---
title: "map(_:)"
framework: createml
role: symbol
role_heading: Instance Method
path: "createml/mldatatable/map(_:)-92wrj"
---

# map(_:)

Creates a new column by applying a given thread-safe transform to every row in the data table.

## Declaration

```swift
func map<T>(_ lazyTransform: @escaping (MLDataTable.Row) -> T) -> MLDataColumn<T> where T : MLDataValueConvertible
```

## Parameters

- `lazyTransform`: A thread-safe row transformation function. The implementation of your transform must accept a row from the data table and return a type that conforms to doc://com.apple.createml/documentation/CreateML/MLDataValueConvertible.

## Return Value

Return Value A new MLDataColumn.

## Discussion

Discussion Use this method to create a new column derived from the existing data in the table. The closure you pass evaluates lazily only when the transformed values are needed for a subsequent operation. Your implementation should accept a data table row and must be thread-safe because the framework may invoke the closure concurrently on unspecified threads.

For example, to perform the column derivation operation shown above, begin by creating a table of data. let data: [String: MLDataValueConvertible] = [     "Day": ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday"],     "Temperature": [91.3, 85.8, 79.5, 83.4, 72.2] ]

var table = try MLDataTable(dictionary: data) After creating the table, use map(_:) to create a new column of data from the original data. The example closure implementation below is stateless and safe to invoke concurrently on any thread, so no synchronization is necessary. let derivedColumn = table.map { row -> String in     guard let day = row["Day"]?.stringValue,           let temperature = row["Temperature"]?.doubleValue else {             fatalError("Missing or invalid columns in row.")     }     return String(format: "%@ %.1fº", day, temperature) } Then use addColumn(_:named:) to add the derived column to a table. table.addColumn(derivedColumn, named: "Description") Mapping elements Preventing Timing Problems When Using Closures map(_:)