我正在尝试编写一个java程序，该程序将两个方程式f（x）和g（x）作为JTextFields中用户的输入。但是，我无法弄清楚我应该如何接受字符串输入并将其转换为可行的等式。有人可以帮我这个吗？我真的无法理解。
如果有人想知道我试图从用户输入中绘制Euler方法，但为了做到这一点，我需要接受一个初始点和一个h值，这很容易，然后我需要这两个方程f（ x）和g（x）来自用户输入，形成系统F（x）。
感谢。
答案 0 :(得分：0)
取自Java Notes 6.0网站，可解决类似问题：
/*
An object belonging to the class Expr is a mathematical expression that
can involve:
-- real numbers such as 2.7, 3, and 12.7e-12
-- the variable x
-- arithmetic operators +, -, *, /, and ^ , where
the last of these represents raising to a power
-- the mathematical functions sin, cos, tan, sec, csc, cot,
arcsin, arccos, arctan, exp, ln, log2, log10, abs, and sqrt,
where ln is the natural log, log2 is the log to the base 2,
log10 is the log to the base 10, abs is absolute value,
and sqrt is the square root
-- parentheses
Some examples would be: x^2 + x + 1
sin(2.3*x-7.1) - cos(7.1*x-2.3)
x - 1
42
exp(x^2) - 1
The trigonometric functions work with radians, not degrees. The parameter
of a function must be enclosed in parentheses, so that "sin x" is NOT allowed.
An Expr is constructed from a String that contains its definition. If an
error is found in this definition, the constructor will throw an
IllegalArgumentException with a message that describes the error and
the position of the error in the string. After an Expr has been
constructed, its defining string can be retrieved using the
method getDefinition().
The main operation on an Expr object is to find its value, given
a value for the variable x. The value is computed by the value() method.
If the expression is undefined at the given value of x, then the
number returned by value() method will be the special "non-a-number number",
Double.NaN. (The boolean-valued function Double.isNaN(v) can be used to
test whether the double value v is the special NaN value. For technical
reasons, you can't just use the == operator to test for this value.)
*/
public class Expr {
//----------------- public interface ---------------------------------------
/**
* Construct an expression, given its definition as a string.
* This will throw an IllegalArgumentException if the string
* does not contain a legal expression.
*/
public Expr(String definition) {
parse(definition);
}
/**
* Computes the value of this expression, when the variable x
* has a specified value. If the expression is undefined
* for the specified value of x, then Double.NaN is returned.
* @param x the value to be used for the variable x in the expression
* @return the computed value of the expression
*/
public double value(double x) {
return eval(x);
}
/**
* Return the original definition string of this expression. This
* is the same string that was provided in the constructor.
*/
public String toString() {
return definition;
}
//------------------- private implementation details ----------------------------------
private String definition; // The original definition of the expression,
// as passed to the constructor.
private byte[] code; // A translated version of the expression, containing
// stack operations that compute the value of the expression.
private double[] stack; // A stack to be used during the evaluation of the expression.
private double[] constants; // An array containing all the constants found in the expression.
private static final byte // values for code array; values >= 0 are indices into constants array
PLUS = -1, MINUS = -2, TIMES = -3, DIVIDE = -4, POWER = -5,
SIN = -6, COS = -7, TAN = -8, COT = -9, SEC = -10,
CSC = -11, ARCSIN = -12, ARCCOS = -13, ARCTAN = -14, EXP = -15,
LN = -16, LOG10 = -17, LOG2 = -18, ABS = -19, SQRT = -20,
UNARYMINUS = -21, VARIABLE = -22;
private static String[] functionNames = { // names of standard functions, used during parsing
"sin", "cos", "tan", "cot", "sec",
"csc", "arcsin", "arccos", "arctan", "exp",
"ln", "log10", "log2", "abs", "sqrt" };
private double eval(double variable) { // evaluate this expression for this value of the variable
try {
int top = 0;
for (int i = 0; i < codeSize; i++) {
if (code[i] >= 0)
stack[top++] = constants[code[i]];
else if (code[i] >= POWER) {
double y = stack[--top];
double x = stack[--top];
double ans = Double.NaN;
switch (code[i]) {
case PLUS: ans = x + y; break;
case MINUS: ans = x - y; break;
case TIMES: ans = x * y; break;
case DIVIDE: ans = x / y; break;
case POWER: ans = Math.pow(x,y); break;
}
if (Double.isNaN(ans))
return ans;
stack[top++] = ans;
}
else if (code[i] == VARIABLE) {
stack[top++] = variable;
}
else {
double x = stack[--top];
double ans = Double.NaN;
switch (code[i]) {
case SIN: ans = Math.sin(x); break;
case COS: ans = Math.cos(x); break;
case TAN: ans = Math.tan(x); break;
case COT: ans = Math.cos(x)/Math.sin(x); break;
case SEC: ans = 1.0/Math.cos(x); break;
case CSC: ans = 1.0/Math.sin(x); break;
case ARCSIN: if (Math.abs(x) <= 1.0) ans = Math.asin(x); break;
case ARCCOS: if (Math.abs(x) <= 1.0) ans = Math.acos(x); break;
case ARCTAN: ans = Math.atan(x); break;
