variable_string_from_math_expression

Define a variable from a mathematical expression.

⚙️ Compatible targets: Linux

Parameters

Name	Documentation
prefix	The prefix of the variable name. This parameter is required.
name	The variable to define, the full name will be prefix.name. This parameter is required.
expression	The mathematical expression to evaluate. This parameter is required.
format	The format string to use. This parameter is required.

Outcome conditions

You need to replace ${name} with its actual canonified value.

✅ Ok: variable_string_from_math_expression_${name}_ok
- ☑️ Already compliant: variable_string_from_math_expression_${name}_kept
- 🟨 Repaired: variable_string_from_math_expression_${name}_repaired
❌ Error: variable_string_from_math_expression_${name}_error

Example

method: variable_string_from_math_expression
params:
  prefix: VALUE
  name: VALUE
  expression: VALUE
  format: VALUE

Documentation

To use the generated variable, you must use the form ${prefix.name} with each name replaced with the parameters of this method.

Be careful that using a global variable can lead to unpredictable content in case of multiple definition, which is implicitly the case when a technique has more than one instance (directive). Please note that only global variables are available within templates.

Usage

This function will evaluate a mathematical expression that may contain variables and format the result according to the provided format string.

The formatting string uses the standard POSIX printf format.

Supported mathematical expressions

All the mathematical computations are done using floats.

The supported infix mathematical syntax, in order of precedence, is:

( and ) parentheses for grouping expressions
^ operator for exponentiation
* and / operators for multiplication and division
% operators for modulo operation
+ and - operators for addition and subtraction
== "close enough" operator to tell if two expressions evaluate to the same number, with a tiny margin to tolerate floating point errors. It returns 1 or 0.
>= "greater or close enough" operator with a tiny margin to tolerate floating point errors. It returns 1 or 0.
> "greater than" operator. It returns 1 or 0.
<= "less than or close enough" operator with a tiny margin to tolerate floating point errors. It returns 1 or 0.
< "less than" operator. It returns 1 or 0.

The numbers can be in any format acceptable to the C scanf function with the %lf format specifier, followed by the k, m, g, t, or p SI units. So e.g. -100 and 2.34m are valid numbers.

In addition, the following constants are recognized:

e: 2.7182818284590452354
log2e: 1.4426950408889634074
log10e: 0.43429448190325182765
ln2: 0.69314718055994530942
ln10: 2.30258509299404568402
pi: 3.14159265358979323846
pi_2: 1.57079632679489661923 (pi over 2)
pi_4: 0.78539816339744830962 (pi over 4)
1_pi: 0.31830988618379067154 (1 over pi)
2_pi: 0.63661977236758134308 (2 over pi)
2_sqrtpi: 1.12837916709551257390 (2 over square root of pi)
sqrt2: 1.41421356237309504880 (square root of 2)
sqrt1_2: 0.70710678118654752440 (square root of 1/2)

The following functions can be used, with parentheses:

ceil and floor: the next highest or the previous highest integer
log10, log2, log
sqrt
sin, cos, tan, asin, acos, atan
abs: absolute value
step: 0 if the argument is negative, 1 otherwise

Formatting options

The format field supports the following specifiers:

%d for decimal integer
%x for hexadecimal integer
%o for octal integer
%f for decimal floating point

You can use usual flags, width and precision syntax.

Examples

If you use:

variable_string("prefix", "var", "10");
variable_string_from_math_expression("prefix", "sum", "2.0+3.0", "%d");
variable_string_from_math_expression("prefix", "product", "3*${prefix.var}", "%d");

The prefix.sum string variable will contain 5 and prefix.product will contain 30.

Rudder techniques