# 21.13: Exercises

Exercise $$\PageIndex{1}$$

A string slice can take a third index that specifies the “step size;” that is, the number of spaces between successive characters. A step size of 2 means every other character; 3 means every third, etc.

>>> fruit = 'banana'
>>> fruit[0:5:2]
'bnn'


A step size of -1 goes through the word backwards, so the slice [::-1] generates a reversed string.

Use this idiom to write a one-line version of is_palindrome from Exercise 6.

Exercise $$\PageIndex{2}$$

The following functions are all intended to check whether a string contains any lowercase letters, but at least some of them are wrong. For each function, describe what the function actually does (assuming that the parameter is a string).

def any_lowercase1(s):
for c in s:
if c.islower():
return True
else:
return False

def any_lowercase2(s):
for c in s:
if 'c'.islower():
return 'True'
else:
return 'False'

def any_lowercase3(s):
for c in s:
flag = c.islower()
return flag

def any_lowercase4(s):
flag = False
for c in s:
flag = flag or c.islower()
return flag

def any_lowercase5(s):
for c in s:
if not c.islower():
return False
return True

Exercise $$\PageIndex{3}$$

ROT13 is a weak form of encryption that involves “rotating” each letter in a word by 13 places. To rotate a letter means to shift it through the alphabet, wrapping around to the beginning if necessary, so ’A’ shifted by 3 is ’D’ and ’Z’ shifted by 1 is ’A’.

Write a function called rotate_word that takes a string and an integer as parameters, and that returns a new string that contains the letters from the original string “rotated” by the given amount.

For example, “cheer” rotated by 7 is “jolly” and “melon” rotated by -10 is “cubed”.

You might want to use the built-in functions ord, which converts a character to a numeric code, and chr, which converts numeric codes to characters.

Potentially offensive jokes on the Internet are sometimes encoded in ROT13. If you are not easily offended, find and decode some of them.

Solution:

http://thinkpython.com/code/rotate.py.