Check if an element is present in a bag?

问题

How can I check in piglatin, if a bag contains an element?

Example : In a bag of chararray, how can I check if a token is present?

回答1:

In Apache Pig you can use statements nested in FOREACH see Pig Basics. Here is example from the documentation: A is a bag in B.

X = FOREACH B {
        S = FILTER A BY 'xyz';
        GENERATE COUNT (S.$0);
}

Instead of COUNT you can use IsEmpty and ?: operator

X = FOREACH B {
        S = FILTER A BY 'xyz';
        GENERATE (IsEmpty(S.$0)) ? 'xyz NOT PRESENT' : 'xyz PRESENT') as present, B;
}

Or only to leave the bags that contain the data:

X = FOREACH B {
        S = FILTER A BY 'xyz';
        GENERATE B, S;
}
F = FILTER X BY not IsEmpty(S);
R = FOREACH F GENERATE B;

This will avoid costly join to itself, as extra joins are extra Map Reduce jobs.

来源：https://stackoverflow.com/questions/26390220/check-if-an-element-is-present-in-a-bag