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Version: Java (Groovy)

Export data to CSV or other delimited files

This guide discusses how to export table data to CSV (or other delimited) files by using writeCsv.

tip

CSV files can also be exported via Table Options > Download CSV in the Deephaven UI.

writeCsv​

The basic syntax for writeCsv follows:

import static io.deephaven.csv.CsvTools.writeCsv

writeCsv(table, "/data/outputFile.csv")
note

Deephaven writes files to locations relative to the base of its Docker container. See Docker data volumes to learn more about the relation between locations in the container and the local file system.

We'll create a table to export by using emptyTable and update. The table contains 100 rows of trigonometric values.

source = emptyTable(100).update(
    "X = 0.1 * i",
    "SinX = sin(X)",
    "CosX = cos(X)",
    "TanX = tan(X)"
)

Standard CSV files​

The simplest way to use writeCsv is to supply two input parameters:

  • The Deephaven source table.
  • The path of the output CSV file.
import static io.deephaven.csv.CsvTools.writeCsv

writeCsv(source, "/data/TrigFunctions.csv")

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The /data mount point​

If you are using Docker-installed Deephaven, you can find a /data folder inside your Deephaven installation's main folder, on the same level as your docker-compose.yml file. This folder is mounted to the /data volume in the running Deephaven container. This means that if the Deephaven console is used to write data to /data/abc/file.csv, that file will be visible at ./data/abc/file.csv on the local file system of your computer.

note

If the ./data directory does not exist when Deephaven is launched, it will be created.

Null values​

Null values are common in tables. How are they handled when exporting data to a CSV? This depends on how you call writeCsv.

First, let's create a table with null values. The example below uses a function to fill the SinX column with a large number of nulls.

sourceWithNulls = emptyTable(100).update(
    "X = 0.1 * i",
    "SinX = X % 0.2 < 0.01 ? NULL_DOUBLE : sin(X)",
    "CosX = cos(X)",
    "TanX = tan(X)"
)

The SinX column contains many null cells. The example below writes this table to a CSV file called TrigFunctionsWithNulls.csv.

import static io.deephaven.csv.CsvTools.writeCsv

writeCsv(sourceWithNulls, "/data/TrigFunctionsWithNulls.csv")

img

Column selection​

In the event you don't want to write every column in the table to a CSV file, you can specify which columns to write. This is done by providing a list of column names, as shown below.

import static io.deephaven.csv.CsvTools.writeCsv

writeCsv(sourceWithNulls, "/data/Cosine.csv", "X", "CosX")

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