I got this line "(123+(45 -3))*4000/-2" read in by a scanner
Scanner scan1= new Scanner(line); //scan by tokens
Scanner scan2 = new Scanner(line).useDelimiter("\\D"); //scan by numbers
The problem is that scan2 takes in the numbers and also white spaces if there are 2 or more arithmetic operators +,-,/,*, or parenthesis ( and )
How do i get rid of the white spaces being read in?
Presumably you are trying to write some sort of a expression parser. If so, you are not on the right track.
Read this thread: http://forum.java.sun.com/thread.jspa?threadID=784915
and especially marlin314's excellent contributions to get a notion of the things you need to do.
Good luck.
marlin314 explanation is indeed detailed!
But what I really need to do is rather simple. I'm required to pass an expression through a scanner and tokenise each operator and operand to be stored in an arraylist.
Unfortunately for me, the tokeniser method cannot be used because its considered a legacy class.
So far everything seems to be going just fine except passing the value 4000 will cause my method to not take in any operators after that. =(
> marlin314 explanation is indeed detailed!
> But what I really need to do is rather simple. I'm
> required to pass an expression through a scanner and
> tokenise each operator and operand to be stored in an
> arraylist.
> Unfortunately for me, the tokeniser method cannot be
> used because its considered a legacy class.
>
> So far everything seems to be going just fine except
> passing the value 4000 will cause my method to not
> take in any operators after that. =(
Something like this?import java.util.regex.*;
// ...
String expression = "(123 +(45 -3))*4000/-2";
String regex = "[0-9]+|\\+|\\*|/|-";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(expression);
while(matcher.find()) {
System.out.println(matcher.group());
}
produces the following output:/*
123
+
45
-
3
*
4000
/
-
2
*/
Almost! But I need to add them into an arraylist so each address in the arraylist will be the running order of the expression.
This is what i have now.
Scanner token = new Scanner(inLine);
while(token.hasNext()){
try{
String line = token.next();
Scanner operands = new Scanner(line).useDelimiter("\\D+");
for(int i=0; i<line.length(); i++){
char sent = line.charAt(i);
switch(sent){
case '+':
tokenLine.add("+");
break;
case '-':
tokenLine.add("-");
break;
case '/':
tokenLine.add("/");
break;
case '*':
tokenLine.add("*");
break;
case '(':
tokenLine.add("(");
break;
case ')':
tokenLine.add(")");
break;
case '=':
tokenLine.add("=");
break;
default:
String num = operands.next();
tokenLine.add(num);
break;
}
}
}catch(Exception e){}
}'>
> Almost! ...
Ah, I see you want to keep the parentheses. Try this:import java.util.*;
import java.util.regex.*;
public class Main {
public static void main(String[] args) {
String expression = "(123 +(45 -3))*4000/-2";
String regex = "[0-9]+|\\+|\\*|/|-|\\(|\\)";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(expression);
List<String> tokens = new ArrayList<String>();
while(matcher.find()) {
tokens.add(matcher.group());
}
System.out.println(tokens);
}
}
Output:/*
[(, 123, +, (, 45, -, 3, ), ), *, 4000, /, -, 2]
*/
> I swear if you're living in my state i'll send you a
> thank you card. =)
; )
> Thanks prometheuzz !!
You're welcome.
> 1 more little thing for me please.
>
> What does this line means? -> String regex = "[0-9]+|\\+|\\*|/|-|\\(|\\)";
It's pretty straight forward actually. The following "groups" get extracted from your expression String:[0-9]+one or more numbers
| OR
\\+the + char
| OR
\\*the * char
| OR
/ the / char
| OR
- the - char
| OR
\\(the ( char
| OR
\\)the ) char
the charcters +, /, ( and ) have a special meaning in the regex-language, so they need to be escaped with \\
> I like to be taught how to fish... =)
Good to hear that.
You can split up the regex for more clarity and even expand it with the introduction of decimal numbers like this:import java.util.*;
import java.util.regex.*;
public class Main {
public static void main(String[] args) {
String expression = "(123.3 +(45 -3))*4000/-2";
String integer = "[0-9]+";
String decimal = integer+"\\."+integer; // '.' is also a special regex-character!
String operator = "\\*|\\+|-|/";
String parenthesis = "\\(|\\)";
// 'decimal' needs to come before 'integer'
String regex = decimal+"|"+integer+"|"+operator+"|"+parenthesis;
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(expression);
List<String> tokens = new ArrayList<String>();
while(matcher.find()) {
tokens.add(matcher.group());
}
System.out.println(tokens);
}
}
Checkout this link:
http://java.sun.com/j2se/1.4.2/docs/api/java/util/regex/Pattern.html
