# BITCOUNT

```json metadata
{
  "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":"examples","title":"Examples"},{"id":"pattern-real-time-metrics-using-bitmaps","title":"Pattern: real-time metrics using bitmaps"},{"id":"performance-considerations","title":"Performance considerations"},{"id":"redis-enterprise-and-redis-cloud-compatibility","title":"Redis Enterprise and Redis Cloud compatibility"},{"id":"return-information","title":"Return information"}]}
}
```













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.

## Examples


SET mykey "foobar"
BITCOUNT mykey
BITCOUNT mykey 0 0
BITCOUNT mykey 1 1
BITCOUNT mykey 1 1 BYTE
BITCOUNT mykey 5 30 BIT


## 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][hbgc212fermurb]".

[hbgc212fermurb]: 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 Enterprise and Redis Cloud compatibility

| Redis<br />Enterprise | Redis<br />Cloud | <span style="min-width: 9em; display: table-cell">Notes</span> |
|:----------------------|:-----------------|:------|
| <span title="Supported">&#x2705; Standard</span><br /><span title="Supported"><nobr>&#x2705; Active-Active</nobr></span> | <span title="Supported">&#x2705; Standard</span><br /><span title="Supported"><nobr>&#x2705; Active-Active</nobr></span> |  |

## Return information

{{< multitabs id="bitcount-return-info" 
    tab1="RESP2" 
    tab2="RESP3" >}}

[Integer reply](../../develop/reference/protocol-spec#integers): the number of bits set to 1.

-tab-sep-

[Integer reply](../../develop/reference/protocol-spec#integers): the number of bits set to 1.