我正在寻找如何计算数学表达式的方法，这些数学表达式可以使用sin(90)和10E8之类的字符串来计算输入，直到我看到这些代码，我无法完全理解它们是如何工作的。我想将这些作为基础，因为我想改进我的MDAS计算器。
我很难理解这些代码。我不熟悉StringBuffer，StringTokenizer，Math.ceil，ans += mul();，( b.toString(), "\t" );，但我对三角函数＆amp; MDAS运作。
更新：我了解StringTokenizer是什么，但它与StringBuffer的关系是什么？
import java.util.*;
public class Expression {
String s, x;
double term() {
double ans = 0;
StringBuffer temp = new StringBuffer();
while (s.length() > 0 && Character.isDigit(s.charAt(0))) {
temp.append(Integer.parseInt("" + s.charAt(0)));
s = s.substring(1);
}
if (s.length() > 0 && s.charAt(0) == '.') {
temp.append('.');
s = s.substring(1);
while (s.length() > 0 && Character.isDigit(s.charAt(0))) {
temp.append(Integer.parseInt("" + s.charAt(0)));
s = s.substring(1);
}
}
if (s.length() > 0 && (s.charAt(0) == 'e' || s.charAt(0) == 'E')) {
temp.append('e');
s = s.substring(1);
temp.append(s.charAt(0));
s = s.substring(1);
while (s.length() > 0 && Character.isDigit(s.charAt(0))) {
temp.append(Integer.parseInt("" + s.charAt(0)));
s = s.substring(1);
}
}
ans = Double.valueOf(temp.toString()).doubleValue();
return ans;
}
double paren() {
double ans;
if (s.charAt(0) == '(') {
s = s.substring(1);
ans = add();
s = s.substring(1);
} else {
ans = term();
}
return ans;
}
double exp() {
boolean neg = false;
if (s.charAt(0) == '-') {
neg = true;
s = s.substring(1);
}
double ans = paren();
while (s.length() > 0) {
if (s.charAt(0) == '^') {
s = s.substring(1);
boolean expNeg = false;
if (s.charAt(0) == '-') {
expNeg = true;
s = s.substring(1);
}
double e = paren();
if (ans < 0) {
double x = 1;
if (Math.ceil(e) == e) {
if (expNeg)
e *= -1;
if (e == 0)
ans = 1;
else if (e > 0)
for (int i = 0; i < e; i++)
x *= ans;
else
for (int i = 0; i < -e; i++)
x /= ans;
ans = x;
} else {
ans = Math.log(-1);
}
} else if (expNeg)
ans = Math.exp(-e * Math.log(ans));
else
ans = Math.exp(e * Math.log(ans));
} else
break;
}
if (neg)
ans *= -1;
return ans;
}
double trig() {
double ans = 0;
boolean found = false;
if (s.indexOf("sin") == 0) {
s = s.substring(3);
ans = Math.sin((trig() * Math.PI) / 180);
found = true;
} else if (s.indexOf("cos") == 0) {
s = s.substring(3);
ans = Math.cos((trig() * Math.PI) / 180);
found = true;
} else if (s.indexOf("tan") == 0) {
s = s.substring(3);
ans = Math.tan((trig() * Math.PI) / 180);
found = true;
}
if (!found) {
ans = exp();
}
return ans;
}
double mul() {
double ans = trig();
if (s.length() > 0) {
while (s.length() > 0) {
if (s.charAt(0) == '*') {
s = s.substring(1);
ans *= trig();
} else if (s.charAt(0) == '/') {
s = s.substring(1);
ans /= trig();
} else
break;
}
}
return ans;
}
double add() {
double ans = mul();
while (s.length() > 0) {
if (s.charAt(0) == '+') {
s = s.substring(1);
ans += mul();
} else if (s.charAt(0) == '-') {
s = s.substring(1);
ans -= mul();
} else {
break;
}
}
return ans;
}
public double evaluate() {
s = x.intern();
double last = add();
return last;
}
public Expression(String s) {
StringBuffer b = new StringBuffer();
StringTokenizer t = new StringTokenizer(s, " ");
while (t.hasMoreElements())
b.append(t.nextToken());
t = new StringTokenizer(b.toString(), "\t");
b = new StringBuffer();
while (t.hasMoreElements())
b.append(t.nextToken());
x = b.toString();
}
public String toString() {
return x.intern();
}
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("Enter expression: ");
Expression e = new Expression(sc.nextLine());
System.out.println("\n" + e + " = " + e.evaluate() + "\n");
}
}
答案 0 :(得分：2)
该程序通常读取数学表达式的字符串表示，并解释和执行该表达式。至于您对以下内容感兴趣的Java元素：
StringBuffer是一个更有效的操作String对象的界面。
StringTokenizer(String, String)是一个将字符串分解为标记的类。在这个构造函数中，第一个参数是一个用于分解标记的字符串，第二个参数是用于创建这些标记的分隔符。
Math.ceil()返回大于或等于参数的最小（最接近负无穷大）double值，并且等于数学整数。
StringBuffer.toString()写出表示缓冲区中数据的String
\t是一个标签
+=和-=是{add / subtract}赋值运算符，它将右操作数{加/减}给左操作数并将结果赋给左操作数。 E.g。
int x = 0;
x += 2; // x is now 2
