Function used to get informed about the presence of data in a catalog.

Ex:

Let i be the cursor of a catalog and x a real variable.

We place the cursor i on the first line of the catalog.

To check the contents of a catalog you have to write the name of the real variable to use followed by the equal sign, the cursor variable of the catalog, a point and the "isvalid" term.

This way, we give to the x variable the value "1" if the catalog contains data and "0" if not.

We generally use this expression to avoid useless nodes if the catalogue is empty. For that, you just have to use a test node after the computing node in order to check the x value. In our example, the expression to check in the test node will be  x==1.