评估数学EX pressions

要实现自己的解析器和前pression评估将针对提供一个给你使用一个链接库的替代品。一个有趣的选择将是一个很容易的嵌入式脚本语言如的Lua 。

An alternative to implementing your own parser and expression evaluator would be to link against a library that provides one for you to use. An interesting choice would be an easily embedded scripting language such as Lua.

这是简单的建立一个Lua间preTER实例，并把它传递前pressions进行评估，取回一个函数来调用，用于评估前pression。你甚至可以让用户有变量...

It is straightforward to set up a Lua interpreter instance, and pass it expressions to be evaluated, getting back a function to call that evaluates the expression. You can even let the user have variables...

下面是一个粗略的实现基于一个Lua间preTER一个简单的前pression评估的。我编这一点，并尝试了一些情况，但它肯定不应该在生产code被信任，而不注意一些错误处理等等。所有常见的警告适用于这里。

Here is a sketchy implementation of a simple expression evaluator based on a Lua interpreter. I compiled this and tried it for a few cases, but it certainly should not be trusted in production code without some attention to error handling and so forth. All the usual caveats apply here.

我编译和使用Lua从的Lua 5.1.4的Windows 测试在Windows操作系统上。在其他平台上，你必须去找Lua从你平时来源，或www.lua.org。

I compiled and tested this on Windows using Lua 5.1.4 from Lua for Windows. On other platforms, you'll have to find Lua from your usual source, or from www.lua.org.

下面是文件 le.h ：

/* Public API for the LE library.
 */
int le_init();
int le_loadexpr(char *expr, char **pmsg);
double le_eval(int cookie, char **pmsg);
void le_unref(int cookie);
void le_setvar(char *name, double value);
double le_getvar(char *name);

样品code。使用LE

下面是文件的T-le.c，展示一个简单的使用这个库。它采用其单命令行参数，加载它作为一个前pression，并与全局变量评估其中X 11级从0.0变为1.0：

Sample code using LE

Here is the file t-le.c, demonstrating a simple use of this library. It takes its single command-line argument, loads it as an expression, and evaluates it with the global variable x changing from 0.0 to 1.0 in 11 steps:

#include <stdio.h>
#include "le.h"

int main(int argc, char **argv)
{
    int cookie;
    int i;
    char *msg = NULL;

    if (!le_init()) {
    printf("can't init LE\n");
    return 1;
    }
    if (argc<2) {
    printf("Usage: t-le \"expression\"\n");
    return 1;
    }
    cookie = le_loadexpr(argv[1], &msg);
    if (msg) {
    printf("can't load: %s\n", msg);
    free(msg);
    return 1;
    }
    printf("  x    %s\n"
       "------ --------\n", argv[1]);
    for (i=0; i<11; ++i) {
    double x = i/10.;
    double y;

    le_setvar("x",x);
    y = le_eval(cookie, &msg);
    if (msg) {
        printf("can't eval: %s\n", msg);
        free(msg);
        return 1;
    }
    printf("%6.2f %.3f\n", x,y);
    }
}

下面是T-乐一些输出：

Here is some output from t-le:

E:...>t-le "math.sin(math.pi * x)"
  x    math.sin(math.pi * x)
------ --------
  0.00 0.000
  0.10 0.309
  0.20 0.588
  0.30 0.809
  0.40 0.951
  0.50 1.000
  0.60 0.951
  0.70 0.809
  0.80 0.588
  0.90 0.309
  1.00 0.000

E:...>

LE的实施

下面是 le.c ，实现Lua的防爆pression评估：

Implementation of LE

Here is le.c, implementing the Lua Expression evaluator:

#include <lua.h>
#include <lauxlib.h>

#include <stdlib.h>
#include <string.h>

static lua_State *L = NULL;

