# ZRANGEBYSCORE

## Syntax

`ZRANGEBYSCORE key min max [WITHSCORES] [LIMIT offset count]`

**Time complexity:** O(log(N)+M) with N being the number of elements in the sorted set and M the number of elements being returned. If M is constant (e.g. always asking for the first 10 elements with LIMIT), you can consider it O(log(N)).

Returns all the elements in the sorted set at `key`

with a score between `min`

and `max`

(including elements with score equal to `min`

or `max`

).
The elements are considered to be ordered from low to high scores.

The elements having the same score are returned in lexicographical order (this follows from a property of the sorted set implementation in Redis and does not involve further computation).

The optional `LIMIT`

argument can be used to only get a range of the matching
elements (similar to *SELECT LIMIT offset, count* in SQL). A negative `count`

returns all elements from the `offset`

.
Keep in mind that if `offset`

is large, the sorted set needs to be traversed for
`offset`

elements before getting to the elements to return, which can add up to
O(N) time complexity.

The optional `WITHSCORES`

argument makes the command return both the element and
its score, instead of the element alone.
This option is available since Redis 2.0.

## Exclusive intervals and infinity

`min`

and `max`

can be `-inf`

and `+inf`

, so that you are not required to know
the highest or lowest score in the sorted set to get all elements from or up to
a certain score.

By default, the interval specified by `min`

and `max`

is closed (inclusive).
It is possible to specify an open interval (exclusive) by prefixing the score
with the character `(`

.
For example:

`ZRANGEBYSCORE zset (1 5`

Will return all elements with `1 < score <= 5`

while:

`ZRANGEBYSCORE zset (5 (10`

Will return all the elements with `5 < score < 10`

(5 and 10 excluded).

## Return

Array reply: list of elements in the specified score range (optionally with their scores).

## Examples

`dragonfly> ZADD myzset 1 "one"`

(integer) 1

dragonfly> ZADD myzset 2 "two"

(integer) 1

dragonfly> ZADD myzset 3 "three"

(integer) 1

dragonfly> ZRANGEBYSCORE myzset -inf +inf

1) "one"

2) "two"

3) "three"

dragonfly> ZRANGEBYSCORE myzset 1 2

1) "one"

2) "two"

dragonfly> ZRANGEBYSCORE myzset (1 2

1) "two"

dragonfly> ZRANGEBYSCORE myzset (1 (2

## Pattern: weighted random selection of an element

Normally `ZRANGEBYSCORE`

is simply used in order to get range of items
where the score is the indexed integer key, however it is possible to do less
obvious things with the command.

For example a common problem when implementing Markov chains and other algorithms is to select an element at random from a set, but different elements may have different weights that change how likely it is they are picked.

This is how we use this command in order to mount such an algorithm:

Imagine you have elements A, B and C with weights 1, 2 and 3. You compute the sum of the weights, which is 1+2+3 = 6

At this point you add all the elements into a sorted set using this algorithm:

`SUM = ELEMENTS.TOTAL_WEIGHT // 6 in this case.`

SCORE = 0

FOREACH ELE in ELEMENTS

SCORE += ELE.weight / SUM

ZADD KEY SCORE ELE

END

This means that you set:

`A to score 0.16`

B to score .5

C to score 1

Since this involves approximations, in order to avoid C is set to, like, 0.998 instead of 1, we just modify the above algorithm to make sure the last score is 1 (left as an exercise for the reader...).

At this point, each time you want to get a weighted random element,
just compute a random number between 0 and 1 (which is like calling
`rand()`

in most languages), so you can just do:

`RANDOM_ELE = ZRANGEBYSCORE key RAND() +inf LIMIT 0 1`