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This extension provides a mathematical expression evaluator. Mathematical expressions can be passed to it as a string, and solved.  This allows mathematical expressions to be stored as strings, and be generated or altered at run-time.

Basic Usage

One-shot calculation

In the most basic usage, variables are loaded into the a new expreval, and an expression is evaluated.  The output variables can then be read.

// create expression
var eval = expreval_create();
// set up variables
expreval_write_variable(eval, "a", 123);
// evaluate expression (intermediate variables are created automatically
expreval_evaluate(eval, "b=2*a;c=sin(b)");
// grab output variables
var retval = expreval_read_variable(eval, "c");
show_debug_message("Returned value: " + string(retval));

Multiple expressions can be evaluated, by separating them with a semicolon in the expression string.

Any temporary or output variables used in the expression will be automatically created, and made available for reading.  Referencing an undefined variable will cause an error.

exprevals must be destroyed after they are no longer used to avoid a memory leak.

Re-running an expression

expreval can load an expression to be run later by using the expreval_load() and expreval_run() functions instead of expreval_evaluate().  This allows

// create expression
var eval = expreval_create();
// load expression (this loads it but does not run it yet
expreval_load(eval, "x=(-b+sqrt(b^2-4*a*c))/(2*a)");
// time expreval
for(var i=0; i<10; i++) {
 expreval_write_variable(eval, "a", 1.0);
 expreval_write_variable(eval, "b", i*2);
 expreval_write_variable(eval, "c", i);
 var retval = expreval_read_variable(eval, "x");
 show_debug_message("Returned value: " + string(retval));

Advanced usage


Creates a new expreval object in the extension.  This returns a numerical index which is used in other functions.  The expreval object will remain available for the duration of the game or until expreval_destroy() is called.  This allows the expression to be modified and evaluated multiple times without having to be recreated from scratch each time


Unloads the expreval object inside the extension.  Use this when the file is no longer needed to avoid memory leaks


Returns true or false depending on whether the provided index exists inside the extension or not.

expreval_write_variable(index, varname, value)

Adds or modifies the value of a variable inside an expression.  This can be called multiple times to update the variable.

expreval_read_variable(index, varname)

Returns the value of the variable. This is used after evaluating an expression to read the output value.  If the variable does not exist, 0 is returned.

expreval_evaluate(index, expression)

Loads and evaluates a mathematical expression.  Returns 1 if successful, returns 0 if unsuccessful. Will also output error messages in the console.

expreval_load(index, expression)

Loads a mathematical expression, but does not evaluate it immediately until exrpeval_run() is called


Runs the previously loaded expression.  Can be run after either expreval_load() or expreval_evaluate().  Returns 1 if successful, returns 0 if unsuccessful.  Will also output error messages in the console.


Expreval will output things in the console.  This sets the verbosity of expreval.  The following values are possible:

  • EXPREVAL_DEBUG_NONE: nothing will be outputted
  • EXPREVAL_DEBUG_ERROR: only errors will output
  • EXPREVAL_DEBUG_WARNING: warnings and errors (this is the default value)
  • EXPREVAL_DEBUG_INFO: info, warnings, and errors
  • EXPREVAL_DEBUG_DEBUG: everything

Available expressions

abs(x) - Absolute value of x

mod(x, y) - modulo x of y (i.e. remainder of x when divided by y)

ipart(x) - the integer part of x

fpart(x) - the fractional part of x

min(x, y, z ...- returns the smallest value of all the parameters

max(x, y, z ...- returns the largest value of all the parameters

sqrt(x) - square root of x

poly(x, ...) - calculates a polynomial, the arguments are coefficients. E.g.: poly(x, 2, 3, 4, 5) becomes 2x^3 + 3x^2 + 4x + 5


ceil(x) - round x to the nearest integer towards positive infinity

floor(x) - round x to the nearest integer towards negative infinity

clip(x, min, max) - returns x clipped to min and max (this is equivalent to GM's clamp() function)

clamp(x, min, max) - wraps x between min and max (note: this is not the same as GM's clamp() function, see above)

rescale(p, o1, o2, n1, n2) - rescales p as a fraction of range o1 and o2 to the range n1 and n2 (this is similar to GM's lerp() function but extends the lerp fraction)


sin(x), cos(x), tan(x) - trigonometric functions, in radian

asin(x), acos(x), atan(x) - inverse trigonometric functions, in radians

atan(x, y) - quadrant-aware inverse tangent function (equivalent to atan2() function in GM and other languages)

sinh(x), cosh(x), tanh(x) - hyperbolic trigonometric functions, in radians

deg(x) - converts x from radians to degrees

rad(x) - converts x from degrees to radians

rect2pol(x, y, &distance, &angle) - converts rectangular coordinates x and y to polar coordinates distance and angle. This is similar to using point_distance() and point_direction() in GM

pol2rect(distance, angle, &x, &y) - converts polar coordinates distance and angle to rectangular coordinates x and y. This is similar to using lengthdir_x() and lengthdir_y() in GM.


log(x) - base 10 logarithm of x

ln(x) - base e logarithm of x

exp(x) - e raised to the power of x

logn(x, y) - base y logarithm of x

Random numbers

rand(&seed) - random number between 0 and 1, also reads and updates seed variable (seed can be written to directly to provide a starting seed)

random(min, max, &seed) - random number between min and max, reads and updates seed variable

randomize(&seed) - randomizes the seed used

Conditionals and Boolean

if(condition, true_exp, false_exp) - provides an if statement.  If condition is non-zero, then true_exp is evaluated and returned, else false_exp is evaluated and returned

select(condition, neg_exp, zero_exp, pos_exp) - selects between neg_exp, zero_exp, pos_exp depending on whether condition is negative, zero, or positive

select(condition, neg_exp, zeropos_exp) - selects between neg_exp, zeropos_exp depending on whether condition is negative, or zero or positive.

equal(x, y) - returns 1 if x and y are equal, returns 0 if not (equivalent to x == y)

above(x, y) - returns 1 if x is greater than y (equivalent to x > y)

below(x, y) - returns 1 if x is less than y (equivalent to x < y)

and(x, y) - returns 1 if x and y evaluate to true (equivalent to x && y)

or(x, y) - returns 1 if x or y evaluate to true (equivalent to x || y)

not(x) - returns 1 if x evaluates to false (equivalent to !x)


PI - value of pi: 3.14159265358979323846

E - value of natural number e: 2.7182818284590452354

Known issues

  • expreval is about 50 times slower than VM native calculations, and probably several times that versus YYC
Rated 4.0 out of 5 stars
(1 total ratings)
Made withGameMaker: Studio
Code licenseMIT License
LinksYoYo Marketplace


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