## Summary

```
count(<tsExpression>[, metrics|sources|sourceTags|pointTags|<pointTagKey>])
count(<hsExpression>)
count(<eventsExpression>)
```

You can use `count()`

with time series, with histograms, and with events.

Time series aggregation function |
Returns the number of reporting time series described by the `tsExpression` at each moment in time.
A time series is counted as reporting even if it has interpolated values. |

Histogram conversion function |
Returns time series that consist of the number of values in each histogram distribution described by the `hsExpression` . |

Event conversion function |
Returns a single time series that reports the number of event boundaries that occur in `eventExpression` over time. |

## Parameters

### Time-Series Aggregation Function

Parameter | Description |
---|---|

tsExpression | Expression describing the set of time series to be counted. |

metrics|sources|sourceTags|pointTags|<pointTagKey> | Optional 'group by' parameter for organizing the time series into subgroups and then returning a count for each subgroup. Use one or more parameters to group by metric names, source names, source tag names, point tag names, values for a particular point tag key, or any combination of these items. Specify point tag keys by name. |

### Histogram Conversion Function

Parameter | Description |
---|---|

hsExpression | Expression describing the histogram distributions to obtain the number of values from. |

### Events Conversion Function

Parameter | Description |
---|---|

eventExpression | Expression describing the events to count over time. |

## Description

You can use `count()`

:

- With time series as an aggregation function for time series.
- With histogram series as a conversion function.
- With event sets as a conversion function.

### Time-Series Aggregation Function

The `count()`

aggregate function adds together the number of reporting time series represented by the expression, at each moment in time.

By default, `count()`

produces a single count across all time series. You can optionally group the time series based on one or more characteristics, and obtain a separate count for each group.

If a time series has data gaps, `count()`

fills them in by interpolation whenever possible. A time series with an interpolated value is considered to be reporting and is included in the current count. When a value cannot be interpolated into a time series (or if the series stops reporting altogether), the series is excluded from the count.

#### Grouping

Like all aggregation functions, `count()`

returns a single series of results by default.

You can include a ‘group by’ parameter to obtain separate counts for groups of time series that share common metric names, source names, source tags, point tags, or values for a particular point tag key. The function returns a separate series of results corresponding to each group.

You can specify multiple ‘group by’ parameters to group the time series based on multiple characteristics. For example, `count(ts("cpu.cpu*"), metrics, Customer)`

first groups by metric names, and then groups by the values of the `Customer`

point tag.

#### Interpolation

If any time series has gaps in its data, Wavefront attempts to fill these gaps with interpolated values before applying the function. A value can be interpolated into a time series only if at least one other time series reports a real data value at the same moment in time.

Within a given time series, an interpolated value is calculated from two real reported values on either side of it.
Sometimes interpolation is not possible–for example, when a new value has not been reported yet in a live-view chart.
In this case, Wavefront finds the last known reported value in the series, and assigns it to any subsequent moment in time for which a real reported data value is present in some other time series. We use the last known reported value only if interpolation can’t occur *and* if the last known reported value has been reported within the last 15% of the query time in the chart window.

You can use `rawcount()`

to suppress interpolation. See Standard Versus Raw Aggregation Functions.

### Histogram Conversion Function

The `count()`

histogram conversion function returns the number of data values in each distribution of each histogram series that is represented by the expression. The counts for a given histogram series are returned as a separate time series that contains a data point corresponding to each input distribution.

`count()`

is a histogram conversion function because it takes histogram distributions as input, and returns time series. You can therefore use a histogram conversion function as a `tsExpression`

parameter in a time series query function.

### Events Conversion Function

The `count()`

events conversion function returns a single time series in which each data point reports the number of event boundaries in `eventExpression`

at that moment in time.

The number of event boundaries at a particular moment in time is equal to: the number of events that started at that time, minus the number of events that ended at that time. Instantaneous events are represented as a single “0” value: 1 started minus 1 ended (instantaneous events are defined as events having their end time equal to their start time).

## Examples

### Time-Series Aggregation Function

The following examples contrast the result you get for two different types of servers. We’re using a Single Stat View chart to first get a count of all servers that have a sample requests latency and the source `app-1*`

(11 servers). Then we get a count of all servers that have the source `app-2*`

While the example is a bit contrived, it illustrates how to use the function.

The following Stacked Area chart groups all sources whose name starts with `"app-1*"`

by the `env`

point tag. The chart shows that 1 server is tagged with `dev`

and 10 servers are tagged with `production`

.

### Histogram Conversion Function

The following example shows the result of applying `count()`

to an `hsExpression`

. As the legend shows, the distribution at 2:40p summarizes 3 values.

**Note:** `count()`

returns a separate time series for each input histogram series. In this example, the `hsExpression`

represents a single histogram series, so the result is a single time series. (In contrast, when `count()`

is used as an aggregation function with a `tsExpression`

, a single returned time series might be the result of counting multiple input time series.)

## Caveats

Count is not a static number, it will change if the number of hosts reporting the metric changes. For some types of analysis, it is better to use `avg()`

instead of `count()`

.