#! /usr/bin/python
# -*- coding: utf-8 -*-
#
#
# “Recently I had a visit with my mom and we realized that the two digits that
# make up my age when reversed resulted in her age. For example, if she’s 73,
# I’m 37. We wondered how often this has happened over the years but we got
# sidetracked with other topics and we never came up with an answer.
# “When I got home I figured out that the digits of our ages have been
# reversible six times so far. I also figured out that if we’re lucky it would
# happen again in a few years, and if we’re really lucky it would happen one
# more time after that. In other words, it would have happened 8 times over all.
# So the question is, how old am I now?” def are_digits_reversed ( v1, v2 ) :
"""Accepts two strings, 2 characters long. Returns True if the two strings
are reversed, that is, character 1 from 1 string is the same as character 2 from
the other string"""
return v1[0] == v2[1] and v1[1] == v2[0] def number_instances ( diff ) :
"""Returns the number of times the digits are reversed given the differnce
in the ages"""
child = 0
age_list = []
while True :
parent = child + diff
parent_age_str = "%02d" % parent
child_age_str = "%02d" % child
if are_digits_reversed ( parent_age_str, child_age_str ) :
age_list.append(child)
if parent > 99 :
break
child += 1
return age_list print ("Diff\t#instances")
for diff in range ( 10, 70 ) : # seems unlikely that the mother would be 10...
age_list = number_instances ( diff )
if len(age_list) > 0 :
print ("%d\t%d" % ( diff, len(age_list) ))
if len(age_list) == 8 :
print ("I am currently %d years old and my parent is %d years olf" \
% ( age_list[5], age_list[5]+diff ))

Result as below:

from Thinking in Python

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