# BITCOUNT ```json metadata { "schema_version": 2, "title": "BITCOUNT", "description": "Counts the number of set bits (population counting) in a string.", "categories": ["docs","develop","stack","oss","rs","rc","oss","kubernetes","clients"], "arguments": [{"display_text":"key","key_spec_index":0,"name":"key","type":"key"},{"arguments":[{"display_text":"start","name":"start","type":"integer"},{"display_text":"end","name":"end","type":"integer"},{"arguments":[{"display_text":"byte","name":"byte","token":"BYTE","type":"pure-token"},{"display_text":"bit","name":"bit","token":"BIT","type":"pure-token"}],"name":"unit","optional":true,"since":"7.0.0","type":"oneof"}],"name":"range","optional":true,"type":"block"}], "syntax_fmt": "BITCOUNT key [start end [BYTE | BIT]]", "complexity": "O(N)", "group": "bitmap", "command_flags": ["readonly"], "acl_categories": ["@read","@bitmap","@slow"], "since": "2.6.0", "arity": -2, "key_specs": [{"RO":true,"access":true,"begin_search":{"spec":{"index":1},"type":"index"},"find_keys":{"spec":{"keystep":1,"lastkey":0,"limit":0},"type":"range"}}], "tableOfContents": {"sections":[{"id":"required-arguments","title":"Required arguments"},{"id":"optional-arguments","title":"Optional arguments"},{"id":"examples","title":"Examples"},{"children":[{"id":"pattern-real-time-metrics-using-bitmaps","title":"Pattern: real-time metrics using bitmaps"},{"id":"performance-considerations","title":"Performance considerations"}],"id":"details","title":"Details"},{"id":"redis-software-and-redis-cloud-compatibility","title":"Redis Software and Redis Cloud compatibility"},{"id":"return-information","title":"Return information"}]} , "codeExamples": [] } ``` Count the number of set bits (population counting) in a string. By default all the bytes contained in the string are examined. It is possible to specify the counting operation only in an interval passing the additional arguments _start_ and _end_. Like for the [`GETRANGE`](https://redis.io/docs/latest/commands/getrange) command start and end can contain negative values in order to index bytes starting from the end of the string, where -1 is the last byte, -2 is the penultimate, and so forth. Non-existent keys are treated as empty strings, so the command will return zero. By default, the additional arguments _start_ and _end_ specify a byte index. We can use an additional argument `BIT` to specify a bit index. So 0 is the first bit, 1 is the second bit, and so forth. For negative values, -1 is the last bit, -2 is the penultimate, and so forth. ## Required arguments
key The name of the key that holds the string.
## Optional arguments
start end Count set bits only within the range from `start` to `end` (inclusive). By default these are byte offsets; negative values count from the end of the string.
BYTE | BIT Interpret `start` and `end` as byte offsets (`BYTE`, the default) or bit offsets (`BIT`).
## Examples redis> SET mykey "foobar" OK redis> BITCOUNT mykey (integer) 26 redis> BITCOUNT mykey 0 0 (integer) 4 redis> BITCOUNT mykey 1 1 (integer) 6 redis> BITCOUNT mykey 1 1 BYTE (integer) 6 redis> BITCOUNT mykey 5 30 BIT (integer) 17 ## Details ### Pattern: real-time metrics using bitmaps Bitmaps are a very space-efficient representation of certain kinds of information. One example is a Web application that needs the history of user visits, so that for instance it is possible to determine what users are good targets of beta features. Using the [`SETBIT`](https://redis.io/docs/latest/commands/setbit) command this is trivial to accomplish, identifying every day with a small progressive integer. For instance day 0 is the first day the application was put online, day 1 the next day, and so forth. Every time a user performs a page view, the application can register that in the current day the user visited the web site using the [`SETBIT`](https://redis.io/docs/latest/commands/setbit) command setting the bit corresponding to the current day. Later it will be trivial to know the number of single days the user visited the web site simply calling the `BITCOUNT` command against the bitmap. A similar pattern where user IDs are used instead of days is described in the article called "[Fast easy realtime metrics using Redis bitmaps](http://blog.getspool.com/2011/11/29/fast-easy-realtime-metrics-using-redis-bitmaps)". ### Performance considerations In the above example of counting days, even after 10 years the application is online we still have just `365*10` bits of data per user, that is just 456 bytes per user. With this amount of data `BITCOUNT` is still as fast as any other O(1) Redis command like [`GET`](https://redis.io/docs/latest/commands/get) or [`INCR`](https://redis.io/docs/latest/commands/incr). When the bitmap is big, there are two alternatives: * Taking a separated key that is incremented every time the bitmap is modified. This can be very efficient and atomic using a small Redis Lua script. * Running the bitmap incrementally using the `BITCOUNT` _start_ and _end_ optional parameters, accumulating the results client-side, and optionally caching the result into a key. ## Redis Software and Redis Cloud compatibility | Redis
Software | Redis
Cloud | Notes | |:----------------------|:-----------------|:------| | ✅ Standard
✅ Active-Active | ✅ Standard
✅ Active-Active | | ## Return information **RESP2:** [Integer reply](../../develop/reference/protocol-spec#integers): the number of bits set to 1. **RESP3:** [Integer reply](../../develop/reference/protocol-spec#integers): the number of bits set to 1.