# Ranking expressions A ranking expression computes a score for each activity, and the feed is ordered by that score, highest first. This page is the reference for what an `expression` or `interest` ranking can use: activity fields, data types, functions, operators, constants and decay functions. For how to configure ranking on a feed group or feed view, see [Ranking](https://getstream.io/activity-feeds/docs/dotnet-csharp/custom-ranking/). ## Activity data It's not possible to access all fields of an `ActivityResponse` inside the ranking context. Below you'll find the list of supported fields: | name | description | | ------------------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `type` | The type of the activity (e.g. the content type string set when creating the activity) | | `time` | UNIX timestamp of when the activity was created | | `popularity` | The popularity score of an activity (formula: `activity.popularity = reactions + comments * 2 + bookmarks * 3 + shares * 3`) | | `reaction_count` | The sum of all reactions | | `reaction_counts` | Reaction counts by type, you can access specific types with dot notation (for example, `reaction_counts.like`) | | `comment_count` | How many comments the activity has | | `bookmark_count` | How many bookmarks the activity has | | `share_count` | How many shares the activity has (Increased when an activity is set as the `parent_id` of another activity) | | `custom` | The `custom` field of the `activity`, nested fields can be accessed with `.` notation, for example: `custom.topic` | | `selector_source` | Which activity selector provided this activity (e.g., `"following"`, `"popular"`, `"interest"`). See [Ranking by Selector Source](#ranking-by-selector-source) for usage examples. | | `interest_score` | Only available when using `interest` type ranking. A number between -1 and 1 reflecting how much a given activity matches the user's interests. See [Interest weights](https://getstream.io/activity-feeds/docs/dotnet-csharp/interest-ranking/) to see how a user's interests are computed/configured. | | `preference_score` | Available when using `expression` or `interest` type ranking. A number reflecting how much a given activity matches the user's preferences based on activity feedback (show more/less). Activities with `interest_tags` that have strong dislike are filtered out. See [Activity feedback](https://getstream.io/activity-feeds/docs/dotnet-csharp/activity-feedback/) for more information. | | `is_read` | Boolean. Available when `track_read` is enabled on a custom feed group or the built-in `notification` group (including flat). `true` if the activity (or its aggregated group) has been marked as read. Uses time-based invalidation. **Enterprise plan only.** See [Ranking by Read/Seen Status](https://getstream.io/activity-feeds/docs/dotnet-csharp/read-seen-ranking/). | | `is_seen` | Boolean. Available when `track_seen` is enabled on a custom feed group or the built-in `notification` group (including flat). Same time-based invalidation as `is_read`. **Enterprise plan only.** See [Ranking by Read/Seen Status](https://getstream.io/activity-feeds/docs/dotnet-csharp/read-seen-ranking/). | Every field the expression references that isn't a built-in field, such as `custom.*`, `external.*`, `metrics.*` and `selector_source`, needs a default value in `defaults`. The default is used for activities that don't have the field, and creating or updating a feed group or feed view without it fails with `expression references fields without defaults: `. Write defaults as nested objects (`{ "custom": { "foo": 0 } }`), not dotted keys (`"custom.foo"`). Built-in fields such as `popularity` or `share_count` can't have defaults. ```csharp label="C#" await _feedsV3Client.CreateFeedGroupAsync(new CreateFeedGroupRequest { ID = "myid", ActivitySelectors = new List { new() { Type = "following" } }, Ranking = new RankingConfig { Type = "expression", Score = "popularity * (custom.isBoosted ? 