A tuple is a sequence of attributes. Each attribute in the sequence is named.For example, the type declaration:

       type Data  = uint32 payloadLength, float64 diffSeconds, uint32 newSourceIP;

defines Data to be a tuple, and the 3 attributes are named payloadLength, diffSeconds and newSourceIP.

An attribute in a tuple is referenced by name. For example, the following statement assigns the attribute payloadLength to have the value stored in the variable x:

       data.payloadLength = x;

A list on the other hand, is a zero-index sequence, which means that the elements in the list are referenced using an index. For example,

       mutable list <int32> SourceIPCount = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0];

intitializes SourceIPCount to be a list with 15 elements, all of which are 0. The first element is set to 12 using the statement SourceIPCount[0]=12. Note that 0 is the index of the first element (hence, the term zero-indexed).

A data stream is an infinite sequence of tuples. It is often useful to use a past value; for example, suppose that Predictor is a tuple and the attribute names are mu0, mu1, mu2,. . .,mu8. The past value of Predictor.mu2 received or computed 200 tuples previously is referenced from the invocation: Predictor[200].mu2.

SPL is strongly typed, which means that a variable’s type must be declared and cannot be changed by assigning a value of a different type, For example, the declaration mutable <float64> flt; declares flt to be 64-bit floating point number, hence, the statement flt = 2; will create a compilation error (since 2 is, by default an (signed) integer). We may convert flt to a 32-bit integer however, using the statement anInteger = (int32)flt. The general form is is x = (type)y;, where type is one of int32, decimal64, float32, etc.

if-then and if-then-else: The purpose of these constructs is to instruct your program to execute a section of code only if a particular test evaluates to true. The structure insures that a statement is executed only if the test statement is true. If an else clause statement is present, then one of two statements will executed based on the test statement.