C++ - Modulo of an Unsigned Integer

Question

Working solution (in-case anyone is doing this or something like it :P) This isn't the most optimized solution, but it does work for 32 bits i'll post a more optimized solution that uses bigNum's for each value instead of unsigned int, because who knows you might have a value of (2^128)*(2^32) :P

void print(){
    //Hold temporary backwards string array
    std::string backwards;
    //Copy of data and remainder to hold previous loop's value
    unsigned int topCopy = topValue, remainder = 0;
    //Loop through each digit of highest 32 bit number
    for(int i = 0; topCopy != 0; i++){
        //Find value of all added max values
        unsigned int value = maxValues*maxDigits[i];
        //Find value of topValue's last digit
        value += topCopy % 10;
        //Add in the remainder from the previous loop
        value += remainder;
        //Create remainder so the printing value is correct
        remainder = value / 10;
        //append the value to the string
        backwards += value % 10;
        //setup for the next digit
        topCopy /= 10;
    }
    //append the last digit
    backwards += remainder;
    //print backwards
    for(int i = backwards.length()-1; i >= 0; i--){
        printf("%i", backwards[i]);
    }
    printf("\n");
}

I am attempting to create a class that will handle arbitrary length unsigned integer values in C++, before anyone says it, I am fully aware that there are already pre-existing libraries that handle this functionality. This is a purely learning experience for me.

Implementation Notes:

I am storing the values in my class in this format:

topValue: stores the remainder of the number % 4294967296  
maxValues: stores how many max values are needed to hold the number  

Example:

maxValues = 4  
topValue = 2820130816  
maxValues *= 4294967296
maxValues == 17179869184
maxValues + topValue == 20000000000  

The Issue:

The issue arises when I am attempting to print the number, I've already implemented method's for addition subtraction etc... When deciding what to print for each digit of the number I am doing this:

1. Take digit of comparison starting with the end (6) 4294967296
2. Add this to topValue % 10 (topValue's end number)

3. Print value and divide the remaining topValue by 10 to get at the next value

    const char maxDigits[] = {6, 9, 2, 7, 6, 9, 4, 9, 2, 4}; void print(){ int topCopy = topValue; for(int i = 0; topCopy != 0; i++){ int value = maxValues*maxDigits[i]; value += topCopy % 10; // RIGHT HERE IS THE ISSUE value %= 10; //print value topCopy /= 10; } } 

When doing the line topCopy % 10 on this unsigned value it gives the answer like it's a signed value and gives me a negative answer that is incorrect, what I need is something that can extract the last digit of the unsigned value.

2,820,130,816 % 10 should be (for my usage) 6 but output is 0 .

TL;DR:

I need a operation that will give me 6 from 2,820,130,816 % 10 instead of 0.

Thanks!

Answer 1

The result of the modulo operation depend on the types of the operand. Since your first operand is an int , and because 2,820,130,816 is greater than the max value that can be stored in a 32-bit int , you get a wrong result.

Changing the type of topCopy to unsigned will fix this problem, and give you back a 6.

Demo on ideone.

Answer 2

Since you are working with some large numbers you might want to use a larger integer type, such as unsigned long long. You are overflowing the range of a 32-bit int.