Extracting bits from uint8_t type array

Question

I have the data: ef324ad13255e219e8110044997cefaa43ff0954800000000000007 stored in an uint8_t type array called lfsr[36].

I want to extract specific bits from the array, e.g. bit no. 96, bit no. 184 etc.

How can I perform this operation?

Answer 1

As noted by barak manos, the proper code is

(lfsr[bit / 8] >> (bit % 8)) & 1

To explain it:

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bit / 8 chooses an element from your array. Each element contains 8 bits, so dividing by 8 is an easy way to convert a bit index to an element index.

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bit % 8 chooses a bit inside the element. This is most straightforward choice of indexing; it counts bits from the least significant bit to most significant bit (little-endian). Another variant is

7 - bit % 8

This variant counts the bits in reverse order (big-endian). Sometimes you have to use it (e.g. in JPEG) for compatibility reasons; if you are free to decide which bit order to choose, use little-endian (because it's easier).

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The syntax (... >> ...) & 1 extracts one bit from a number. See here for details.