PHP实现逆波兰式

近期一个小项目需要用到公式运算， 所以就进行一些了解，以下内容均属于个人经验。

在PHP中实现公式表达式四则运算大概有两种方法:

1）使用系统函数eval

2）将表达式转换成逆波兰表达式进行计算。

<?php
//使用系统函数eval
$str = 'L*((k-J)-(C+k))/M';
$param = array('L' => 0.5, 'k' => 2, 'J' => 1, 'C' => 6, 'M' => 4);

$str2 = '';
for($i = 0; $i < strlen($str); $i++) {
	$tmp = substr($str, $i, 1);
	if (array_key_exists($tmp, $param)) {
		$str2 .= $param[$tmp];
	} else {
		$str2 .= $tmp;
	}
}
eval("\$str = $str2;");
printf('%5.2d', $str);
/*End of php*/

<?php
/**
 * math_rpn 
 *
 * 实现逆波兰式算法
 *   
 * @author   sparkHuang 260558820@qq.com
 * @version  RPN 1.0.0 
 * 
 */

class math_rpn {
	//初始的计算表达式
	private $_expression = '';
	//处理后的逆波兰表达式
	private $_rpnexp = array();
	
	//模拟栈结构的数组
	private $_stack  = array('#');
	
	//正则判断
	//private $_reg    = '/^([A-Za-z0-9\(\)\+\-\*\/])*$/';
	
	//优先级
	private $_priority = array('#' => 0, '(' => 10, '+' => 20, '-' => 20, '*' => 30, '/' => 30);
	
	//四则运算
	private $_operator = array('(', '+', '-', '*', '/', ')');
	
	public function __construct($expression) {
		$this->_init($expression);
	}
	
	private function _init($expression) {
		$this->_expression = $expression;
	}
	
	public function exp2rpn() {
		$len = strlen($this->_expression);
		
		for($i = 0; $i < $len; $i++) {
			$char = substr($this->_expression, $i, 1);
			
			if ($char == '(') {
				$this->_stack[] = $char;
				continue;
			} else if ( ! in_array($char, $this->_operator)) {
				$this->_rpnexp[] = $char;
				continue;
			} else if ($char == ')') {
				for($j = count($this->_stack); $j >= 0; $j--) {
					$tmp = array_pop($this->_stack);
					if ($tmp == "(") {
						break;	
					} else {
						$this->_rpnexp[] = $tmp;
					}
				}
				continue;
			} else if ($this->_priority[$char] <= $this->_priority[end($this->_stack)]) {
				$this->_rpnexp[] = array_pop($this->_stack);
				$this->_stack[]  = $char;
				continue;
			} else {
				$this->_stack[] = $char;
				continue;
			}
		}
		for($i = count($this->_stack); $i >= 0; $i--) {
			if (end($this->_stack) == '#') break;
			$this->_rpnexp[] = array_pop($this->_stack);	
		}
		return $this->_rpnexp;
	}
}

$expression = "(A*(B+C)-E+F)*G";
var_dump($expression);
$mathrpn = new math_rpn($expression);
var_dump($mathrpn->exp2rpn());

/*End of php*/

来源：oschina

链接：https://my.oschina.net/u/167160/blog/33052