how to store ascii digit 4 bits instead of the whole byte

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hello
i am trying my own implementation of a big int class using string manipulation

but since each string digit takes up 8 bits of memory i want to use 4 bits for the digit, instead of the whole byte. Thus, the number 123,456 might be represented by the bytes 12 34 56 instead of the string 123456. (Three bytes as opposed to six.)

help please ;

Posted 24-Dec-15 6:55am

Member 12223678

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Sergey Alexandrovich Kryukov 24-Dec-15 13:29pm

It's easy: back two digits in one byte. But I'm not sure your binary-decimal approach is the best for big int.
Why not using the system with the base 256, for example, in exact same way? Then you would need conversion to decimal and from decimal only when you first enter data or output as string. You never really need decimal for calculations.
—SA

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bling · Accepted Answer · 2015-12-24T07:27:00

Solution 1

What you want is BCD.

Binary-coded decimal - Wikipedia, the free encyclopedia[^]

When viewed as hexadecimal, the decimal digit pair appear like decimal.

C++

const char *text = "123456";

std::vector<unsigned char> number;

while (*text)
{
    unsigned char pair = 0;
    unsigned char digit = *text++;
    digit -= '0';
    pair = digit;
    if (*text)
    {
        digit = *text++;
        digit -= '0';
        pair <<= 4;
        pair |= digit;
    }
    number.push_back(pair);
}

for (int i = 0; i < number.size())
{
    printf("%02X", number[i]);
}

Posted 24-Dec-15 7:27am

bling

Updated 24-Dec-15 7:33am

v2

Comments

Sergey Alexandrovich Kryukov 24-Dec-15 14:01pm

Nice explanation, a 5.
—SA

Member 12223678 24-Dec-15 14:53pm

how can i implement that inside my big integer class

Philippe Mori 24-Dec-15 20:44pm

Write code, more code and a lot of code...

[no name] 24-Dec-15 15:18pm

I have no information on your big integer class.

PJ Arends · Accepted Answer · 2015-12-24T09:23:00

Solution 2

Have a look at my article An introduction to bitwise operators[^]. You can accomplish what you want using the AND, OR, and SHIFT operators.

Posted 24-Dec-15 9:23am

PJ Arends