Practice Quesitons for E03, CS5NM, Fall 2008

Topic: Boolean Expressions

1. Consider the following segment of code. It is part of the body of a function that checks three variables, x, y and z, each representing a side of a triangle. The function should return -1 if any of the sides is less than or equal to zero:

   if x <= 0:
      return -1
   elif y <= 0:
      return -1
   elif z <= 0:
      return -1  
     

   Fred notices that this can be simplified into a single if—we don't need the complication of the if/elif/elif here. What should we fill in for the condition of this single if? Fill in the blank below:

   
   if (_______________________________________):
      return -1
   
   

   Hint: think carefully: do you need and or do you need or?
   
   Here are some possible answers. Which of these are correct, and which of them are incorrect, and why?
   
   

   if x > 0 and y > 0 and z > 0:
       return -1
   
   

   
   

   if ( x or y or z <= 0 ):
       return -1
   
   

   
   

   if (x <=0 or  y<= 0  or z <= 0 ):
       return -1
   
   

   
   

   if not ( x > 0 and  y > 0  and z > 0 ):
       return -1
   
   

Topic: Recursion

We've gone over recursion in this course as it applies to lists in Python. We've looked at how to use recursion to do the following things in Python:

1. Count all the things in a list that meet some criteria.
• The countOdd function in the file of examples.
• Define a Python function countBigOnes() that takes a list of integers, and counts how many integers in that list are greater than 100.
• Define a Python function howManySWords() that takes a list of words (each represented as a string), and counts how many of the words in that list start with the letter 's' (or 'S').
  
  
2. Sum the things in a list.
   
   • The function sumNeg in the examples.
   • Define a Python function sumList() that adds up all the numbers in a list.
   • Define a Python function sumOdd() that adds up all the odd numbers in a list.
     
     
3. Find the minimum or the maximum of the items in a list.
• For example, the smallest function from the examples.
• Define a python function minOfList() that finds the minimum value in a list.
• Define a python function that finds the smallest odd number in a list.
• Define a python function that finds the smallest number that is bigger than 100 (e.g. from the list [23,105,234,76] you should return 105. If no number on the list is bigger than 100, raise an exception 'No number on the list is >100'
• Write a Python function largestAbs() that returns the item on the a list of numbers with the largest absolute value. (In Python, abs() finds the absolute value of a number). For example, largestAbs([3,-9,2,5]) returns -9, and largestAbs([3,-9,10,2,5]) returns 10. Hint: be sure that you return the number itself, and not the absolute value of the largest number.
  
4. Change one list into another.
• Example the lengths function from the file of examples that turns a list of strings into a list of lengths of those strings. (e.g. lengths(["This", "is" , "a", "test"]) returns [4, 2, 1, 4]
• Define a Python funciton that turns a list of numeric grades (as a list of integers) into a list of letter grades (as a list of strings)
• The function from ex16 that turns a list of integers into the same values, but mod 5, e.g. turn [3,6,67,2,8,15,19] into [3,1,2,2,3,0,4]
  
  
5. Remove certain items from a list.
• Example: consume a list of strings, and produce the same list, but with all the strings of length 4 or less removed
  • This is the bigWords function from the file of examples.
• Example: Define a function that takes a list of integers and returns the same list, with all the odd numbers removed
  
  
6. Retain only certain items from a list
   • consume a list of strings, and produce a list that only contains strings from the original list that have length >=5
     • This is the bigWords function from the file of examples.
   • Define a function that takes a list of integers and returns the same list, with only the numbers from the original list that are even.
   • Note that a function that "retains even numbers" is the same as one that "removes odd numbers"
   • A function that "retains big words" is the same as one the "removes small words".