Expression Syntax

The expressions used to define Synthetic Columns and some Row Subsets are actually expressions in the Java language, which are compiled into Java bytecode before evaluation. However, this does not mean that you need to be a Java programmer to write them. The syntax is pretty similar to C.

The following explanation gives some guidance and examples for writing these expressions. Unfortunately a complete tutorial on writing Java expressions is beyond the scope of this document, but it should provide enough information for even a novice to write useful expressions.

Referencing cell values

To create a useful expression for a cell in a column, you will have to refer to other cells in different columns of the same table row. You can do this in two ways:

By Name

The Name of the column may be used if it is unique (no other column in the table has the same name) and if it has a suitable form. This means that it must have the form of a Java variable - basically starting with a letter and continuuing with letters or numbers. In particular it cannot have any spaces in it. The underscore and currency symbols count as letters for this purpose. Column names are treated case-insensitively.

By $ID

The "$ID" identifier of the column may always be used to refer to it. This is just a "$" sign followed by a unique integer assigned by the viewer to each column when it is first encountered. You can find out the $ID identifier by looking in the Column Metadata Window.

There is a special column whose name is "Index" and whose ID is "$0". The value of this is the same as the row number in the unsorted table (the grey numbers on the left of the grid in the main browser window).

The value of the variables so referenced will be a primitive (boolean, byte, short, char, int, long, float, double) if the column contains one of the corresponding types. Otherwise it will be an Object of the type held by the column.

Referencing Row Subset flags

If you have any Row Subsets defined you can also access the value of the boolean (true/false) flag indicating whether the current row is in that each subset. Again there are two ways of doing this:

By Name

The name assigned to the subset when it was created can be used if it is unique and if it has a suitable form. The same comments apply as to column names above.

By #ID

The "#ID" identifier of the subset may always be used to refer to it. As for the "$ID" identifier for columns above, this is a unique index preceded by a special symbol, "#".

Operators

The operators are pretty much the same as in C. The common ones are:

Arithmetic

+ (add)

- (subtract)

* (mutliply)

/ (divide)

% (modulus)

Logical

!  (not)

&& (and)

|| (or)

== (identity)

!= (non-identity)

<  (less than)

>  (greater than)

<= (less than or equal)

>= (greater than or equal)

Other

[] (array dereferencing)

?: (conditional switch)

instanceof (class membership)

Math static methods

abs(x) (absolute value)

cos(x) (cosine)

sqrt(x) (square root)

max(a,b) (maximum)

pow(a,b) (exponentiation)

toRadians(deg) (angle conversion)

String instance methods

startsWith(s) (comparison)

equals(s) (equality)

equalsIgnoreCase(s) (case-insensitive)

matches(regex) (regular expression matching)

Examples

Here are some examples for synthetic columns:

Average

    ( first + second ) * 0.5
    

Outlier clipping

    min( 1000, max( value, 0 ) )
    

Within a circle

    sqrt( pow($2,2) + pow($3,2) ) < 0.01
    

String matching

    CONSTELLATION.equalsIgnoreCase( "cygnus" )
    

    ( #1 && #2 ) && ! #3