How Asterisk Works

This guide teaches you how to use expression evaluation in Asterisk, a key tool for evaluating arithmetic and logical expressions. It includes syntax, practical examples, important notes, and combined usage with variables.

Basic Syntax

$[expr1 operator expr2]

Logical Operators

These operators allow you to combine boolean conditions:

• expr1 | expr2 (Logical OR)

  • Returns expr1 if it evaluates as non-empty/non-zero ("true").
  • Otherwise, evaluates and returns expr2.

• expr1 & expr2 (Logical AND)

  • Returns expr1 if both expressions evaluate as non-empty/non-zero ("true").
  • Otherwise, returns zero ("false").

• !expr (Unary Logical NOT)

  • Note: There must be no space between ! and expr.
  • Not available in Asterisk versions earlier than 1.1.

Using Logical Operators

You can combine conditions within GotoIf or similar commands:

$[ $[ "${FOO}" = "1" ] & $[ "${BAR}" = "2" ] ]

In this example, the expression evaluates whether both variables (FOO and BAR) meet their respective values. If they do, the full expression returns 1.

Comparison Operators

These allow value comparisons to control the call flow based on conditions:

• expr1 = expr2
• expr1 != expr2
• expr1 < expr2
• expr1 > expr2
• expr1 <= expr2
• expr1 >= expr2

Important Notes

• If both arguments are integers, an integer comparison is performed.
• Otherwise, a string comparison is performed using the system’s local sort order.
• Returns 1 if true, 0 if false.

Arithmetic Operators

Asterisk also supports basic arithmetic with integers:

• expr1 + expr2: Addition
• expr1 - expr2: Subtraction
• -expr: Negation (Not available in Asterisk < 1.1)
• expr1 * expr2: Multiplication
• expr1 / expr2: Integer division
• expr1 % expr2: Modulo (remainder)

These operators only work with integer values.

Asterisk Arithmetic Functions

Functions for rounding and truncation are available and work with integers:

• FLOOR(x): Rounds down to the nearest integer.
• CEIL(x): Rounds up to the nearest integer.
• ROUND(x): Rounds to the nearest integer. If exactly halfway, it rounds away from zero.
• RINT(x): Rounds to the nearest integer, preferring the even number in case of a tie.
• TRUNC(x): Removes the decimal part, rounding toward zero.

Clarifications

• Dialplan functions can be called directly within $[...] without ${...}.

  • Example: $[3 + FLOOR(2.9)]

• Variables, however, must use ${...}.

  • Example: $[${a} + 2]

• Arguments in functions must be separated by commas , when using $[...].

Available Math Functions

Most functions from the C math library are available for more advanced calculations:

• Trigonometric (input in radians):

  • COS(x)
  • SIN(x)
  • TAN(x)

• Inverse Trigonometric (input between -1 and 1):

  • ACOS(x)
  • ASIN(x)

• ATAN(x) (returns radians between -π/2 and π/2)

• ATAN2(x, y) (returns between -π and π)

• Exponents and Logarithms:

  • POW(x, y): x to the power of y (x^y)
  • EXP(x): e to the power of x
  • EXP2(x): 2 to the power of x
  • LOG(x): Natural logarithm (base e)
  • LOG2(x): Base 2 logarithm
  • LOG10(x): Base 10 logarithm

• Others:

  • SQRT(x): Square root
  • REMAINDER(x, y): Returns the mathematical remainder (unlike %). Calculated as x - n*y where n = x/y rounded to the nearest even integer.