October 27, 2023

Utilizing Loops in KNIME Analytics Platform

By John Emery

Effective data manipulation and analysis can become a complex and time-consuming task in the evolving world of data science. The era of automated data processing has empowered us with tools that simplify these tasks, allowing us to spend more time interpreting results than preparing data. 

One such tool that has gained substantial traction recently is the KNIME Analytics Platform. This post will explore the integral aspect of KNIME – loops, their types, and a practical approach to constructing a loop in KNIME.

KNIME Analytics Platform: A Brief Introduction

KNIME is an open-source data analytics, reporting, and integration platform that integrates various machine learning and data mining components. Built on a modular data pipelining concept, KNIME allows users to create data flows visually, selectively execute a few or all analysis steps, and inspect the results, models, and interactive views later.

What distinguishes KNIME from other data analysis platforms is its intuitive graphical interface for designing and implementing data workflows. This attribute brings us to our main topic for the day – loops.

What are Loops?

Loops, a core concept in programming, are all about repetition. They automate the execution of a sequence of instructions until a specified condition is satisfied. Think of it like baking cookies; instead of making one cookie at a time, you use a cookie cutter to stamp out several cookies at once.

In computing, each repetition or execution of the instruction set is called an ‘iteration.’ The set of instructions is known as a ‘block’ or ‘body’ of the loop. The condition that governs the continuation of the loop is the ‘loop condition.’ As long as this condition holds true, the loop persists.

In data analysis, loops are indispensable. They break down complex tasks into manageable, repeatable units, which is particularly useful when performing operations on each element within a large array or list. Loops reduce redundancy and enhance efficiency, underscoring the ‘write once, run many times’ principle in programming and data analysis. 

Mastering loops is a crucial step towards practical data analysis.

What Loop Types Are Available in KNIME Analytics Platform?

KNIME Analytics Platform provides over ten types of loops from which to choose. While the selection may initially feel overwhelming, the good news is that each loop type is constructed similarly. 

When building loops in KNIME, the most important thing to understand is the appropriate loop for a given situation. Below, we have listed the types of loops available in KNIME and their uses.

  • Counting Loop: These are the simplest forms of loops, performing a task for a defined number of times. It’s perfect for scenarios where the exact number of iterations is known in advance. These are akin to ‘for’ loops in computer programming.

  • Chunk Loop: When dealing with a large dataset, it might be more efficient to divide the data into manageable chunks or batches and then perform the operations. Chunk loops allow you to split each iteration into a set of consecutive records from the input table. For example, the first iteration could cover records 1-100, the second iteration records 101-200, etc.

  • Column List Loop: This type of loop is a bit of an oddity. Instead of partitioning some set of records from the input table, different sets of columns are included or excluded in each iteration. For example, you can set this loop to have only numeric columns to perform some desired manipulations.

  • Generic Loop: The Generic loop iterates until some condition has been satisfied. This is accomplished through the use of flow variables and the special Variable Condition Loop End node. KNIME’s generic loop is akin to the ‘while’ loop in computer programming.

  • Table Row to Variable Loop: In KNIME, flow variables are special parameters that can be used to control configuration settings in downstream nodes. The Table Row to Variable loop changes the value of a flow variable based on a selected column for each iteration of the loop. This loop type is beneficial when reading the data from multiple Excel sheets and concatenating them into a single table.

  • Group Loop: Similar to the Chunk loop, each iteration of the Group loop takes a set of records from an input table. Users select one or more columns whose unique values are grouped.

  • Interval Loop: Each iteration of this loop increments a variable by a specified amount. For example, you can have a variable iterate from 1 to 10 in increments of 0.1 for 100 total iterations. This variable can be used in calculations and updating node configuration settings in each iteration.

  • Recursive Loop: Recursive loops are a bit more complex, utilizing the output of one iteration as the input for the next. These are particularly useful in situations like predictive modeling, where future predictions depend on past data. The Recursive loop has a particular loop end node: Recursive Loop End. This node contains a collection port, which holds data that is done being iterated over, and a recursion port, which sends data back to the loop start for the next iteration.

  • Window Loop: The Window loop provides two options for users: the ability to iterate over a specific number of rows or a specific amount of time compared to a datetime field. The Window loop is very effective if you have a loop structure where you want each iteration to cover one year at a time.

Constructing a Loop in KNIME

Building, debugging, and analyzing loops in KNIME is extremely easy. There are three components to any loop:

  1. Loop Start: This is the initial node where the loop begins. With most loop types in KNIME, you will connect a standard data table or a flow variable. Depending on the type of loop you are constructing, there may be configuration settings that you have to enter. For example, if you build a Group loop, you must select the columns to include in your grouping mechanism. On the other hand, the Counting loop only requires you to indicate how many iterations the loop will have.

  2. Loop Body: This is the meat of the loop, the set of nodes whose procedures will be repeated for each iteration. Depending on your needs, the loop body may be simple or complex. You can also next loops inside of loops for particularly challenging processes, although the performance of such workflows may suffer.

  3. Loop End: This is the node that ends the loop. You will often use the standard Loop End node, although several other options depend on your specific needs. For example, the Generic loop must end with the Variable Condition Loop End node. The loop end nodes provide a few configuration settings, including iteration numbers as a column, passing flow variables from within the loop, and allowing changing table specifications.

For example, imagine you have an Excel workbook that contains many sheets of yearly sales data. Since each sheet has the same schema, you want to combine them into a single table in KNIME. Rather than manually combining these sheets, which is time-consuming and not dynamic (what if new sheets are added or a sheet is removed?), you can utilize a loop to handle it.

The image below depicts a simple example of how to do this. First, use the Read Excel Sheet Names node to return a table of the names of each sheet in the workbook. The great thing about this node is that it is fully dynamic; if the sheet names change over time, that will be reflected when you run the workflow.

Next, the Table Row to Variable Loop Start allows us to turn each sheet name value into a flow variable. So, if the workbook has ten sheets, that will become a table with ten records. The subsequent sheet name will become the variable value for each loop iteration.

We connect this flow variable to an Excel Reader node within the loop. By telling KNIME to open an Excel sheet based on the value of that variable, we can dynamically open the workbook’s contents for each sheet.

Finally, the Loop End node collects the records from each iteration and concatenates them into a single output table. If you suspect the schema may change from one iteration to the next, select the appropriate option in the node’s configuration menu.

A pop up screen that's titled, "Dialog - 3:18 - Loop End."

Admittedly, this was a simple example. But the great thing about building loops in KNIME is that the general structure is always this simple. Certainly, the loop’s body may be considerably more complex than this example, but that would be the case with or without a loop.


Loops in KNIME are a powerful feature that helps streamline data processing, particularly with large or complex datasets. By mastering the use of loops in KNIME, you can reduce the amount of manual work required and make your data workflows more efficient and easier to manage. The platform’s intuitive interface and extensive resources make learning relatively simple, even for beginners in data science.

Loops are just one aspect of what makes KNIME a dynamic and flexible tool for data analysis. As you dive deeper into KNIME, you’ll discover various features designed to simplify and optimize your data analytics journey. Happy KNIME-ing!

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