Python equivalent to C strtod

Question

I am working on converting parts of a C++ program to Python, but I have some trouble replacing the C function strtod. The strings I'm working on consists of simple mathmatical-ish equations, such as "KM/1000.0". The problem is that the both constants and numbers are mixed and I'm therefore unable to use float().

How can a Python function be written to simulate strtod which returns both the converted number and the position of the next character?

Answer 1

I'm not aware of any existing functions that would do that.

However, it's pretty easy to write one using regular expressions:

import re

# returns (float,endpos)
def strtod(s, pos):
  m = re.match(r'[+-]?\d*[.]?\d*(?:[eE][+-]?\d+)?', s[pos:])
  if m.group(0) == '': raise ValueError('bad float: %s' % s[pos:])
  return float(m.group(0)), pos + m.end()

print strtod('(a+2.0)/1e-1', 3)
print strtod('(a+2.0)/1e-1', 8)

A better overall approach might be to build a lexical scanner that would tokenize the expression first, and then work with a sequence of tokens rather than directly with the string (or indeed go the whole hog and build a yacc-style parser).

Answer 2

You can create a simple C strtod wrapper:

#include <stdlib.h>

double strtod_wrap(const char *nptr, char **endptr)
{
   return strtod(nptr, endptr);
}

compile with:

gcc -fPIC -shared -o libstrtod.dll strtod.c

(if you're using Python 64 bit, the compiler must be 64-bit as well)

and call it using ctypes from python (linux: change .dll to .so in the lib target and in the code below, this was tested on Windows):

import ctypes

_strtod = ctypes.CDLL('libstrtod.dll')
_strtod.strtod_wrap.argtypes = (ctypes.c_char_p, ctypes.POINTER(ctypes.c_char_p))
_strtod.strtod_wrap.restype = ctypes.c_double

def strtod(s):
    p = ctypes.c_char_p(0)
    s = ctypes.create_string_buffer(s.encode('utf-8'))
    result = _strtod.strtod_wrap(s, ctypes.byref(p))
    return result,ctypes.string_at(p)

print(strtod("12.5hello"))

prints:

(12.5, b'hello')

(It's not as hard as it seems, since I learned how to do that just 10 minutes ago)

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