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Application Programming: Expressions and Operators |
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Variables, constants, and function results are combined into expressions using operators. The value of an expression depends on the values of the operands and the operator involved. Expressions can be combined with other expressions, variables, and constants to yield more complex expressions. In IDL, unlike FORTRAN or C, expressions can be scalar- or array-valued.
IDL has a large number of different operators. In addition to the usual operators — addition, subtraction, multiplication, division, exponentiation, relations (EQ, NE, GT, etc.), and logical arithmetic (&&, ||, ~, AND, OR, NOT, and XOR) — other operators exist to find minima, maxima, select scalars and subarrays from arrays (subscripting), and to concatenate scalars and arrays to form new arrays.
Functions, which are operators in themselves, perform operations that are usually of a more complex nature than those denoted by simple operators. Functions exist in IDL for data smoothing, shifting, transforming, evaluation of transcendental functions, and other operations.
Expressions can be arguments to functions or procedures. For example, the expression SIN(A*!PI) evaluates the variable A multiplied by the value of p, then applies the trigonometric sine function. This result can be used as an operand to form a more complex expression or as an argument to yet another function (e.g., EXP(SIN(A*!PI) evaluates esin(ap).
IDL Online Help (March 06, 2007)