Analyzing the subset of data

To determine the highest and lowest trading price, you need to write the logic for determining those values. In addition, you need some local variables to track the highest and lowest price that is processed so far. You can use the Custom operator to accomplish these tasks. As shown, the input stream is only the IBM transactions; there is no output stream for the customized operator.

The SPL code for the Custom operator is shown. Because the Custom operator does not produce an output stream, specify "() as Sink" before the equals sign.

      () as Sink = Custom(IBMTransactions) {
         logic
            state : {
               mutable decimal64 highest = 0.0d, lowest = 999.99d;
            }
            onTuple IBMTransactions : {
               if (price > highest)
                  highest = price;
               if (price < lowest)
                  lowest = price;
            }
            onPunct IBMTransactions : {
               if (currentPunct() == Sys.FinalMarker) {
                  printStringLn("highest = " + (rstring)highest);
                  printStringLn("lowest = " + (rstring)lowest);
               }
            }
      }

In general, the Custom operator performs the statements that are specified in the state, onTuple, and onPunct sections of the logic clause. Local variables are initialized in the state section. Logic to be performed for each tuple from the input stream is specified in the onTuple section. Logic to be performed for punctuation from the input stream is specified in the onPunct section. A final punctuation marks the end of a stream.

In this example, the Custom operator performs the following steps:

1. Initializes the local variables (highest and lowest).
2. Receives a tuple from the input stream (IBMTransactions).
3. Checks whether the price is greater than the highest price. If so, sets the new highest price.
4. Checks whether the price is less than the lowest price. If so, sets the new lowest price.
5. Repeats Steps 2 to 4 until all the tuples from the input stream are processed.
6. When the end of the input stream is reached, prints the highest and lowest prices.