MagicPlot Manual

Plotting and nonlinear fitting software

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Formula Syntax

Formula editor is used in the following cases:

MagicPlot uses standard IEEE 754 double precision floating-point arithmetic. Double precision floating point takes 8 bytes per number and provides a relative precision of about 16 decimal digits and magnitude range from about 10-308 to about 10+308.

Syntax Highlighting

MagicPlot formula editor highlights expression syntax. It also marks matching brackets:

General Rules

Case Sensitivity

MagicPlot formula translator is generally case sensitive, i.e. you can write sin but not Sin.

Note that x and X are different variables. You can use this feature when naming Custom Equation Fit Curve parameters.

Entering Numbers

  • You can use dot (.) or comma (,) as decimal separator, and separate function arguments with a semicolon (;) in the following cases:

You can use e or E for scientific notation: 1.45e-3 or 1.45E-3.

Using Spaces and Line Breaks

You can freely insert space characters and line breaks in formula, but do not break function names, numbers, operators. You do not need to enter special characters to indicate line break.

Functions

You can see a list of all available functions and their descriptions in Functions tab in Set Column Formula window and in Help on Functions window which can be opened from menu in calculator window.

MagicPlot uses functions of Java programming language library StrictMath to evaluate sin, cos, exp, etc. These functions are available from the well-known network library netlib as a “Freely Distributable Math Library”, fdlibm package. The same library is widely used in many scientific computing applications.

Trigonometric Functions

MagicPlot supports all standard trigonometric functions (sin, cos, etc.). All angles are always measured in radians for clarity.

You can use the following functions to convert angles units:

  • deg(a) — converts angles input in radians to an equivalent measure in degrees.
  • rad(a) — converts angles input in degrees to an equivalent measure in radians.
Examples
  • sin(rad(90))
  • deg(asin(1))

Constants

The predefined constants are:

  • pi, Pi, PIπ = 3.1416… value (the ratio of circumference of a circle to its diameter).
  • ee = 2.7183… value (the base of the natural logarithms). Note: expression e^a is evaluated as exp(a).
  • nan, NaN, NANNot-a-Number value.
  • inf, Inf, infinity, Infinity — positive infinity value which may be used in some calculations. Note: write -inf for negative infinity.
  • epsmachine epsilon, gives an upper bound of the relative error due to rounding in floating point arithmetic. Note: eps = ulp(1) = 2^(-52) = 2,2204E-16. (52 is the number of bits used to store fractional part of a number.)

Boolean Logic

MagicPlot can interpret boolean logic expressions. Zero and negative values (<=0) are interpreted as false and positive values (>0) are interpreted as true similarly to C programming language. You can use simple logical operators which are described below. Use 1 as true and 0 as false.

'if' Function

The basic logical function is if(condition, a, b). If condition argument is true (greater than 0) it returns the second argument (a), else returns the third argument (b).

Examples
  • if(col(A) >= 0, col(A), -col(A)) — evaluates absolute value of column A (you can use abs(col(A)) for that, of course).
  • if(col(B) >= 0, col(B), NaN) — returns only positive values from column B. Negative values are replaced with NaN value (empty cell). You can use this expression to filter negative values if you do not want to use them in future calculations. Note that “Not-a-Number returned at row #” warning can be shown for such expression.
  • if(col(A) > 0 & col(B) > 0, max(col(A), col(B)), NaN)
  • a * if(x >= 0, x, -x) – custom fit function example for abs.

Equality Checking

You have to be careful if you need to check equality of two values. Due to inaccuracy of computer floating-point calculations the result of evaluation is always approximate. For example, result of sqrt(3)^2 is number 2.9999999999999996, not exactly 3. The expression sqrt(3)^2 == 3 is false (it returns 0). Keep in mind that for convenience MagicPlot rounds numbers when showing on the screen, so this value will be shown as 3 in table if the number of shown fractional digits in MagicPlot preferences is not big enough.

Generally, if you want to check equality of two values you need to use some equality threshold for relative difference. That is, you should compare the modulus of relative difference of two values a and b with threshold t: if(abs((a-b)/a) < t, …, …).

Examples
  • sqrt(3)^2 - 3 results something about -4,4409E-16
  • if(abs(sqrt(3)^2 - 3) / 3 < 1e-10, …, …) — checks equality of sqrt(3)^2 and 3 with a threshold of 1e-10.

Operators

Operator Description Operator Description
+ addition == equal to
- subtraction != not equal to
* multiplication < less than
/ division > greater than
^ power <= less than or equal to
| or >= greater than or equal to
& and

Operations Priority

Operators with lower precedence value are evaluated earlier. You can use brackets to change calculation sequence.

Expression is evaluated left-to-right, excluding repeated exponentiation operator ^. The ^ operator is right-associative like in Fortran language (evaluated right-to-left; note that in general case a^(b^c) ≠ (a^b)^c). Hence a^b^c is evaluated as a^(b^c).

The reason for exponentiation being right-associative is that a repeated left-associative exponentiation operation would be less useful. Multiple appearances could (and would) be rewritten with multiplication: (a^b)^c = a^(b*c).

Operations Precedence Associativity
foo() (function call) 1
^ 2 Right-to-left
- (unary minus) 3
*, / 4 Left-to-right
+, - 5 Left-to-right
<, >, <=, >= 6 Left-to-right
==, != 7 Left-to-right
& 8 Left-to-right
| 9 Left-to-right
= (assignment) 10 Left-to-right
Examples
  • 1 + 2 * 3 returns 7.
  • (1 + 2) * 3 returns 9.
  • 2*-3 returns -6.
  • -3^2 is equal to -(3^2), because ^ priority is higher than that of unary minus. The result is -9.
  • (-3)^2 returns 9.
  • 2^2^3 is equal to 2^(2^3), because ^ is right-associative operator. The result is 256.
formula.txt · Last modified: Sun Nov 8 12:21:24 2015 (external edit)