Prefix calculator.

Author: calebmadrigal

prefix_calc.py

@@ -0,0 +1,61 @@
+""" Prefix calculator. Takes a space-delimited string like '* ( / 10 5 ) ( + ( - 5 2 ) ( * 1 3 ) )'
+and parses it, builds an AST, and reduces the AST to a number by running specified operators. """
+
+__author__ = "Caleb Madrigal"
+__date__ = "2015-03-04"
+
+
+def calc(node):
+    if node == ():
+        return
+    (op, a, b) = node[0], node[1], node[2]
+    if type(a) == tuple:
+        a = calc(a)
+    if type(b) == tuple:
+        b = calc(b)
+    if op == '+':
+        return a + b
+    elif op == '-':
+        return a - b
+    elif op == '*':
+        return a * b
+    elif op == '/':
+        return a / b
+
+
+def build_ast(token_list):
+    if not token_list:
+        return (), ()
+    token, *rest = token_list
+    if token == '(':
+        internal, remainder = build_ast(rest)
+        internal2, remainder2 = build_ast(remainder)
+        return (internal,) + internal2, remainder2
+    elif token == ')':
+        return (), rest
+    elif token in ['+', '-', '*', '/']:
+        internal, remainder = build_ast(rest)
+        return (token,) + internal, remainder
+    else:  # Token is number
+        internal, remainder = build_ast(rest)
+        return (float(token),) + internal, remainder
+
+
+def parse(calc_str):
+    return calc_str.split()
+
+if __name__ == '__main__':
+    calc_str = '+ ( * 2 2 ) ( - ( + 3 2 ) 4 )'
+    parsed = parse(calc_str)
+    ast, _ = build_ast(parsed)
+    print("Original: '{}'".format(calc_str))
+    print("Parsed:", parsed)
+    print("AST:", ast)
+    print("Result:", calc(ast))
+
+    calc_str2 = '* ( / 10 5 ) ( + ( - 5 2 ) ( * 1 3 ) )'
+    ast, _ = build_ast(parse(calc_str2))
+    print("\nOriginal: '{}'".format(calc_str2))
+    print("AST:", ast)
+    print("Result:", calc(ast))
+