449,018 Members | 870 Online Need help? Post your question and get tips & solutions from a community of 449,018 IT Pros & Developers. It's quick & easy.

# lisper learning python, ... could you please comment my first python program?

 P: n/a Hi, to extend my skills, I am learning python. I have written small program which computes math expression like "1+2*sin(y^10)/cos(x*y)" and similar, so far only + - * / ^ sin con tan sqrt are supported. But my program is quite inextensible, I have to change the code to add new functions... Could some fellow experienced pythonista give me some tips how to make my program shorter, and more extensible? to use it, try something like compute("1+x+sin(x)", {"x" : 10}), the second param is environment so variables like x or y are looked for value here... below is the code.... thanks for any tips! ----- code here ---- import sys import string import math def normalize(string): tmp = "".join([c for c in string if c != " "]) return "(" + tmp + ")" def most_nested_expression(string): start = index = 0 end = len(string) - 1 level = max_level = 0 most_nested = False for c in string: if c == "(": level += 1 if level max_level: most_nested = True max_level = level start = index elif c == ")": level -= 1 if most_nested == True: most_nested = False end = index index += 1 if level != 0: raise IOError("mismatched parens") if max_level == 0: return (0, len(string), string) else: return (start + 1, end - start - 1, string[start + 1:end]) def is_reduced_expression(string): for c in string: if c == "^" or c == "*" or c == "/" or c == "+" or c == "-": return False return True def find_first(string, operators): positions = [] for op in operators: pos = string.find(op) if pos != -1: positions += [pos] if positions == []: return None else: return min(positions) def find_operator(string): for ops in [["^"], ["*", "/"], ["+", "-"]]: pos = find_first(string, ops) if pos != None: if string[pos + 1] == "+" or string[pos + 1] == "-": return pos + 1 else: return pos return None def left_operand(string, operator_pos): left = None operator = string[operator_pos] candidates = [pos for pos in [string.rfind(op, 0, operator_pos) for op in ["(", ")", "^", "*", "/", "+", "-"]] if pos != -1] if candidates != []: left = max(candidates) if left == None: if operator == "^" or operator == "*" or operator == "/": raise IOError("invalid expression %s" % string) else: # + or - return ("0", operator_pos) else: if left + 1 == operator_pos: if operator == "+" or operator == "-": return ("0", left) else: raise IOError("invalid expression %s" % string) else: return (string[left + 1:operator_pos], left) def right_operand(string, operator_pos): right = None candidates = [pos for pos in [string.find(op, operator_pos + 1) for op in ["(", ")", "^", "*", "/", "+", "-"]] if pos != -1] if candidates == []: if operator_pos == len(string) - 1: raise IOError("invalid expression %s" % string) else: return (string[operator_pos + 1:], len(string)) else: right = min(candidates) if operator_pos + 1 == right: raise IOError("invalid expression %s" % string) else: return (string[operator_pos + 1:right], right) def function_name(string, left_paren_pos): candidates = [pos for pos in [string.rfind(op, 0, left_paren_pos) for op in ["(", "^", "*", "/", "+", "-"]] if pos != -1] if candidates == []: return (None, None) else: left = max(candidates) name = string[left + 1:left_paren_pos] fun_names = ["sin", "cos", "tan", "sqrt"] for f in fun_names: if f == name: return (left + 1, name) return (None, None) def reduce_step(string, index): (left, exp_len, exp) = most_nested_expression(string) #print "most nested %s" % exp if is_reduced_expression(exp): (left1, name) = function_name(string, left - 1) if left1 != None: return ((name, string[left:left + exp_len], None), string[0:left1] + "\$%s" % index + string[left + exp_len + 1:], True) else: return ((None, None, None), string[0:left - 1] + exp + string[left + exp_len + 1:], False) else: operator_pos = find_operator(exp) + left (left_op, left_mark) = left_operand(string, operator_pos) (right_op, right_mark) = right_operand(string, operator_pos) return ((string[operator_pos], left_op, right_op), string[0:left_mark + 1] + "\$%s" % index + string[right_mark:], True) def reduce(string): chain = [] index = 0 while string == "(": ((function, left_op, right_op), new_string, is_expr) = reduce_step(string, index) if is_expr: chain += [(function, left_op, right_op)] index += 1 string = new_string return chain def add(a, b): return a + b def sub(a, b): return a - b def mul(a, b): return a * b def div(a, b): return a / b def translate_function(fn_str): if fn_str == "+": return add elif fn_str == "-": return sub elif fn_str == "*": return mul elif fn_str == "/": return div elif fn_str == "^": return math.pow elif fn_str == "sin": return math.sin elif fn_str == "cos": return math.cos elif fn_str == "tan": return math.tan elif fn_str == "sqrt": return math.sqrt else: raise IOError("unknown function %s" % fn_str) def translate_operand(op_str): if op_str == "\$": result_idx = int(op_str[1:]) return lambda results, env: results[result_idx] else: try: value = float(op_str) return lambda results, env: value except ValueError: return lambda results, env: env[op_str] def translate(chain): res = [] for (fn_str, left_op_str, right_op_str) in chain: fn = translate_function(fn_str) left_op = translate_operand(left_op_str) if right_op_str != None: res += [(fn, [left_op, translate_operand(right_op_str)])] else: res += [(fn, [left_op])] return res def compute_value(chain, env, results): assert len(chain) == len(results) index = 0 for (fn, operand_fns) in chain: operands = [op_fn(results, env) for op_fn in operand_fns] results[index] = apply(fn, operands) index += 1 return results[index - 1] def compute(string, env): print "input: %s" % string string = normalize(string) print "normalized: %s" % string string_chain = reduce(string) print "reduced: %s" % string_chain chain = translate(string_chain) print "translated: %s" % chain return compute_value(chain, env, range(len(chain))) Aug 26 '07 #1
4 Replies

