read/write CSV boolean

Hello, I’m writing a CSV file that has a few columns with the boolean datatype.

The data is saved as 1’s and 0’s when written, as expected.

I’m reading this same CSV file in with a different workflow, however, the boolean option is not available for these columns in the transformation tab.

For some reason dates and small strings are available to change to boolean?

What is the requirement for reading a boolean column? And why are 1’s and 0’s not included if that’s how the CSV writer writes them?


Hi @klenner , welcome to the KNIME forum!

Life’s a mystery sometimes :wink: I’d not noticed that behaviour before but you’re right it is inconsistent.

From some quick experimentation, it seems you need to convert a boolean to a string before sending it to the csv writer, so that it gets written as “true” or “false” rather than 1 or 0, and then in the CSV Reader it is then possible to transform that string into a boolean. Either that, or following the CSV Reader you’d have to convert 0 and 1 back to false and true, using an additional node.

To convert the boolean to a string, you can use String Manipulation prior to CSV writer, if this is just a single boolean column. You can use the function:

and select to have it replace the existing boolean column.

Likewise, if reading 1 or 0 as an integer in CSV Reader, it could be followed by a Rule Engine to convert back to boolean, e.g:

$mybool$ = 1 => TRUE
$mybool$ = 0 => FALSE

So the workaround is a little annoying, but simple, for a single boolean column.

Unfortunately the situation isn’t quite so straightforward if you have multiple boolean columns. You could obviously have a series of String Manipulation nodes or Rule Engine nodes. Ideally you could use String Manipulation (Multi Column) but unfortunately that node cannot handle boolean data (it “breaks” if it encounters a boolean). It seems that Boolean support wasn’t given top priority!

I’m trying to think of a simple way to handle booleans in a multi-column environment. This would be where we’d want to make use of a Multi-Column Rule Engine, but such a node doesn’t currently exist. Yes you could use a column list loop, but that a lot of work for something “simple” like this.

I’m out of suggestions for core nodes! I know it can be achieved with my “Rule Engine (Multi-Column)” component, which basically wraps the Rule Engine node, and adds some additional features:


Although the following script looks like it wouldn’t do anything, it will actually convert all booleans into strings of either “true” or “false”.

"<CURRENTCOLUMNTYPE>" =  "Boolean" AND $CURRENTCOLUMN$ = "true" => "true"
"<CURRENTCOLUMNTYPE>" =  "Boolean" AND $CURRENTCOLUMN$ = "false" =>  "false"

This would then enable you to read the Booleans in after configuring the CSV Reader:

Sorry I haven’t currently got a simpler (core KNIME) solution to your question. Maybe somebody else can spot a better way that I’ve missed.


@klenner , I’ve put some more thought to it…

Although the String Manipulation (Multi-Column) doesn’t like reading booleans, it can write them!

So, you could follow your CSV Reader with String Manipulation (Multi-Column) and give it this script:

toBoolean( $$CURRENTCOLUMN$$ ==0 ? "false" : "true") 

You’d need to configure it and select only the specific columns to be converted, but this actually might be more straightforward than if you’d had to configure the specific columns on CSV Reader anyway…

If you need the node to be able to handle the eventuality that the incoming data from the CSV reader could be either in (String) “true”,“false” format or (Integer) 1,0 format, then the following script should work instead:

	string($$CURRENTCOLUMN$$).equals("false") || 
	? "false" 
	: "true") 
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This seems to work really well.

It might be a fool’s errand, but I’d like to make my 1.7 million row file as small as possible!



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Great. Glad it worked for you @klenner and thanks for marking the solution :slight_smile:

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