1.5 : 1)", Defaults = new Dictionary { ["custom"] = new Dictionary { ["isBoosted"] = false } } } }); ``` ## Data types The following data types are supported in the ranking expression context: | name | example | | ------- | ----------------- | | Boolean | `true`, `false` | | Integer | `36`, `0b101010` | | Float | `0.2` | | String | `"foo"` | | Array | `["a", "b", "c"]` | | Map | `{foo: "bar"}` | | Nil | `nil` | ## Functions The following functions can be used in ranking expressions: | name | description | | ------------------------------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ln(x) | Natural logarithm function | | log10(x) | Logarithm function base 10 | | sin(x) | Trigonometric sine function | | cos(x) | Trigonometric cosine function | | tan(x) | Trigonometric tangent function | | asin(x) | Arcsine function | | acos(x) | Arccosine function | | atan(x) | Arctangent function | | abs(x) | Absolute value | | min(a,b) | Minimum value | | max(a,b) | Maximum value | | trunc(x) | Truncates to the nearest integer value | | round(x) | Rounds to the nearest integer value | | decay_linear(t) | Linear decay, see [Decay & Ranking](#decay--ranking) for more information | | decay_exp(t) | Exponential decay, see [Decay & Ranking](#decay--ranking) for more information | | decay_gauss(t) | Gaussian decay, see [Decay & Ranking](#decay--ranking) for more information | | rand_normal() | Returns a normally distributed number in the range [-inf, +inf] with standard normal distribution (stddev = 1, mean = 0) | | rand_normal(a,b,σ,µ) | Returns a normally distributed number in the range [a, b] with specific normal distribution (stddev = σ, mean = µ) | | rand() | Returns a random number in the range [0, 1.0) | | to_unix_timestamp(t) | Converts a time value to a UNIX timestamp | | dist(lat1,lng1,lat2,lng2,unit) | Returns the distance between the two points given by (lat1,lng1) and (lat2,lng2). By default the unit is in kilometers, but the unit can also be `"M"` for miles or `"N"` for nautical miles. Coordinates written in the expression must be decimals (`51.0`, not `51`). This function can be combined with the external ranking parameters to rank activities based on a user's distance to them. | | map_lookup(key, map) | Look up value from a map by key (`0.0` if the key is missing), see [Arrays and Maps](https://getstream.io/activity-feeds/docs/dotnet-csharp/external-ranking-data/#arrays-and-maps) for more information | | sum_map_lookup(keys, map) | Sum values from a map for multiple keys, see [Arrays and Maps](https://getstream.io/activity-feeds/docs/dotnet-csharp/external-ranking-data/#arrays-and-maps) for more information | | in_array(needle, haystack) | Returns `1.0` if a value exists in an array and `0.0` otherwise, see [Arrays and Maps](https://getstream.io/activity-feeds/docs/dotnet-csharp/external-ranking-data/#arrays-and-maps) for more information | ## Operators The following operators can be used in ranking expressions: - Arithmetic operators: `+`, `-`, `*`, `/`, `%` (modulus), `^`, `**` (exponent) - Logical operators: `not`/`!`, `&&`/ `and`, `||`/`or` - Conditional: `x ? y : z`, `??`, `if`/`else` - Comparison: `==`, `!=`, `<`, `>`, `<=`, `>=` - Control order of evaluation with parentheses `()` - Array/map operators: `[]` (index), `.`, `?.`, `in` (checks if an array/map has an item/key) - String operators: `+` (concatenation), `contains`, `startsWith`, `endsWith` This lets you construct a scoring algorithm like this: ```text "score":"(a > 2 || (b > 4 && c > 3)) ? 1 : -1" ``` ## Constants The following constants and helpers are also available to use in ranking expressions: | name | description | | -------------- | ---------------------------------- | | `e` / `E` | Euler's number | | `pi` / `PI` | π | | `current_time` | UNIX timestamp of the current time | ## Decay & Ranking Stream supports linear, exponential, and Gaussian decay. The built-in `decay_linear(t)`, `decay_exp(t)` and `decay_gauss(t)` take one argument and always use the default parameters below (origin now, scale 5 days, no offset, decay 0.5); extra arguments are ignored. To change the parameters, define your own function under `functions` with a `base` of `decay_linear`, `decay_exp` or `decay_gauss`, and call it by name in `score`. All parameter values are strings. ![](https://getstream.imgix.net/images/docs/decay_explainer.png?auto=compress&fit=clip&w=800&h=600) ### Parameters | name | description | default | | --------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------- | | base | Required. The decay function