case EXP: ans = Math.exp(x); break;
case LN: if (x > 0.0) ans = Math.log(x); break;
case LOG2: if (x > 0.0) ans = Math.log(x)/Math.log(2); break;
case LOG10: if (x > 0.0) ans = Math.log(x)/Math.log(10); break;
case ABS: ans = Math.abs(x); break;
case SQRT: if (x >= 0.0) ans = Math.sqrt(x); break;
case UNARYMINUS: ans = -x; break;
}
if (Double.isNaN(ans))
return ans;
stack[top++] = ans;
}
}
}
catch (Exception e) {
return Double.NaN;
}
if (Double.isInfinite(stack[0]))
return Double.NaN;
else
return stack[0];
}
private int pos = 0, constantCt = 0, codeSize = 0; // data for use during parsing
private void error(String message) { // called when an error occurs during parsing
throw new IllegalArgumentException("Parse error: " + message + " (Position in data = " + pos + ".)");
}
private int computeStackUsage() { // call after code[] is computed
int s = 0; // stack size after each operation
int max = 0; // maximum stack size seen
for (int i = 0; i < codeSize; i++) {
if (code[i] >= 0 || code[i] == VARIABLE) {
s++;
if (s > max)
max = s;
}
else if (code[i] >= POWER)
s--;
}
return max;
}
private void parse(String definition) { // Parse the definition and produce all
// the data that represents the expression
// internally; can throw IllegalArgumentException
if (definition == null || definition.trim().equals(""))
error("No data provided to Expr constructor");
this.definition = definition;
code = new byte[definition.length()];
constants = new double[definition.length()];
parseExpression();
skip();
if (next() != 0)
error("Extra data found after the end of the expression.");
int stackSize = computeStackUsage();
stack = new double[stackSize];
byte[] c = new byte[codeSize];
System.arraycopy(code,0,c,0,codeSize);
code = c;
double[] A = new double[constantCt];
System.arraycopy(constants,0,A,0,constantCt);
constants = A;
}
private char next() { // return next char in data or 0 if data is all used up
if (pos >= definition.length())
return 0;
else
return definition.charAt(pos);
}
private void skip() { // skip over white space in data
while(Character.isWhitespace(next()))
pos++;
}
// remaining routines do a standard recursive parse of the expression
private void parseExpression() {
boolean neg = false;
skip();
if (next() == '+' || next() == '-') {
neg = (next() == '-');
pos++;
skip();
}
parseTerm();
if (neg)
code[codeSize++] = UNARYMINUS;
skip();
while (next() == '+' || next() == '-') {
char op = next();
pos++;
parseTerm();
code[codeSize++] = (op == '+')? PLUS : MINUS;
skip();
}
}
private void parseTerm() {
parseFactor();
skip();
while (next() == '*' || next() == '/') {
char op = next();
pos++;
parseFactor();
code[codeSize++] = (op == '*')? TIMES : DIVIDE;
skip();
}
}
private void parseFactor() {
parsePrimary();
skip();
while (next() == '^') {
pos++;
parsePrimary();
code[codeSize++] = POWER;
skip();
}
}
private void parsePrimary() {
skip();
char ch = next();
if (ch == 'x' || ch == 'X') {
pos++;
code[codeSize++] = VARIABLE;
}
else if (Character.isLetter(ch))
parseWord();
else if (Character.isDigit(ch) || ch == '.')
parseNumber();
else if (ch == '(') {
pos++;
parseExpression();
skip();
if (next() != ')')
error("Exprected a right parenthesis.");
pos++;
}
else if (ch == ')')
error("Unmatched right parenthesis.");
else if (ch == '+' || ch == '-' || ch == '*' || ch == '/' || ch == '^')
error("Operator '" + ch + "' found in an unexpected position.");
else if (ch == 0)
error("Unexpected end of data in the middle of an expression.");
else
error("Illegal character '" + ch + "' found in data.");
}
private void parseWord() {
String w = "";
while (Character.isLetterOrDigit(next())) {
w += next();
pos++;
}
w = w.toLowerCase();
for (int i = 0; i < functionNames.length; i++) {
if (w.equals(functionNames[i])) {
skip();
if (next() != '(')
error("Function name '" + w + "' must be followed by its parameter in parentheses.");
pos++;
parseExpression();
skip();
if (next() != ')')
error("Missing right parenthesis after parameter of function '" + w + "'.");
pos++;
code[codeSize++] = (byte)(SIN - i);
return;
}
}
error("Unknown word '" + w + "' found in data.");
}
private void parseNumber() {
String w = "";
while (Character.isDigit(next())) {
w += next();
pos++;
}
if (next() == '.') {
w += next();
pos++;
while (Character.isDigit(next())) {
w += next();
pos++;
}
}
if (w.equals("."))
error("Illegal number found, consisting of decimal point only.");
if (next() == 'E' || next() == 'e') {
w += next();
pos++;
if (next() == '+' || next() == '-') {
w += next();
pos++;
}
if (! Character.isDigit(next()))
error("Illegal number found, with no digits in its exponent.");
while (Character.isDigit(next())) {
w += next();
pos++;
}
}
double d = Double.NaN;
try {
d = Double.valueOf(w).doubleValue();
}
catch (Exception e) {
}
if (Double.isNaN(d))
error("Illegal number '" + w + "' found in data.");
code[codeSize++] = (byte)constantCt;
constants[constantCt++] = d;
}
} // end class Expr