15-110 Spring 2011 Homework 2 Practice

These are optional practice problems to help you prepare for hw2.  Solutions are provided, along with short video tutorials.

Here are the files you'll need:

• hw2-practice.py:  starter code (download this and replace the stubs with your answers)
• hw2-practice-solutions.py:  sample solutions (note that there are many other perfectly valid solutions!)
• hw2-practice-videos.html:  short video tutorials explaining the sample solutions, and a brief overview of how to get started on hw2.

Note:  This week, you may not use "if" statements (conditionals), "for" or "while" statements (loops), recursion, or collections (lists, dictionaries, etc).  Since we have not covered these yet, this should not be a concern for most of you.  That said, solutions that use "if", "for", "while", etc, will receive half credit for those portions.

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1. rotateStringLeft
2. rotateStringRight
   
3. hasThreeWords
4. getTimeString

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1. rotateStringLeft
   Write the function rotateStringLeft that takes a string and a non-negative integer, and returns the string rotated that many places to the left.  Here are some test cases for you:
       assert(rotateStringLeft("abcde", 0) == "abcde")
       assert(rotateStringLeft("abcde", 1) == "bcdea")
       assert(rotateStringLeft("abcde", 2) == "cdeab")
       assert(rotateStringLeft("abcde", 3) == "deabc")
       assert(rotateStringLeft("abcde", 4) == "eabcd")
       assert(rotateStringLeft("abcde", 5) == "abcde")
       assert(rotateStringLeft("abcde", 25) == "abcde")
       assert(rotateStringLeft("abcde", 28) == "deabc")
   
   
2. rotateStringRight
   Write the function rotateStringRight that takes a string and a non-negative integer, and returns the string rotated that many places to the right.  Here are some test cases for you:
       assert(rotateStringRight("abcde", 0) == "abcde")
       assert(rotateStringRight("abcde", 1) == "eabcd")
       assert(rotateStringRight("abcde", 2) == "deabc")
       assert(rotateStringRight("abcde", 3) == "cdeab")
       assert(rotateStringRight("abcde", 4) == "bcdea")
       assert(rotateStringRight("abcde", 5) == "abcde")
       assert(rotateStringRight("abcde", 25) == "abcde")
       assert(rotateStringRight("abcde", 28) == "cdeab")
   
   
3. hasThreeWords
   Write the function hasThreeWords that takes a string (which you may assume only contains letters or spaces) and returns true if the string contains exactly three words, with no leading spaces, and with each word separated by exactly one space.  Here are some test cases for you:
       assert(hasThreeWords("a b c") == True)
       assert(hasThreeWords("this should work") == True)
       assert(hasThreeWords("but this should fail") == False)
       assert(hasThreeWords("this too") == False)
       assert(hasThreeWords("likewise") == False)
       assert(hasThreeWords(" leading space fails") == False)
       assert(hasThreeWords("no double  spaces") == False)
   
   
4. getTimeString
   Write the function getTimeString that takes (as a non-negative integer) the number of minutes since midnight, and returns the time in "hh:mm" format.  Here are some test cases for you:
       assert(getTimeString(0) == "12:00")
       assert(getTimeString(5) == "12:05")
       assert(getTimeString(60) == "01:00")
       assert(getTimeString(65) == "01:05")
       assert(getTimeString(605) == "10:05")
       assert(getTimeString(665) == "11:05")
       assert(getTimeString(725) == "12:05")
       assert(getTimeString(785) == "01:05")
       assert(getTimeString(665 + 60*12*1000) == "11:05")
   

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