to use: `decay_linear`, `decay_exp` or `decay_gauss`. | | | origin | The best possible value, as a number (for example a UNIX timestamp, or a popularity). If the value is equal to the origin, the decay function will return 1. | now | | scale | Determines how quickly the score drops from 1.0 to the decay value. If the scale is set to "3d", the score is equal to the decay value 3 days beyond the offset. | 5d | | offset | Values within the offset of the origin score 1. The decay starts beyond the offset. | 0 | | decay | The score that a value at scale distance from the origin should receive. | 0.5 | | direction | `both`, `right` or `left`. With `right`, only values below the origin decay (for timestamps, older activities) and values above it score 1. With `left`, only values above the origin decay. | both | > > **Info:** For `scale` and `offset` you can use s, m, h, d, w and y to specify seconds, minutes, hours, days, weeks and years, with a whole number (for example `36h`, not `1.5d`). A plain number is used as is. > The example below defines a simple_gauss with the following parameters: - **scale**: 5 days - **offset**: 1 day - **decay**: 0.3 This means that an activity younger than 1 day (the offset) will return a score of 1. An activity that is exactly 6 days old (offset + scale) will get a score of 0.3 (the decay factor). The example also defines a `popularity_gauss` that scores activities with a popularity of 100 or more as 1 (`origin: "100"` with `direction: "right"`), and lowers the score of less popular activities. A function applied to something other than time needs an `origin`, because the default origin is the current time. The full example: ```csharp label="C#" await _feedsV3Client.CreateFeedGroupAsync(new CreateFeedGroupRequest { ID = "myid", ActivitySelectors = new List { new() { Type = "following" } }, Ranking = new RankingConfig { Type = "expression", Score = "simple_gauss(time)*popularity_gauss(popularity)", Functions = new Dictionary { ["simple_gauss"] = new() { Base = "decay_gauss", Scale = "5d", Offset = "1d", Decay = "0.3" }, ["popularity_gauss"] = new() { Base = "decay_gauss", Origin = "100", Direction = "right", Scale = "100", Offset = "5", Decay = "0.5" } } } }); ``` ## Ranking by Selector Source When using multiple [activity selectors](https://getstream.io/activity-feeds/docs/dotnet-csharp/activity-selectors/), each activity is tagged with a `selector_source` field indicating which selector provided it. This enables you to prioritize activities from specific sources in your ranking expressions. ```csharp label="C#" // Example 1: Simple boost - following activities get 2x multiplier await _feedsV3Client.CreateFeedGroupAsync(new CreateFeedGroupRequest { ID = "selector_timeline", ActivitySelectors = new List { new() { Type = "following" }, new() { Type = "popular", MinPopularity = 1 } }, Ranking = new RankingConfig { Type = "expression", Score = "selector_source == \"following\" ? popularity * 2 : popularity", // Required when using selector_source in expressions Defaults = new Dictionary { ["selector_source"] = "" } } }); // Example 2: Multiple selectors with different bonuses await _feedsV3Client.CreateFeedGroupAsync(new CreateFeedGroupRequest { ID = "multi_selector_timeline", ActivitySelectors = new List { new() { Type = "following" }, new() { Type = "interest" }, new() { Type = "popular", MinPopularity = 1 } }, Ranking = new RankingConfig { Type = "expression", Score = "(selector_source == \"following\" ? 10 : 0) + " + "(selector_source == \"interest\" ? 5 : 0) + " + "popularity", Defaults = new Dictionary { ["selector_source"] = "" } } }); ``` **Important notes:** - **Defaults required:** You must provide `selector_source` in the `defaults` map (typically as an empty string `""`) - **Duplicate activities:** If an activity matches multiple selectors, it keeps a single `selector_source`. `following` takes precedence over the other selectors - **Valid sources:** `following`, `popular`, `interest`, `query`, `proximity`, `current_feed`, `follow_suggestion` --- For the most recent version of this documentation, visit [https://getstream.io/activity-feeds/docs/dotnet-csharp/ranking-expressions/](https://getstream.io/activity-feeds/docs/dotnet-csharp/ranking-expressions/).