Practice with Loops

Most of today's exercises are more practice with loops. For these exercises, you don't need to include error-checking. But, if you have time, and you want more experience, after you've written these functions so that they work, go back and add appropriate error-checking code to the definitions.

• Write maximum() which takes a list and returns the largest element in the list. Do not use the built-in max() function.

  Test your maximum function using OwlTest

  What happens if you use this on a string, like maximum("abracadabra")? What about a list of strings? What about a list containing both numbers and strings?

• Write evens_list() which takes a list of numbers and returns a list of all of the even integers in the list.

  As a first step, how can you use the % operator to determine if a number is even?

  Test your evens_list function using OwlTest

• Write n_squares() that takes a number n and returns a list of the squares 0² through (n-1)².

  Hint: When you know exactly how many times you want to loop, you can loop over a list generated by range().

  Test your n_squares function using OwlTest

• Mixing flavored drinks, both non-alcoholic and alcoholic, is currently popular. First, define two lists of strings, each containing various drink flavors.

  Define mixed_drinks() that takes your two lists. It returns a list of all the strings formed by concatenating a flavor from the first list with one from the second list, plus a hyphen to separate them. For example, mixed_drinks(["apple", "strawberry"], ["Coke", "lemonade"]) should return ["apple-Coke", "apple-lemonade", "strawberry-Coke", "strawberry-lemonade"].

  Hint: You'll want two loops — one nested inside the other.

  Test your mixed_drinks function using OwlTest

• As another example with nested loops, write a function print_triangle that takes a string and prints a triangle like the following. For the input "hello" it should print

  h
  h e
  h e l
  h e l l
  h e l l o
  

  Hint: Decompose the problem into easier pieces. First write a loop that prints one line of this triangle, given the input string and a number of characters to print. Next write an outer loop that prints multiple lines of increasing size. How can you use range() in this outer loop?

  Hint: If you use print with a trailing comma, the next print output goes on the same line. For a simple example, try

  print 1,
  print 2,
  print 3
  

  Create a modification of your print_triangle function called list_triangle that returns the output strings in a list.   That is, list_triangle("hello") would return ["h", "he", "hel", "hell", "hello"]    (Note the lack of spaces between the letters.)

  Test your list_triangle function using OwlTest

Plotting

We've recently added a function to CodeSkulptor to plot functions. In the CodeSkulptor documentation, read the Graphics / SimplePlot section.

• Use simpleplot.plot_lines() on some sample lists until you feel comfortable using it. For example, you could plot a series of numbers and their squares:

  dataset = [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25), (6, 36)]
  

  By the way, we haven't explained what the difference is between (2, 4) and [2, 4] yet. For right now, the difference doesn't matter, but we'll explain it very soon.

• Look at the Predator-prey slides from last Friday. On slide 10 you'll see a detailed algorithm that uses the math formulas from slide 9.

  You'll need a working version of this to use on your current assignment. But, don't worry, we'll give you one in case you have problems writing it yourself.