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C based Expression Evaluation Library

, 9 Aug 2004
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Fast, powerful, useful expression evaluation library.

Introduction

ExprEval is a C based expression evaluation library designed to be fast and powerful. It is written in ANSI compliant C to be able to work with any C/C++ compiler.

How it works

ExprEval takes an expression entered as a string and parses it down into an expression tree. This tree, once parsed, can be evaluated over and over without the need to parse it again. Variables can still be changed by the application and/or the expression.

Fast variable access

Variable access is fast within the ExprEval library. At parse time, any variables found in the expression string are added to the variable list if they are not already there. Then the memory address of the variable is stored in the tree. This way, at evaluation time, the variable can be directly accessed without needing to lookup the variable using string comparisons. Functions are also provided to enable the application to do the same thing.

The expression string

The expression string consists of one or more expressions, each terminated with a semicolon. The expressions appear much like they would on paper. An expression can do assignments, basic math operators (+, -, *, /, negate), and even call functions.

Order of operators

The parser parses the expression in a way that the order of operators should work correctly. First, parenthesis are done, then multiplication and division, then addition and then subtraction.

Assignment

Assignments can occur about anywhere in an expression. If an assignment is inside a function parameter, the function solver must evaluate that parameter for the assignment to take place.

x = sin(y = 50);

Functions

The ExprEval library provides many default functions. Functions can take normal parameters and reference parameters, depending on the function solver. Normal parameters are not pre-evaluated by the library. Instead, the function solver needs to evaluate them. This means more work for the function solver, but also more power because the function solver can evaluate and execute the normal parameters in any order and any number of times. Reference parameters are passed to the function solver as addresses to the variable being referenced. Their main purpose is to enable the function solver to 'return' more than one value by changing the value of some other variables. Normal and reference parameters can be mixed together. The only thing that matters is that the normal parameters are in a certain order, and the reference parameters are in a certain order, defined by the function solver.

x = func1(a, b, c);
//Normal parameters
x = func2(&a, &b);
//Reference parameters referencing variables a and b
x = func3(1, 2, &a, 3, &b);
//Normal and reference parameters can be mixed
x = func3(1, &a, &b, 2, 3);
//This is the same as the above
x = func3(2, &b, &a, 3, 1);
//This is not the same

Custom functions

Custom functions can easily be added to the library. The functions are written in C, and then added to the function list.

Constants

The ExprEval library also provides many constants such as M_PI, M_E, etc.

Function, variable and constant lists

Function lists, variable lists, and constant lists are all stored separately from each other so that the developer can choose how to bind them to the expression objects. You can create multiple expressions objects which share the same variables or that have there own private variables or constants or functions or any mix and match.

Sample Usage

The following is a sample of how to use ExprEval:

#include "expreval.h"

...
...

/* Breaker function to break out of long expression functions
   such as the 'for' function */
int breaker(exprObj *o)
    {
    /* Return nonzero to break out */
    return need_to_stop;
    }

/* Custom function using macros */
EXPR_FUNCTIONSOLVER(my_func)
    {
    EXPRTYPE tmp;
    int err; /* Need to declare this variable to use macros */

    EXPR_REQUIREREFCOUNT(1); /* Need 1 reference parameter */
    EXPR_REQUIRECOUNT(1); /* Need 1 normal parameter */

    /* Evaluate the first normal parameter */
    EXPR_EVALNODE(0, tmp);

    /* Set the ref parameter to the value of the normal parameter */
    *refitems[0] = tmp;

    /* Set the return value */
    *val = tmp;

    /* No error occured */
    return EXPR_ERROR_NOERROR;
    }

void DoExpression(char *expr)
    {
    exprObj *e = NULL;
    exprFuncList *f = NULL;
    exprValList *v = NULL;
    exprValList *c = NULL;
    EXPRTYPE *e_x, *e_y; /* EXPRTYPE is typedef for double */
    EXPRTYPE dummy;
    double pos;

    /* Create function list */
    err = exprFuncListCreate(&f);
    if(err != EXPR_ERROR_NOERROR)
        goto error;

    /* Init function list with internal functions */
    err = exprFuncListInit(f);
    if(err != EXPR_ERROR_NOERROR)
        goto error;

    /* Add custom function */
    err = exprFuncListAdd(f, my_func, "myfunc", 1, 1, 1, 1);
    if(err != EXPR_ERROR_NOERROR)
        goto error;

    /* Create constant list */
    err = exprValListCreate(&c);
    if(err != EXPR_ERROR_NOERROR)
        goto error;

    /* Init constant list with internal constants */
    err = exprValListInit(c);
    if(err != EXPR_ERROR_NOERROR)
        goto error;

    /* Create variable list */
    err = exprValListCreate(&v);
    if(err != EXPR_ERROR_NOERROR)
        goto error;

    /* Add variables x and y */
    exprValListAdd(v, "x", 0.0);
    exprValListAdd(v, "y", 0.0);

    exprValListGetAddress(v, "x", &e_x);
    exprValListGetAddress(v, "y", &e_y);

    if(e_x == NULL || e_y = NULL)
        goto error;

    /* Create expression object */
    err = exprCreate(&e, f, v, c, NULL, breaker, 0);
    if(err != EXPR_ERROR_NOERROR)
        goto error;

    /* Parse expression */
    err = exprParse(e, expr);
    if(err != EXPR_ERROR_NOERROR)
        goto error;

    /* Enable soft errors */
    exprSetSoftErrors(e, 1);

    /* The expression should set the variable y based on x */
    for(pos = 0.0; pos < 10.0; pos += 0.1)
        {
        /* Directly access and set variable x */
        *e_x = (EXPRTYPE)pos;

        /* Evaluate the expression */
        exprEval(e, &dummy);

        /* Do something with the results */
        do_something(*e_x, *e_y);        
        }

    goto done;

error:
    /* Alert user of error */
    printf("An error occured\n");

done:
    /* Do cleanup */
    if(e)
        exprFree(e); 

    if(f)
        exprFuncListFree(f);

    if(v)
        exprValListFree(v);

    if(c)
        exprValListFree(c);

    return;
    }

Documentation and examples

The download contains a more complete documentation and reference for the ExprEval library as well as two examples. One is basically a speed test, and the other generates an image based on a math expression.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

bavander

United States United States
No Biography provided

Comments and Discussions

 
GeneralAdditional Changes Pinmemberbavander1-Jul-06 13:41 
GeneralChanges to ExprEval Pinmemberbavander29-Dec-05 18:54 
GeneralRe: Changes to ExprEval Pinmemberccheng.g.g3-Nov-08 18:02 
GeneralError Pinmembersteboond14-Dec-05 23:21 
Questiondatatype of vaiables Pinmemberwaqqas_jabbar13-Dec-05 20:38 
Generalgood code, got my 5! PinmemberPhil. Invoker13-Nov-05 21:45 
Questionhuh? wheres the article? PinmemberTheGlynn10-Aug-04 22:40 

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