in c++ how do you set the first 4 bits in a byte

[Adversus]

I have two integer alpha values and I want to set the first four bits of a byte to one and the second 4 bits of the byte to the second alpha.

I think I need to use bitwise operators but can't figure it out. Does anyone else know how?

[greenya]

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            int varResult = 0;
            int varWith4bitsHI = 0xa;
            int varWith4bitsLO = 0xb;
 
            varResult = varWith4bitsHI << 4 | varWith4bitsLO;
            // now varResult == 0xab

[hendu]

value = secondalpha;
value |= 15;

@greenya
0xb would be 11 in decimal or 1011 in binary, not all four 1111 = 15 = 0xf

[Adversus]

thanks, what would be the functions for adding and getting them individually?

[greenya]

@greenya
0xb would be 11 in decimal or 1011 in binary, not all four 1111 = 15 = 0xf

Didn't understand you.
4 bits is 1111 and value range is 0-15 inclusive.
8 bits makes byte.
In my post i showed how individually set first 4 bits and last 4 bits.

thanks, what would be the functions for adding and getting them individually?

Refering to my code above, you can do that next way:

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            int varResult11110000 = (varResult & 0xf0) >> 4; // now its 0xa
            int varResult00001111 = varResult & 0xf; // now its 0xb

[hendu]

Yep, but he asked for all 4 low bits set

[Mel]

the bitwise operations that may help are the shift left and shift right ops. But i never remember how they work exactly, and leads me to errors sometimes, the good thing is that both have an equivalent in multiplying or dividing a number by the power of 2 you want to shift the bits, if you want to store your alpha values, this would work pretty well:

combinedAlpha = alpha1*16 + alpha2;

if you want to recover your original alphas:

alpha1 = combinedAlpha/16;
alpha2 = combinedAlpha%16;

and you are done. With integer values they are as fast as the bitwise operations. Just keep in mind that your combined alpha should be treated as an unsigned char.

[zerochen]

or u use something like this

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    void setBit(int& val, int bitToSet)
    {
        val |=  1<<bitToSet;
    }
 
    bool isBitSet(int& val, int bitToTest)
    {
        return (1<<bitToTest) == (val &&  1<<bitToTest);
    }
 

[Adversus]

Thanks, bit help. I think I even understand what that does