CODEWITHSUNDEEP
carraysbit
user656925
user656925

Reputation:

How do I implement a bit array?

Current direction:

Start with and unsigned char which is 1 Byte on my system using sizeof. Range is 0-255. If length is the number of bits I need then elements is the number of elements (bytes) I need in my array.

constant unsigned int elements = length/8 + (length % y > 0 ? 1 : 0);  
unsigned char bit_arr[elements];

Now I add basic functionality such as set, unset, and test. Where j is the bit per byte index, i is the byte index and h = bit index. We have i = h / 8 and j = i % 8.

Psuedo-Code : 

bit_arr[i] |= (1 << j); // Set 
bit_arr[i] &= ~(1 << j);  // Unset
if( bit_arr[i] & (1 << j) ) // Test

Upvotes: 1

Views: 2132

Answers (2)

cHao
cHao

Reputation: 86505

pow() will give you floating-point values, which you don't want. At all. It might work for you, as you use powers of two, but it can get weird as j gets bigger.

You'd do a bit better to use 1 << j instead. Removes any chance of float weirdness, and it probably performs better, too.

Upvotes: 2

QuantumMechanic
QuantumMechanic

Reputation: 13946

Looks like you have a very good idea of what needs to be done. Though instead of pow(2, j), use 1 << j. You also need to change your test code. You don't want the test to do an assignment to the array.

Upvotes: 6

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