/* Initialize the LE library by creating a Lua state.
 *
 * The new Lua interpreter state has the "usual" standard libraries
 * open.
 */
int le_init()
{
    L = luaL_newstate();
    if (L) 
    luaL_openlibs(L);
    return !!L;
}

/* Load an expression, returning a cookie that can be used later to
 * select this expression for evaluation by le_eval(). Note that
 * le_unref() must eventually be called to free the expression.
 *
 * The cookie is a lua_ref() reference to a function that evaluates the
 * expression when called. Any variables in the expression are assumed
 * to refer to the global environment, which is _G in the interpreter.
 * A refinement might be to isolate the function envioronment from the
 * globals.
 *
 * The implementation rewrites the expr as "return "..expr so that the
 * anonymous function actually produced by lua_load() looks like:
 *
 *     function() return expr end
 *
 *
 * If there is an error and the pmsg parameter is non-NULL, the char *
 * it points to is filled with an error message. The message is
 * allocated by strdup() so the caller is responsible for freeing the
 * storage.
 * 
 * Returns a valid cookie or the constant LUA_NOREF (-2).
 */
int le_loadexpr(char *expr, char **pmsg)
{
    int err;
    char *buf;

    if (!L) {
    if (pmsg)
        *pmsg = strdup("LE library not initialized");
    return LUA_NOREF;
    }
    buf = malloc(strlen(expr)+8);
    if (!buf) {
    if (pmsg)
        *pmsg = strdup("Insufficient memory");
    return LUA_NOREF;
    }
    strcpy(buf, "return ");
    strcat(buf, expr);
    err = luaL_loadstring(L,buf);
    free(buf);
    if (err) {
    if (pmsg)
        *pmsg = strdup(lua_tostring(L,-1));
    lua_pop(L,1);
    return LUA_NOREF;
    }
    if (pmsg)
    *pmsg = NULL;
    return luaL_ref(L, LUA_REGISTRYINDEX);
}

/* Evaluate the loaded expression.
 * 
 * If there is an error and the pmsg parameter is non-NULL, the char *
 * it points to is filled with an error message. The message is
 * allocated by strdup() so the caller is responsible for freeing the
 * storage.
 * 
 * Returns the result or 0 on error.
 */
double le_eval(int cookie, char **pmsg)
{
    int err;
    double ret;

    if (!L) {
    if (pmsg)
        *pmsg = strdup("LE library not initialized");
    return 0;
    }
    lua_rawgeti(L, LUA_REGISTRYINDEX, cookie);
    err = lua_pcall(L,0,1,0);
    if (err) {
    if (pmsg)
        *pmsg = strdup(lua_tostring(L,-1));
    lua_pop(L,1);
    return 0;
    }
    if (pmsg)
    *pmsg = NULL;
    ret = (double)lua_tonumber(L,-1);
    lua_pop(L,1);
    return ret;
}


/* Free the loaded expression.
 */
void le_unref(int cookie)
{
    if (!L)
    return;
    luaL_unref(L, LUA_REGISTRYINDEX, cookie);    
}

/* Set a variable for use in an expression.
 */
void le_setvar(char *name, double value)
{
    if (!L)
    return;
    lua_pushnumber(L,value);
    lua_setglobal(L,name);
}

/* Retrieve the current value of a variable.
 */
double le_getvar(char *name)
{
    double ret;

    if (!L)
    return 0;
    lua_getglobal(L,name);
    ret = (double)lua_tonumber(L,-1);
    lua_pop(L,1);
    return ret;
}

说明

以上的样本包括189线code总，包括注释，空行飞溅和示范。不坏，知道如何评价一个变量的合理任意EX pressions，并具有快速功能评估的富人的标准数学函数库的在其使唤。

您拥有这一切之下的图灵完备的语言，这将是一个很容易的扩展，允许用户定义完整的功能以及评估简单的前pressions。

You have a Turing-complete language underneath it all, and it would be an easy extension to allow the user to define complete functions as well as to evaluate simple expressions.