> ## Documentation Index > Fetch the complete documentation index at: https://docs.prophecy.ai/llms.txt > Use this file to discover all available pages before exploring further. # WhileIterator > Recursively processes rows **Dependencies:** * ProphecySparkAlteryxPython 0.0.1+ **Cluster requirements:** * UC dedicated clusters 14.3+ supported * UC standard clusters 14.3+ supported * Livy clusters not supported The WhileIterator allows you to group a set of gems together, and then run them one after another repeatedly in a loop. WhileIterator ## Input The input of the WhileIterator can be any dataset. Importantly, the output schema of the WhileIterator **must** match or be a subset of the input schema. This is because the output feeds back into the input as the subgraph is iterating (looping). ## Configuration Before setting the WhileIterator configuration, make sure you are familiar with [pipeline configurations](/data-engineering/development/pipelines/configuration). Once you have a WhileIterator subgraph on your canvas, set configurations by opening the subgraph and clicking on **Configuration**. The following table describes the parameters you need to set. | Parameter | Tab | Description | | -------------------------------------------------------- | ------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | Max iterations | Settings | The maximum number of loops that will be performed. | | Populate iteration number in config variable | Settings | A checkbox to enable if you want to keep track of iteration numbers in a variable. | | Select config variable name to populate iteration number | Settings | The name of the config variable that will store the iteration number. | | Schema | Configuration | A table where you define configuration variables. You can **Copy Pipeline Configs** to inherit these from your pipeline. If you want to populate the iteration number in a config variable, make sure to include that variable here. | | Config | Configuration | An area where you can define default values for variables in one or more configurations. | ## Subgraph Within the WhileIterator, you can design a flow with multiple gems to create your desired output. Importantly, you also need to understand the two inputs of the WhileIterator\_Output. The order of your inputs is important: * The **in0** dataset contains the DataFrame for the **following** iteration. If this dataset becomes empty, the loop is terminated. * The **in1** dataset passes on an individual iteration. You'll see the entire list of iterations as the output of the WhileIterator gem. ## Break condition There are two scenarios in which the WhileIterator will break: * The **in0** dataset of the WhileIterator\_Output becomes empty. * The maximum number of iterations is reached. ## Output There are two outputs of the WhileIterator: * The **out0** dataset that contains the output of the **last iteration** of the WhileIterator. * The **out1** dataset that contains the whole list of iterations produced by the WhileIterator. This is produced by performing a union on all output iterations. *** ## Example Imagine you are simulating an investment that earns a fixed percentage of interest per year. You want to keep compounding the interest until your investment reaches a desired amount. You can use the WhileIterator gem to produce a dataset demonstrating this yearly growth. WhileIterator For this example, the input of the WhileIterator will be a simple table that describes your starting investment and the fixed interest rate.
| `year` | `principal` | `interest_rate` | | ------ | ----------- | --------------- | | 0 | 1000 | 0.05 |
### Add Reformat gem Now, let's use a Reformat gem in the WhileIterator subgraph to update the **Year** and **Principal** each iteration. 1. Add the Reformat gem inside the WhileIterator. 2. Connect WhileIterator\_Input to the Reformat gem. 3. Connect the Reformat gem to the WhileIterator\_Output. 4. Configure the **Expressions** table as it is shown below. | Target Column | Expression | | --------------- | ---------------------------------------- | | `year` | `year + 1 ` | | `principal` | `principal + principal \* interest_rate` | | `interest_rate` | `interest_rate` | ### Terminate loop The while loop will stop if the **in0** dataset becomes empty. How can the **in0** dataset become empty? In this example: 1. Add a Filter gem with the condition `principal < 2000`. 2. Connect the Reformat gem output to the input of the Filter gem. 3. Connect the Filter gem output to the input of WhileIterator\_Output. This way, the Filter gem will return data as-is until the principal reaches 2000. After that, the record is filtered out, and the loop will terminate. WhileIterator ### View output data The **out0** dataset should be empty because we used a Filter gem to return an empty set when the `principal > 2000`. Below is an example **out1** dataset. This lets you project how your principal will grow over the years.
| `year` | `principal` | `interest_rate` | | ------ | -------------- | --------------- | | 1 | 1050 | 0.05 | | 2 | 1102.5 | 0.05 | | 3 | 1157.625 | 0.05 | | 4 | 1215.50625 | 0.05 | | 5 | 1276.2815625 | 0.05 | | 6 | 1340.095640625 | 0.05 | | 7 | 1407.100422656 | 0.05 | | 8 | 1477.455443789 | 0.05 | | 9 | 1551.328215978 | 0.05 | | 10 | 1628.894626777 | 0.05 | | 11 | 1710.339358116 | 0.05 | | 12 | 1795.856326022 | 0.05 | | 13 | 1885.649142323 | 0.05 | | 14 | 1979.931599439 | 0.05 | | 15 | 2078.928179411 | 0.05 |