find_and_replace.py

# Question 7: Find and Replace

# For this question you need to define two procedures:
#  make_converter(match, replacement)
#     Takes as input two strings and returns a converter. It doesn't have
#     to make a specific type of thing. Itcan 
#     return anything you would find useful in apply_converter.
#  apply_converter(converter, string)
#     Takes as input a converter (produced by make_converter), and 
#     a string, and returns the result of applying the converter to the 
#     input string. This replaces all occurrences of the match used to 
#     build the converter, with the replacement.  It keeps doing 
#     replacements until there are no more opportunities for replacements.


def make_converter(match, replacement):
	return match,replacement


def apply_converter(converter, string):
	s=[]
	s.append(string)
	begin=0
	while True:
		record=""
		begin=s[-1].find(converter[0],0)
		if begin==-1:
			break
		end=begin+len(converter[0])
		record=s[-1][:begin]+converter[1]+s[-1][end:]
		s.append(record)
	return s[-1]


# For example,

c1 = make_converter('aa', 'a')
print apply_converter(c1, 'aaaa')
#>>> a

c = make_converter('aba', 'b')
print apply_converter(c, 'aaaaaabaaaaa')
#>>> ab

# Note that this process is not guaranteed to terminate for all inputs
# (for example, apply_converter(make_converter('a', 'aa'), 'a') would 
# run forever).