User defined variables are defined by the user. Their names are a sequence of letters, digits, and underscores that do not begin with a digit. These variables are dynamic in that they come into existence when they are first used. Their values are either numbers or strings (a sequence of characters), or both, depending upon the type of values they are initially assigned. In CXL, any variable that is used before giving it a value is a string variable and is initialized to empty string. CXL also allows the user to define one-dimensional arrays. Two-, three- and four-dimensional arrays may be simulated. For related information, see Multidimensional Arrays.