Difference between revisions of "Media Center expression language-NEW"

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Revision as of 12:44, 16 August 2010


The J. River Media Core database engine supports Excel-style functions for use in view schemes, searches, displayed columns, and tag editing.

An expression is a mixture of text, fields, and functions. A function allows special operations to be performed. Functions are all listed with a name followed by an opening and closing parenthesis. When building expressions, the instructions you wish to pass to the function are placed inside these parenthesis, with multiple instructions separated by commas. When your expression is evaluated, any spaces entered after a comma are ignored. This allows you to compose complex expressions on multiple lines, making it easier to keep track of where you are in your expression. Occasionally, you will find that you want a space, or parenthesis character to be treated literally as part of your instructions, rather than expression syntax, and in these cases, the character is "escaped" by using a preceding forward slash. As you progress with your expression building, you will begin nesting multiple functions into a single expression; always remember that a completed expression must contain a matching number of opening and closing parenthesis in order to work reliably.

The functions available to us range from highly functional, with a broad range of applicable uses, through specialised, limited use functions that only appeal to a select group of users, to seemingly redundant functions and obscure functions that MEDIA CENTER uses internally. The functions below are listed with those with the broadest appeal at the top, and the more obscure at the bottom.

Expressions can be used in many places within MEDIA CENTER, for varying reasons. Places that you might consider using expressions include:

  • The player information bar
  • The image playback caption
  • The "Rename, Move or Copy Files" tool
  • The tag window
  • Panes (aka categories)
  • List columns
  • Thumbnail text
  • The search field
  • The library field manager (Create expression based library fields)
  • Theatre View
  • The links manager (Use expressions to format the URLs for your links)

Any given expression can only work on any given single file. It is not possible to compare a field in one file with a field in another file.


Any text between brackets [] will be replaced with the corresponding field from your library. As an example, [Artist] would be replaced by Bob Dylan for any Bob Dylan tracks. If the text between brackets doesn't match any known fields, it will be left alone. After the field name, a comma can be placed followed by a 0 or 1 for whether the field should get formatted. So, [Duration] and [Duration, 1] will give "4:02" while [Duration, 0] will give "242". This is particularly important when working with the "Format" functions, where most times you will want the evaluator to work with the raw field contents rather than the formatted contents you see in the file list.

Function and field values in expressions are not case-sensitive. Available functions with descriptions and examples are listed below.


If(...): Ask closed questions and control output dependent upon result.

Function Description Construction Examples
If() This will be the function you will likely use more than any other. It is typically used in conjunction with one or more other functions and allows you to give specific instructions depending upon whether the result is positive (1) or negative (0). The positive instruction is always given first. if(expression to test,instructions if positive,instructions if negative) if(isequal([artist],bob dylan,1),Genius,Mediocre)

Wherever this expression is applied, be it as an expression column or category, as part of a renaming rule, thumbnail text, if the artist tag is Bob Dylan, MEDIA CENTER will produce Genius, and for all other artists, it will produce Mediocre. The "IsEqual()" function is described in the first table below.

if(isequal([artist],bob dylan,1),Genius,if(isequal([album],Joshua Tree,8),Great Album,Mediocre))

Here, we have two nested "If" functions. First we ask if the artist is Bob Dylan, if the result is positive, write "Genius". If the artist is not Dylan, we then ask if the album is "Joshua Tree", if yes, write "Great Album", and if no, write mediocre

On it's own, "If()" is rather boring and not a lot of use. When used in conjunction with other functions, however, it becomes very powerful indeed. In the two examples above, the numbers 1 and 8 have been used. These are comparison mode switches used by the IsEqual function. There are 9 different compare modes in total, all of which are detailed below. Note how the second example ends with two closing parenthesis, one for each "If" function.

Query, Test and Compare using "Is" Functions

All of the functions in this section, when used on their own, will only ever return one of two values, 1 (true) or 0 (false). There is a "FormatBoolean()" function that can be wrapped around these and allows us to specify the output for each case and is discussed in more depth in the Format section. The real power and versatility of these "Is" functions is released when they are wrapped inside the If() function discussed above.

Function Description Construction Examples
IsEqual() Compares two values in one of nine specified modes ("exact match", "Is greater than" etc.) and outputs 1 for a positive match and 0 for a negative match. If no compare mode is specified, the compare will default to 0. isequal(1st value to compare,2nd value to compare,compare mode)

Available Compare Modes:

  • 0: case-sensitive string compare for equality
  • 1: case-insensitive string compare for equality
  • 2: numeric compare for equality
  • 3: numeric less than
  • 4: numeric less than or equal to
  • 5: numeric greater than
  • 6: numeric greater than or equal to
  • 7: substring search (case sensitive)
  • 8: substring search (case insensitive)

If the 'artist' and 'album' values are the same, the output will be 1, otherwise, the output will be 0.

if(isequal([artist],[album],1),Self Titled,[album])

Wrapped inside an 'If' function, if the 'artist' and 'album' values are the same, the output will be 'Self Titled', otherwise, the output will be the 'album' value.

if(isequal([artist],[album],1),Self Titled/,,[album]/))

This example demonstrates the character 'escaping' mentioned in the overview. Here, we want the output to be either "Self Titled," (note the inclusion of the comma) or the album value with a closing parenthesis. In order to achieve this, the comma, and the closing parenthesis, are escaped using a preceding forward-slash character. This informs the expression evaluator that these characters are not part of the expression syntax and are to be treated literally

IsEmpty() Tests any given field for data. If the field is empty, the function returns 1, and if populated, the function returns 0. There are two different test modes available, a "string" test, and a "number" test. This is because as far as MEDIA CENTER is concerned, fields designated as containing numerical values that are populated with the number zero, are empty. If no test mode is specified, the function will default to 0.

Pay particular attention to the third example offered on the right, as it covers a caveat that comes with this particular function.

isempty([field to test],test mode)

Available test modes:

  • 0: String test (field must be empty to get a positive result)
  • 1: Numerical test (field must be empty, or contain 0 to get a positive result)

If the comment field is empty, this expression will return 1, but if the comment field contains data, the expression will return 0.

isempty([track #],1)

This expression will perform a numerical test for data in the [track #] field. If the field is empty, or contains 0, the expression will return a positive result, 1, otherwise, the result will be negative, 0.

if(isempty([disc #]),,[disc #])

Here, by not entering any instructions to carry out for a positive result, the expression will do just that, and output nothing, but if the [disc #] field does contain data, the expression will write that out. Upon first look, you would think that this could work really well in the "Rename, Move and Copy Files" tool for only creating a \Disc #\ directory if needed. This will not work. When asking the rename tool to create directories from field data, it is smart enough to know that directories with no name are illegal in Windows, so when it encounters an empty field, it automatically converts it to "Unknown [Field Name]". It does this before the expression engine gets to see it, which means that the IsEmpty function will always return a negative result when used in the rename tool, even when the specified field is indeed empty. As we now know that empty fields are passed through the rename tool as "Unknown [field name]" we can use if(isequal([disc #],unknown,8),,[disc #]) to achieve the desired result. Note that this only applies when using the rename files tool.

IsRange() IsRange allows us to test if any given field falls inside any given range of values. B C
IsMissing() A B C
IsRemovable() A B C
IsInPlayingNow() A B C
IsPlaying() A B C

Change how existing data is displayed using Format functions

Function Description Construction Examples
FormatDate() A B C
FormatNumber() A B C
FormatDuration() A B C
FormatFilesize() A B C
FormatRange() A B C
FormatBoolean() A B C