#!/usr/bin/python
"""A collection of my personal
utility functions."""


def reverse(a_to_b):
    """reverse is a function to "reverse" a 
dictionary that has no duplicate values.  
It takes a dictionary as its argument.  It 
returns another dictionary whose keys are 
the values of the input dictionary, and whose
values are the corresponding keys of the
input dictionary."""

    b_to_a = {}
    for a in a_to_b:
        b = a_to_b[a]
        b_to_a[b] = a
    return b_to_a


def print_dict(dict):
    """print_dict is a function to print
the contents of a dictionary.  It takes a
dictionary as its argument.  For each key 
in the dictionary it prints the key followed 
by the value."""

    for key in dict:
        print key, dict[key]


def find_root(
    function,
    lower,
    upper,
    tolerance
    ):
    """find_root is a function that finds 
the interval in which the root of a function
lies by repeated bisection.  It takes the
following arguments:

The function name,

The lower and upper values of
    the interval in which the root lies, and

The size of the returned interval (the tolerance).

It returns an interval containing the root whose size
is less than or equal to the supplied tolerance."""

    while upper - lower > tolerance:

        middle = (lower + upper)/2.0

        if function(upper)*function(middle) > 0.0:
            upper = middle
        else:
            lower = middle

    return (lower, upper)


def file2dict(filename):
    """file2dict takes a file name as its argument.
This file must contain pairs of words.
It returns the equivalent Python dictionary."""

    import sys

    dict={}

    data = None

    try:
        data = open(filename)

        for line in data:
            try:
                [ key, value ] = line.split()
            except ValueError:
                print "File %s is not in the correct format." % filename
                dict = None
                break

            dict[key] = value

        data.close()

    except IOError, error:
	(errno, errdetails) = error
        print "Problem with file %s: %s" % (filename, errdetails)
        print "Aborting!"
	if type(data) == file:
            data.close()
        sys.exit(1)

    return dict