 P: n/a On Sun, 26 Aug 2007 15:56:08 +0000, neptundancer wrote: Hi, to extend my skills, I am learning python. I have written small program which computes math expression like "1+2*sin(y^10)/cos(x*y)" and similar, so far only + - * / ^ sin con tan sqrt are supported. But my program is quite inextensible, I have to change the code to add new functions... Could some fellow experienced pythonista give me some tips how to make my program shorter, and more extensible? Just a few comments at random. This is certainly not meant to be exhaustive: def normalize(string): tmp = "".join([c for c in string if c != " "]) return "(" + tmp + ")" Try this instead: def normalize(astring): return "(" + astring.replace(" ", "") + ")" def most_nested_expression(string): [snip code] if level != 0: raise IOError("mismatched parens") You raise IOError quite often, but that's a misuse of it. IOError is a subclass of EnvironmentError, and is meant to indicate (e.g.) a failed read from a disk. If you execute help(IOError) at the interactive prompt, you will see: class IOError(EnvironmentError) | I/O operation failed. I suggest a better exception to use would be ValueError, or even create your own: class MyCustomError(ValueError): pass would be a minimal example. def is_reduced_expression(string): for c in string: if c == "^" or c == "*" or c == "/" or c == "+" or c == "-": return False return True Change that to: for c in string: if c in "^*/+-": return False return True [snip] def add(a, b): return a + b def sub(a, b): return a - b def mul(a, b): return a * b def div(a, b): return a / b Replace the above four functions with: from operator import add, sub, mul from operator import truediv as div def translate_function(fn_str): if fn_str == "+": return add elif fn_str == "-": return sub elif fn_str == "*": return mul elif fn_str == "/": return div elif fn_str == "^": return math.pow elif fn_str == "sin": return math.sin elif fn_str == "cos": return math.cos elif fn_str == "tan": return math.tan elif fn_str == "sqrt": return math.sqrt else: raise IOError("unknown function %s" % fn_str) fn_map = {"+": add, "-": sub, "*": mul, "/": div, "^": math.pow, "sin": math.sin } # etc. def translate_function(fn_str): try: return fn_map[fn_str] except KeyError: raise ValueError("unknown function '%s'" % fn_str) Hope that helps. -- Steven. Aug 26 '07 #2

 P: n/a Thanks a lot for comments! I am going to fix the source according to your advices ;) Nep On Aug 26, 6:32 pm, Steven D'Aprano

 P: n/a On Aug 26, 5:56 pm, neptundan...@gmail.com wrote: Hi, to extend my skills, I am learning python. I have written small program which computes math expression like "1+2*sin(y^10)/cos(x*y)" and similar, so far only + - * / ^ sin con tan sqrt are supported. But my program is quite inextensible, I have to change the code to add new functions... Could some fellow experienced pythonista give me some tips how to make my program shorter, and more extensible? I understand that you are doing this as a learning exercise. Still, I would recommend you to have a look at the shlex module in the standard library and to pyparsing. Looking at their source code should should give you some idea. BTW, since you are familiar with Lisp, I would recommend you IPython for a better interactive experience. Michele Simionato Aug 26 '07 #4

 P: n/a On Aug 26, 7:40 pm, Michele Simionato wrote: On Aug 26, 5:56 pm, neptundan...@gmail.com wrote: Hi, to extend my skills, I am learning python. I have written small program which computes math expression like "1+2*sin(y^10)/cos(x*y)" and similar, so far only + - * / ^ sin con tan sqrt are supported. But my program is quite inextensible, I have to change the code to add new functions... Could some fellow experienced pythonista give me some tips how to make my program shorter, and more extensible? I understand that you are doing this as a learning exercise. Still, I would recommend you to have a look at the shlex module in the standard library and to pyparsing. Looking at their source code should should give you some idea. BTW, since you are familiar with Lisp, I would recommend you IPython for a better interactive experience. Michele Simionato thanks for the tip! Now I have IPython working nicely. The shell now looks a little like Christmas tree but I think it can be reduced a bit ;) the ?var and auto completion is exactly what I was looking for! Nep Aug 26 '07 #5

### This discussion thread is closed

Replies have been disabled for this discussion. 