First commit

math_exp_parser.py

@@ -0,0 +1,226 @@
+# A really simple expression evaluator supporting the 
+# four basic math functions, parentheses, and variables. 
+from math import *
+
+class Parser:
+    def __init__(self, string, vars={}):
+        self.string = string
+        self.index = 0
+        self.vars = {
+            'pi' : 3.141592653589793,
+            'e' : 2.718281828459045
+        }
+
+        for var in vars.keys():
+            if self.vars.get(var) != None:
+                raise Exception(
+                        "Cannot redefine the value of " + var
+                )
+            self.vars[var] = vars[var]
+
+    def getValue(self):
+        value = self.parseExpression()
+        self.skipWhitespace()
+
+        if self.hasNext():
+            raise Exception(
+                "Unexpected character found: '" + self.peek() + "' at index " + str(self.index)
+            )
+        return value
+
+    def peek(self):
+        return self.string[self.index:self.index + 1]
+
+    def hasNext(self):
+        return self.index < len(self.string)
+
+    def skipWhitespace(self):
+        while self.hasNext():
+            if self.peek() in ' \t\n\r':
+                self.index += 1
+            else:
+                return
+
+    def parseExpression(self):
+        return self.parseAddition()
+
+    def parseAddition(self):
+        values = [self.parseMultiplication()]
+
+        while True:
+            self.skipWhitespace()
+            char = self.peek()
+
+            if char == '+':
+                self.index += 1
+                values.append(self.parseMultiplication())
+            elif char == '-':
+                self.index += 1
+                values.append(-1 * self.parseMultiplication())
+            else:
+                break
+
+        return sum(values)
+
+    def parseMultiplication(self):
+        values = [self.parseParenthesis()]
+
+        while True:
+            self.skipWhitespace()
+            char = self.peek()
+
+            if char == '*':
+                self.index += 1
+                values.append(self.parseParenthesis())
+            elif char == '/':
+                div_index = self.index
+                self.index += 1
+                denominator = self.parseParenthesis()
+
+                if denominator == 0:
+                    raise Exception(
+                        "Division by 0 kills baby whales (occured at index " + str(div_index) + ")"
+                    )
+                values.append(1.0 / denominator)
+            else:
+                break
+
+        value = 1.0
+
+        for factor in values:
+            value *= factor
+        return value
+
+    def parseParenthesis(self):
+        self.skipWhitespace()
+        char = self.peek()
+
+        if char == '(':
+            self.index += 1
+            value = self.parseExpression()
+            self.skipWhitespace()
+
+            if self.peek() != ')':
+                raise Exception(
+                    "No closing parenthesis found at character " + str(self.index)
+                )
+            self.index += 1
+            return value
+        else:
+            return self.parseNegative()
+
+    def parseNegative(self):
+        self.skipWhitespace()
+        char = self.peek()
+
+        if char == '-':
+            self.index += 1
+            return -1 * self.parseParenthesis()
+        else:
+            return self.parseValue()
+
+    def parseValue(self):
+        self.skipWhitespace()
+        char = self.peek()
+
+        if char in '0123456789.':
+            return self.parseNumber()
+        else:
+            return self.parseVariable()
+
+    def parseVariable(self):
+        self.skipWhitespace()
+        var = ''
+        functions = ['math','abs','acos' 'asin','atan','atan2','ceil','cos','cosh','degrees','exp','fabs','floor','fmod','frexp','hypot','ldexp','log','log10','modf','pow','radians','sin','sinh','sqrt','tan','tanh']
+
+        while self.hasNext():
+            char = self.peek()
+
+            if char.lower() in '_abcdefghijklmnopqrstuvwxyz0123456789':
+                var += char
+                self.index += 1
+            else:
+                break
+
+        if len(var) < 3:
+            value = self.vars.get(var, None)
+
+            if value == None:
+                raise Exception(
+                    "Unrecognized variable: '" + var + "'"
+                )
+            return float(value)
+        else:
+            if var in functions:
+                value = self.parseParenthesis()
+                value = eval(var+"("+str(value)+")")
+            else:
+                raise Exception(
+                         "Unrecognized function: '" + var + "'"
+                )
+
+            return float(value)
+
+    def parseNumber(self):
+        self.skipWhitespace()
+        strValue = ''
+        decimal_found = False
+        char = ''
+
+        while self.hasNext():
+            char = self.peek()            
+
+            if char == '.':
+                if decimal_found:
+                    raise Exception(
+                        "Found an extra period in a number at character " + str(self.index) + ". Are you European?"
+                    )
+                decimal_found = True
+                strValue += '.'
+            elif char in '0123456789':
+                strValue += char
+            else:
+                break
+            self.index += 1
+
+        if len(strValue) == 0:
+            if char == '':
+                raise Exception("Unexpected end found")
+            else:
+                raise Exception(
+                    "I was expecting to find a number at character " + str(self.index) + " but instead I found a '" + char + "'. What's up with that?")
+
+        return float(strValue)
+
+def evaluate(expression, vars={}):
+    try:
+        p = Parser(expression, vars)
+        value = p.getValue()
+    except Exception as (ex):
+        msg = ex.message
+        raise Exception(msg)
+
+    # Return an integer type if the answer is an integer 
+    if int(value) == value:
+        return int(value)
+
+    # If Python made some silly precision error 
+    # like x.99999999999996, just return x+1 as an integer 
+    epsilon = 0.0000000001
+    if int(value + epsilon) != int(value):
+        return int(value + epsilon)
+    elif int(value - epsilon) != int(value):
+        return int(value)
+
+    return value
+
+
+print evaluate("cos(x+4*3) + 2 * 3", { 'x': 5  })
+print evaluate("exp(0)")
+print evaluate("-(1 + 2) * 3")
+print evaluate("(1-2)/3.0 + 0.0000")
+print evaluate("abs(-2) + pi / 4")
+print evaluate("(x + e * 10) / 10", { 'x' : 3 })
+print evaluate("1.0 / 3 * 6")
+print evaluate("(1 - 1 + -1) * pi")
+print evaluate("cos(pi) * 1")