Editing java Snippet node to check if column exist or create it

[tommasopalomba]

Hi all, I have a java Snippet node set as below:

if (c_Column3_Arr1==-10){
	out_New_CLASS =  "Not_subs";
} else {
	out_New_CLASS = c_Column3;
}

I’d like to add a check before the cmd searching if “Column3_Arr1” exist. If exist the node should be follow the setting already present otherwise have to create the column “Column3_Arr1” filled by all -10 values and then follow the cmd out_New_CLASS = “Not_subs”.

Could someone suggest the right correction to the script?

[ipazin]

Hello @tommasopalomba,

check here:

Check if Column Exists in Column Expressions Node

Hello, I have a column that exists within some files and not in others. Thus, when running a loop through reading all the files, it does not always appear. I am running into a problem with the Column expressions node that it throws an error because the column I am wanting to use (if it exists) does not exist. Essentially, I want to write an expression that says: If the column(“column1”) exists, then do something, else, do something different I have tried something along the lines of with no …

Also check this topic by using Table Validator node(s):

Add column if doesn't exist

Hi @TigerCole yes 5 years later there are better solutions for this I would now use the Table Validator for this. You can in the dialog define what happens if certain columns are missing, and add them with Missing Values in case they are not there. The Missing Value can than add you a fixed value into these missing values. [image] If you have a second reference table, you can also use the Table Validator (Reference) Than you do not have to take care of configuring the node…

Br,
Ivan

[tommasopalomba]

The first example you mentioned is something that I already try, but the java snippet node configured as in the example doesn’t work and I really don’t understand why.

The saw also the second solution, but I have a doubt because in the example the column to look for is use in the input file, but if this is missing how it can be use as input? Strange.

[mlauber71]

@tommasopalomba a pretty basic code with columnExists:

out_colExists = columnExists(v_var_column_to_check) ? 1 : 0;

will give you a flow variable that you can then further use to steer a switch or something.

Use Java Snippet to check if Column is there, fill a FlowVariable and then...

[takbb]

Hi @tommasopalomba , as an addendum to @mlauber71 's solution, you cannot directly access the value of Column3_Arr1 if there is a chance it won’t exist because that would then result in a configuration error.

Instead, to access the contents of your column which might exist, could write something like this:

	try{

		out_New_CLASS = this.getCell("Column3_Arr1", ""); 
		
		
	}	
	catch(ColumnException ex)
	{
		out_New_CLASS = "Not_subs";
	}

This does not result in an error because Column3_Arr1 is referenced only via a call to the getCell function rather than directly through a variable name. The getCell function returns a run-time exception which can be trapped, rather than a compilation/config error which cannot.

Note that you cannot, within a single java snippet optionally choose to create a new column, so you would need to have a flow containing additional nodes to perform actions such as creating new columns, as @mlauber71’s flow indicates.

[s.roughley]

With the new Array-type variables which I think have happened since this original question, along with the new-ish @Vernalis Configurable IF/CASE... nodes, it is possible to do this with no scripting requirements:

The Extract Column Header / GroupBy / Table Row to Variable sequence puts the column names into an array variable, and then the Configurable IF/CASE Switch (Flow Variable Value) node can test various options for members of the array, including the selected Any Equals option for an exact match:

NB Also the options around case sensitivity etc

See If Column Exists – KNIME Community Hub for example workflow

Steve

[takbb]

@s.roughley 's workflow reminded me I have a component on the hub from a few months ago that does something similar…

IF Column Exists – takbb

(You will need to have the Column Expressions node installed to use this component)