[英]Efficient algorithm for expression parser (Java)
我正在嘗試設計一種可以解析字符串形式的表達式的算法。 我希望能夠從給定的表達式中提取操作數和操作。 另外,我希望算法能夠識別括號平衡。 不需要操作的優先級,因為如果有超過 1 個二進制操作,算法的輸入將包含括號。 對於一元運算,如果括號前出現“-”,則表示相應括號內的整個表達式為操作數。 例子:
-parsing "a+b" gives "a" and "b" as operands and "+" as operation.
-parsing "(a/b) - (c*v)" gives "a/b" and "c*v" as operands and "-" as operation.
-parsing "((a/(b))) - (((c)*v))" gives the same result as above
-parsing "-a" gives operand as "a" and operation as "-"
-parsing "a + (-c/v)" gives "a" and "-c/v" as operands and "+" as operation
-parsing "-(c)" gives "c" is operand and "-" as operands
-parsing "(-(c))" gives same result as above
謝謝
嘗試這個。
record Node(String name, Node left, Node right) {
@Override
public String toString() {
return "Node[" + name
+ (left != null ? ", " + left : "")
+ (right != null ? ", " + right : "") + "]";
}
}
和
static Node parse(String input) {
return new Object() {
int index = 0;
int ch() { return index < input.length() ? input.charAt(index) : -1; }
boolean eat(char expected) {
while (Character.isWhitespace(ch())) ++index;
if (ch() == expected) {
++index;
return true;
}
return false;
}
Node factor() {
Node node;
boolean minus = eat('-');
if (eat('(')) {
node = expression();
if (!eat(')'))
throw new RuntimeException("')' expected");
} else if (Character.isAlphabetic(ch())) {
node = new Node(Character.toString(ch()), null, null);
++index;
} else
throw new RuntimeException("unknown char '" + (char)ch() + "'");
if (minus) node = new Node("-", node, null);
return node;
}
Node expression() {
Node node = factor();
while (true)
if (eat('*')) node = new Node("*", node, factor());
else if (eat('/')) node = new Node("/", node, factor());
else if (eat('+')) node = new Node("+", node, factor());
else if (eat('-')) node = new Node("-", node, factor());
else break;
return node;
}
}.expression();
}
測試:
static void testParse(String input) {
System.out.printf("%-22s -> %s%n", input, parse(input));
}
public static void main(String[] args) {
testParse("a+b");
testParse("(a/b) - (c*v)");
testParse("((a/(b))) - (((c)*v))");
testParse("-a");
testParse("-a + (-c/v)");
testParse("-(c)");
testParse("(-(c))");
}
output:
a+b -> Node[+, Node[a], Node[b]]
(a/b) - (c*v) -> Node[-, Node[/, Node[a], Node[b]], Node[*, Node[c], Node[v]]]
((a/(b))) - (((c)*v)) -> Node[-, Node[/, Node[a], Node[b]], Node[*, Node[c], Node[v]]]
-a -> Node[-, Node[a]]
-a + (-c/v) -> Node[+, Node[-, Node[a]], Node[/, Node[-, Node[c]], Node[v]]]
-(c) -> Node[-, Node[c]]
(-(c)) -> Node[-, Node[c]]
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