15-110 Spring 2011 Homework 3 Practice

These are optional practice problems to help you prepare for hw3.  Solutions are provided, along with short video tutorials.

Here are the files you'll need:

• hw3-practice.py:  starter code (download this and replace the stubs with your answers)
• hw3-practice-solutions.py:  sample solutions (note that there are many other perfectly valid solutions!)
• hw3-practice-videos.html:  short video tutorials explaining the sample solutions, and a brief overview of how to get started on hw3.

Note:  This week, you may use "if" statements (conditionals), and "for" or "while" statements (loops), but you may not use recursion, or collections (lists, dictionaries, etc).  Since we have not covered these yet, this should not be a concern for most of you. 

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1. maxChar
2. evenChars
3. reverseString
4. vowelCount
   

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1. maxChar
   Without using max(s), return the character with the maximum Unicode value in the string s.  If s has no characters, return None.  Here are some test cases for you:
   
       assert(maxChar("abcdefabcd") == "f")
       assert(maxChar("abcdefabfcd") == "f")
       assert(maxChar("ABDFDSFA") == "S")
       assert(maxChar("ABDFqDSFA") == "q")
       assert(maxChar("ABD{FqDSFA98754") == "{")
       assert(maxChar("") == None)
   
   
2. evenChars
   Write a function that takes a string s and returns the string formed by the characters at even indexes of s.  Here are some test cases for you:
   
       assert(evenChars("abcdefg") == "aceg")
       assert(evenChars("") == "")
       assert(evenChars("a") == "a")
       assert(evenChars("ab") == "a")
   
   
3. reverseString
   Write the function reverseString that takes a string and returns the reverse of that string (so "abc" becomes "cba").  Do this from first principles, with loops and conditionals, and not using a built-in reverse function or slicing like s[::-1] or converting the string to a list or anything else of that nature. Here are some test cases for you:
   
       assert(reverseString("abcde") == "edcba")
       assert(reverseString("abcdef GH") == "HG fedcba")   
       assert(reverseString("a") == "a")   
       assert(reverseString("") == "")  
   
   
4. vowelCount
   Write the function vowelCount that takes a string and returns the number of vowels in it (case-insensitively). Here are some test cases for you:
   
       assert(vowelCount("abcdefg") == 2)
       assert(vowelCount("ABCDEFG") == 2)
       assert(vowelCount("") == 0)
       assert(vowelCount("This is a test.  12345.") == 4)
   

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