Java 如何计算此树中的表达式?
下面是我正在使用的一个解析xml文件的示例,它将xml文件标记为树形Java 如何计算此树中的表达式?,java,xml,recursion,xml-parsing,recursive-descent,Java,Xml,Recursion,Xml Parsing,Recursive Descent,下面是我正在使用的一个解析xml文件的示例,它将xml文件标记为树形 commandList assign variable #text[a] expression-int #text[1] assign variable #text[b] expression-int #text[2] assign variable #text[c] expression-operat
commandList
assign
variable
#text[a]
expression-int
#text[1]
assign
variable
#text[b]
expression-int
#text[2]
assign
variable
#text[c]
expression-operation
operator
#text[OP_SET]
arguments
expression-variable
variable
#text[a]
expression-variable
variable
#text[b]
assign
variable
#text[d]
expression-operation
operator
#text[OP_SET]
arguments
expression-operation
operator
#text[OP_TUPLE]
arguments
expression-int
#text[1]
expression-int
#text[2]
expression-operation
operator
#text[OP_TUPLE]
arguments
expression-int
#text[3]
expression-int
#text[4]
我希望这个输入不难理解。以下是不从XML文件解析时的正常情况:
a := 1;
b := 2;
c := {1,2};
d := {(1,2),(3,4)};
等等
所有赋值对(即一个值和一个变量)都将存储在hashmap中,以便该值可以由其变量查找并在以后的表达式中使用。我将使用递归下降求值器(我想是吧?)根据语法来解析表达式
在过去的24小时里,我已经在谷歌上搜索了各种各样的东西,看到了很多基本算法的树评估器(例如2+3*8等),但还没有看到这对我的特定树是如何起作用的
到目前为止,我编写的代码只找到变量名(a、b、c、d、e等),但我无法开始考虑如何对递归进行编码,从而为哈希映射提供正确的值
public void evaluate(Node node){
HashMap<String, String> assignments = new HashMap<String, String>();
NodeList assignment = node.getChildNodes();
for (int i=0; i < assignment.getLength(); i++){ //1 to 13
Node assign = assignment.item(i);
Node variable = this.getChild(assign, 0);
Node varValNode = this.getChild(variable, 0);
String varVal = varValNode.getNodeValue();
Node expression = this.getChild(assign, 1);
假设所有值都是整数类型,则应创建一个
HashMap
来存储变量值,并将其传递给evaluate
方法:
public static void main(String[] args) {
NodeList commandList = ... // get your XML from somewhere
Map<string,Integer> vars = new HashMap<string,Integer>();
for (Node node : commandList) {
evaluate(node, vars);
}
// At this point, vars contains values of all variables assigned in commands
// from the command list
}
publicstaticvoidmain(字符串[]args){
NodeList commandList=…//从某处获取XML
Map vars=newhashmap();
用于(节点:commandList){
评估(节点、变量);
}
//此时,vars包含命令中指定的所有变量的值
//从命令列表中
}
评估应该变得相对简单:
private static Integer evaluate(Node node, Map<string,Integer> vars) {
if (node is an assignment) {
String varName = ... // get variable name from node
Node expr = ... // get the node representing expression being assigned
Integer value = evaluate(expr, vars);
vars.put(varName, value);
return value;
}
if (node is a variable reference) {
String varName = ... // get variable name from node
return vars.get(varName);
}
if (node is an integer constant) {
String constVal = ... // Get the representation from XML
return Integer.decode(constVal);
}
if (node is a binary expression) {
Node lhs = ... // Get the left-hand side expression from the node
Node rhs = ... // Get the right-hand side expression from the node
Integer lhsVal = evaluate(lhs, vars);
Integer rhsVal = evaluate(rhs, vars);
if (operator is plus) {
return new Integer(((int)lhsVal) + ((int)rhsVal));
}
if (operator is minus) {
return new Integer(((int)lhsVal) - ((int)rhsVal));
}
if (operator is multiply) {
return new Integer(((int)lhsVal) * ((int)rhsVal));
}
if (operator is divide) {
return new Integer(((int)lhsVal) / ((int)rhsVal));
}
// ... and so on
}
// ... and so on
}
私有静态整数求值(节点,映射变量){
if(节点是一个赋值){
字符串varName=…//从节点获取变量名
Node expr=…//获取表示要分配的表达式的节点
整数值=评估(expr,vars);
vars.put(varName,value);
返回值;
}
if(节点是变量引用){
字符串varName=…//从节点获取变量名
返回vars.get(varName);
}
if(节点为整数常量){
String constVal=…//从XML获取表示形式
返回整数。解码(consval);
}
if(节点是二进制表达式){
Node lhs=…//从节点获取左侧表达式
Node rhs=…//从节点获取右侧表达式
整数lhsVal=evaluate(lhs,vars);
整数rhsVal=评估(rhs,vars);
如果(操作员为加号){
返回新整数(((int)lhsVal)+((int)rhsVal));
}
if(运算符为负){
返回新整数(((int)lhsVal)-(int)rhsVal));
}
if(运算符为乘法){
返回新整数(((int)lhsVal)*((int)rhsVal));
}
if(运算符为divide){
返回新整数(((int)lhsVal)/((int)rhsVal));
}
//……等等
}
//……等等
}
我应该说,当涉及函数时,a、b、c和d后面的行确实会变得更复杂一些,如集合、元组等。不过,我会试试你给我的,它看起来是一个很好的布局!
private static Integer evaluate(Node node, Map<string,Integer> vars) {
if (node is an assignment) {
String varName = ... // get variable name from node
Node expr = ... // get the node representing expression being assigned
Integer value = evaluate(expr, vars);
vars.put(varName, value);
return value;
}
if (node is a variable reference) {
String varName = ... // get variable name from node
return vars.get(varName);
}
if (node is an integer constant) {
String constVal = ... // Get the representation from XML
return Integer.decode(constVal);
}
if (node is a binary expression) {
Node lhs = ... // Get the left-hand side expression from the node
Node rhs = ... // Get the right-hand side expression from the node
Integer lhsVal = evaluate(lhs, vars);
Integer rhsVal = evaluate(rhs, vars);
if (operator is plus) {
return new Integer(((int)lhsVal) + ((int)rhsVal));
}
if (operator is minus) {
return new Integer(((int)lhsVal) - ((int)rhsVal));
}
if (operator is multiply) {
return new Integer(((int)lhsVal) * ((int)rhsVal));
}
if (operator is divide) {
return new Integer(((int)lhsVal) / ((int)rhsVal));
}
// ... and so on
}
// ... and so on
}