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BITCOUNT

Counts the number of set bits (1s) in the string stored at a key within the specified range.

Syntax​

BITCOUNT key [start end]

Parameters​

  • key: The name of the key containing the string to count bits in.
  • start: Optional. The starting byte index for counting. The index is inclusive and can be negative to indicate an offset from the end of the string.
  • end: Optional. The ending byte index for counting. The index is inclusive and can be negative to indicate an offset from the end of the string.

Details​

  • Available since: 2.6.0
  • Time complexity: O(N) where N is the length of the string.
  • ACL categories: @read, @bitmap, @slow

The BITCOUNT command counts the number of bits set to 1 in the string stored at key. By default, it counts the bits in the entire string. If start and end parameters are provided, it counts the bits only within the specified byte range. The range is inclusive, meaning that both the start and end bytes are included in the count.

Byte Range Indexing​

  • 0: The first byte in the string.
  • -1: The last byte in the string.
  • -2: The second last byte, and so on.

Examples​

Counting Bits in the Entire String​

Assume we have a key called mykey with a binary value:

SETBIT mykey 0 1
SETBIT mykey 1 1
SETBIT mykey 2 0
SETBIT mykey 3 1

To count the number of set bits in the entire string:

BITCOUNT mykey

This will return:

(integer) 3

Counting Bits in a Specific Range​

To count the number of set bits between the first and second bytes:

BITCOUNT mykey 0 1

This will return:

(integer) 2

Handling Non-Existent Key​

If the key does not exist:

BITCOUNT nonexistingkey

This will return:

(integer) 0

Edge Cases​

  • If the key does not exist, BITCOUNT returns 0.
  • If the start and end parameters are out of range, BITCOUNT returns 0.
  • The start and end parameters are inclusive.
  • If the key exists but is not a string, an error is returned.

RESP2/RESP3 Reply​

  • Integer reply: the number of bits set to 1 in the specified range.