = newNumbers;
    else if(input > 10)
        newInput = (newNumbers * 10) + newInput;

you're not taking into account the numbers 100 and 10 themselves in the TRUE condition, whereas you should be counting them, too. You need to change the if condition to use >=, like

if(input >= 100)
        newInput = newNumbers;
    else if(input >= 10)
        newInput = (newNumbers * 10) + newInput;

Answer 2

Hi an easier solution is just this one:

output = (input + 666) % 1000; //Add 6 to all numbers
if(input % 10 > 3)             //Correct units carry
    output-=10;
if(input % 100 > 30)           //Correct tents carry 
    output-= 100;

It works and is easy scalable :)