How Can I Efficiently Merge and Decode Eight Boolean Values into a Single Byte?

Merging and Decoding Boolean Values into Bytes

Introduction

Often, programmers encounter the need to represent boolean values in a compact form, such as a single byte. This article explores two approaches to merging and decoding boolean values into and from bytes.

Merging Boolean Values

To merge eight boolean values into a byte, one can use the following approach:

unsigned char ToByte(bool b[8])
{
    unsigned char c = 0;
    for (int i = 0; i < 8; i++)
        if (b[i])
            c |= 1 << i;
    return c;
}

This code iterates over the boolean values, setting the corresponding bits in the byte using bitwise OR operations.

Decoding Bytes

To decode a byte into eight separate boolean values, one can use this function:

void FromByte(unsigned char c, bool b[8])
{
    for (int i = 0; i < 8; i++)
        b[i] = (c & (1 << i)) != 0;
}

This function iterates over the bits in the byte, checking if each bit is set and assigning the corresponding boolean value.

Cool Alternative: Union

Alternatively, one can use a union to represent both boolean values and a byte:

struct Bits
{
    unsigned b0:1, b1:1, b2:1, b3:1, b4:1, b5:1, b6:1, b7:1;
};

union CBits
{
    Bits bits;
    unsigned char byte;
};

By writing to one member of the union and reading from the other, one can merge and decode boolean values efficiently. However, it's important to note that the order of bits in the Bits structure is implementation-defined.

Implementation Notes

It's crucial to note that reading one member of a union after writing to another is well-defined in ISO C99 and as an extension in major C compilers. However, it's Undefined Behaviour in ISO C . To ensure portability, it's recommended to use memcpy or C 20 std::bit_cast for type-punning in C .

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