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POSplit a mathematical expression while handling negative numbers with Java
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copied!<p>I'm working on an expression calculator in Java. I decided to first write a code for conversion to postfix and then write an reverse polish notation calculator. So far my calculator works great and can handle any expression including operators <code>+ - * / %</code>.</p> <p>The problem I'm having however is that it splits the expression using a space <code>input.split(" ")</code> so this means the expression must be entered <code>( 4 + ( 2 * ( -2 - 1 ) ) ) * 1.5</code> when I it should be able to be entered <code>(4+(2*(-2-1)))*1.5</code>.</p> <p>After hours of tinkering and I now know it cant work regex but would it be able to write a for loop that loops through two tokens of the string at a time and if they both equal an operator then you can assume that the second must be a negative value. Or if the Equation starts with an operator then it must be a negative value? Iterate through the string like this until the second operator gets to the end of the expression?</p> <p>Here is some code I have been playing with trying to make a start at this but since I'm still quite new to programming I can't seem to get it to work.</p> <pre><code>String expression = "(4+(2*(-2--16)))*-1.5"; ArrayList<String> tokens = new ArrayList<String>(); String orig = null; String regex = "[-+/*()]+"; String first = Character.toString(expression.charAt(0)); tokens.add(first); for (int i = 0; i < expression.length(); i++) { char x = expression.charAt(i); String a = Character.toString(x); if (i >= 1){ //Check i is greater than or equal to 1 char y = expression.charAt(i-1); String b = Character.toString(y); if(b.matches(regex) && x == '-'){ orig = a; }else if(orig != null && orig.equals("-")){ System.out.println(orig + a); tokens.add(orig + a); orig = null; }else{ tokens.add(a); } } } for(String t:tokens){ System.out.print(t+" "); } </code></pre> <p>Thanks for any help, Ciaran.</p> <p>Edit:</p> <p>My question is how can I write a method to split a mathematical expression which while splitting can tell the difference '-' as a binary operator and '-' as a unary operator? Am I on the right lines with the idea of iterating through a string and comparing the two tokens? – Ciaran Ashton 6 mins ago </p> <p>What I am trying to achieve I want to turn <code>String expression = (4+(2*(-2-1)))</code> into <code>String[] expression = (, 4, (, 2, *, (, -2, -, 1, ), ), )</code></p>

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