Step-by-step guide

You have to use the "by rank" syntax i.e. "#n#key". For example using the bufr_filter rules:

set #4#airTemperature = 283.95;

This sets the 4th instance of the key "airTemperature" to 283.95 K.

Note: Although you can use the "by condition" syntax for getting values of keys, this method cannot be used for setting such keys i.e. The following will not work:

# Does not work
set /subsetNumber=4/airTemperature = 283.95; ## Error


Also see Setting keys by rank.