# TERRAggregation_Real

In the Spotfire Custom Expressions dialog box, you can select the pre-defined expression `TERRAggregation_Real` from the Function list. This expression function sets the variable `output` to a scalar value of the TERR data type `numeric`, which Spotfire converts to the corresponding Spotfire data type Real. The TERR script is called once for each group of data to be aggregated. No special handling for aggregation is necessary in your TERR script.

The expression function has at least two arguments.
Argument | Argument description
--- | ---
A TERR script. | The TERR script contains the following. A number of variables using the naming convention that Spotfire requires: `input1` to `inputN`, where `inputN` is the highest number of the specified inputs, numbered sequentally. A TERR assignment operator (`<-`) that assigns the results of the TERR evaluation to an object named `output` (also using the naming convention that Spotfire requires).
Spotfire column names. | Passed as additional arguments, these are the data column names that `input1` to `inputN` represent. All columns must be the same length.

The output type is returned from TERR and converted by Spotfire.
Returned by TERR | Converted in Spotfire
--- | ---
A single aggregated value of data type `numeric`. | A single aggregated value of data type Real.

## TERRAggregation_Real example

You can work through the example in Aggregating binned weather data using TERR in Spotfire. That example adds a horizontal line on the the Y axis for data provided in air data set for Spotfire examples.

    TERRAggregation_Real("output <- mean(input1)",[Y])

The resulting table in Spotfire shows the following.