答案 1 :(得分：1)
请参阅http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuffer.html
处的StringBuffer的javadoc请参阅StringTokenize http://docs.oracle.com/javase/7/docs/api/java/util/StringTokenizer.html
的javadoc除了显而易见的行之外，我会尝试发表评论
import java.util.*;
public class Expression {
String s, x;
double term() {
double ans = 0;
StringBuffer temp = new StringBuffer(); //Efficient than simple String
while (s.length() > 0 && Character.isDigit(s.charAt(0))) { //Check if the first character is a digit
temp.append(Integer.parseInt("" + s.charAt(0))); //If true, add to temp String
s = s.substring(1);
}
if (s.length() > 0 && s.charAt(0) == '.') {
temp.append('.');
s = s.substring(1);
while (s.length() > 0 && Character.isDigit(s.charAt(0))) {
temp.append(Integer.parseInt("" + s.charAt(0)));
s = s.substring(1);
}
}
if (s.length() > 0 && (s.charAt(0) == 'e' || s.charAt(0) == 'E')) {
temp.append('e');
s = s.substring(1);
temp.append(s.charAt(0));
s = s.substring(1);
while (s.length() > 0 && Character.isDigit(s.charAt(0))) {
temp.append(Integer.parseInt("" + s.charAt(0)));
s = s.substring(1);
}
}
ans = Double.valueOf(temp.toString()).doubleValue();
return ans;
}
double paren() {
double ans;
if (s.charAt(0) == '(') {
s = s.substring(1);
ans = add();
s = s.substring(1);
} else {
ans = term();
}
return ans;
}
double exp() {
boolean neg = false;
if (s.charAt(0) == '-') {
neg = true;
s = s.substring(1);
}
double ans = paren();
while (s.length() > 0) {
if (s.charAt(0) == '^') {
s = s.substring(1);
boolean expNeg = false;
if (s.charAt(0) == '-') {
expNeg = true;
s = s.substring(1);
}
double e = paren();
if (ans < 0) {
double x = 1;
if (Math.ceil(e) == e) { //Check Math.ceil documentation at http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#ceil(double)
if (expNeg)
e *= -1;
if (e == 0)
ans = 1;
else if (e > 0)
for (int i = 0; i < e; i++)
x *= ans;
else
for (int i = 0; i < -e; i++)
x /= ans;
ans = x;
} else {
ans = Math.log(-1); //http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#log(double)
}
} else if (expNeg)
ans = Math.exp(-e * Math.log(ans));
else
ans = Math.exp(e * Math.log(ans)); //http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#exp(double)
} else
break;
}
if (neg)
ans *= -1;
return ans;
}
double trig() {
double ans = 0;
boolean found = false;
if (s.indexOf("sin") == 0) {
s = s.substring(3);
ans = Math.sin((trig() * Math.PI) / 180);
found = true;
} else if (s.indexOf("cos") == 0) {
s = s.substring(3);
ans = Math.cos((trig() * Math.PI) / 180);
found = true;
} else if (s.indexOf("tan") == 0) {
s = s.substring(3);
ans = Math.tan((trig() * Math.PI) / 180);
found = true;
}
if (!found) {
ans = exp();
}
return ans;
}
double mul() {
double ans = trig();
if (s.length() > 0) {
while (s.length() > 0) {
if (s.charAt(0) == '*') {
s = s.substring(1);
ans *= trig();
} else if (s.charAt(0) == '/') {
s = s.substring(1);
ans /= trig();
} else
break;
}
}
return ans;
}
double add() {
double ans = mul();
while (s.length() > 0) {
if (s.charAt(0) == '+') {
s = s.substring(1);
ans += mul();
} else if (s.charAt(0) == '-') {
s = s.substring(1);
ans -= mul();
} else {
break;
}
}
return ans;
}
public double evaluate() {
s = x.intern();
double last = add();
return last;
}
public Expression(String s) {
StringBuffer b = new StringBuffer();
StringTokenizer t = new StringTokenizer(s, " "); //Creates a iterable t object so you can iterate over each "word" separate by space
while (t.hasMoreElements())
b.append(t.nextToken());
t = new StringTokenizer(b.toString(), "\t");
b = new StringBuffer();
while (t.hasMoreElements())
b.append(t.nextToken());
x = b.toString();
}
public String toString() {
return x.intern();
}
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("Enter expression: ");
Expression e = new Expression(sc.nextLine());
System.out.println("\n" + e + " = " + e.evaluate() + "\n");
}
}