Week 3 Lab: Using Lists

Git Repository: https://classroom.github.com/a/hdHTCu3l

Text-based Drawing, with for loops

The first thing we are going to do today is use for loops to do some basic text-based drawing. We are going to write programs that draw lines, squares, rectangles, triangles, and pyramids. This will help us practice loops.

In the lab, there is a program called “line.py”. Run it with the command

python line.py

It should print out a horizontal line of eight * characters.

Exercise 1: User-input length line

Modify the line program to ask the user how long of a line they want (much like last week’s counter exercise asked for how high to count), and then use a for loop to print out a line of the length requested by the user.

Hint: Remember the range() function, which is commonly used in for loops.

Hint 2: When the user types in a number, python stores it as a string. You might need to convert it to an integer with the int() function.

Exercise 2: Square

OK, now you’ve got a program that prints out a line of different lengths. Create a copy of this program and call it square.py. Modify the program so that it prints out a square instead of a line — it should print out multiple lines that are all the same length. So a square of size 3 looks like this:

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Be sure to test this program with different sizes.

Hint: You can have for loops inside for loops. Remember that python uses indentation to show what is inside the loop.

Hint 2: Don’t forget to add a new line character at the end (by running print("") without the end= parameter). Should this be inside the inner for loop, or the outer for loop?

Add the square program to git. Don’t forget to commit and push.

Exercise 3: Triangle

Create a copy of the square program and call it triangle.py. Modify the program so that it prints out a triangle. A right triangle of size 4 should look like this:

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Be sure to test this program with different sizes.

Add the triangle program to git. Don’t forget to commit and push.

Exercise 4: Rectangle

Create a copy of the square program and call it rectangle.py. Modity the program so that it asks the user separately for a length and a width, and prints out a rectangle.

Be sure to test this program with different sizes.

Add the rectangle program to git. Don’t forget to commit and push. Now is a good time to swap driver and navigator if you haven’t already.

Names, names, and more names

Exercise 5: Add your names to a list

Start by running the program names.py. This prints out a list with the names of the instructor and TAs for this class.

Add both your name to the list, and your partner’s name to the end of the list. Run the program and make sure it prints out all six names correctly.

Exercise 6: Adding another name

You have hard coded a set of names in the program by explicitly specifying a list. However, you can also get python to add things to a list using .append(). So if you have a variables called stuff that contains a list, and you want to add “basketball” to that list you can say:

stuff.append("basketball")

Of course, you can replace “basketball” with a variable that contains a string also.

Use this to modify your program to ask the user for another name with input(). Then .append() that name to your list, and print out the full list with 7 names on it.

Exercise 7: Alphabetical order

The function .sort() will sort any list you have. By default, it sorts things alphabetically, with all the capital letters coming before any lowercase letter. So if you have a variable stuff that contains a list, then you can say stuff.sort() to sort that list.

The names program should print out 7 names now. Modify the program to sort the names, and print out the list of names in alphabetical order. Test your program by entering in different names, and seeing if it sorts them differently.

Exercise 8: Beginning of the Alphabet

In your program, you can retrieve specific items from the list using []. So, to get the 2nd item from the list stuff, you can say stuff[1]. (Note: Lists in python always start at 0, so 0 is the first item, 1 is the second, and so on.)

Modify your program to print out only one name: the name that comes first in alphabetical order.

Exercise 9: End of the Alphabet

Let’s give some love to the end of the alphabet. Change your program so it prints out the person’s name who appears last in alphabetical order.

Hint: Modify your program so it prints out the name that appears last in the list, after the list has been sorted into alphabetical order. OR, first in the list after sorting by reverse alphabetical order.

Random Number Generator

Next, we are going to use the random package to build a couple of useful tools to help us make choices. As a reminder, before you can use the random package, you need to import random into your program. I usually put the import lines at the beginning of my program.

The git repository has a really simple coin flip program called coin.py. Run this program. What happens? Run it again. Does it always print out the same thing? Now, open up the code and look at how it works.

Notice that you had to run the program more than once to see what it was doing. That is true in general when dealing with randomness; you have to run the program more than once to verify that the random numbers are working correctly.

Exercise 10: Random Number Generator

Coin flips are great when you only have 2 options to choose from. But sometimes you have more than two options to choose from. The program rand_num.py assumes you need to choose among 5 things, and picks a random one of those 5 things.

Modify this program to ask the user how many things they are trying to choose between. It should then pick a random number between 1 and that number and print it out, to help the user make a choice. For example, if the user is trying to decide between 6 things, the program will ask how many things, the user will enter “6”, and the program should pick a random number between 1 and 6

Note: Remember the difference between strings and integers. When a user enters a response to input(), it comes as a string. If you want to use it as a number, for example in a range() command, then you need to convert it to a number with int().

Note 2: Remember to run the program more than once to make sure it’s giving you a random number.

Combining Loops and Randomness

Exercise 11: Coin flipper

The coin.py program flips a single coin and prints out either heads or tails. Let’s test to see if this program is biased. Modify the program to flip the coin 20 times and print out the results of each flip.

Hint: You will need to use a loop for this. Look back at the for loop lab for help.

Exercise 12: Coin flipper pt. 2

Modify your coin flipping program to count how many heads and tails it gets. At the end of the program, it should print out the counts – how many heads came up, and how many tails came up.

Hint: You can use a counter to do this (which is sometimes known as an “accumulator”). You create a variable, and then set it to zero as a place to start. Then, each time through a loop, you add one to it. Something like this:

counter = 0
while looping:
	counter = counter + 1
print(counter)

Hint 2: Sometimes you don’t want to count every time in a loop, but only some loops (like the loops where it comes up heads). You can use an if statement to only add one to the counter when that happens.

Exercise 13: Coin flipper pt. 3

Modify the program to ask the user how many times it should flip the coin. Then have the program flip a coin that many times, and count how many heads and tails it gets. At the end of the program, it should print out the counts – how many heads came up, and how many tails came up.

Be sure to commit your changes to the git repository. Also, if you haven’t already, now is a good time to swap driver and navigator.

Choosing a restaurant

It is always difficult to choose where to go for lunch when you are with a bunch of friends. We are going to build a restaurant choosing program to help with the choice. Let’s start by looking at restaurant.py and running it. It should ask the user for 3 restaurants, and then just print out that list.

Exercise 14: User experience of counting

Python counts starting a zero: 0, 1, 2, 3, … Regular human beings, on the other hand, start counting at 1: 1, 2, 3, 4, …

Right now, the program asks for “restaurant 0” first. Modify the program so that it looks like human beings would expect, starting with 1.

Note: the program should still ask for 3 restaurants.

Exercise 15: Random Choice

The function random.choice() will pick a random item out of a list, and return that item. So if you have a list of stuff, you can say random.choice(stuff) to get a random thing out of that list.

Modify the restaurant program to use this to randomly choose a restaurant for lunch, and print out which restaurant the computer chose.

Note: Remember to import the random library (import random) at the beginning of your program.

Exercise 16: More (or less) than 3

This is where lists really shine. Lists are great because you don’t have to know ahead of time how many items are going to be in the list. They get bigger or smaller as needed.

Right now, the restaurant.py program always asks for exactly three restaurants. In programmer terminology, we say that “three” is “hard-coded” into the program.

Modify the program to ask the user how many restaurants they are thinking about for lunch, and then have it ask them for that many restaurant names, and then pick a random restaurant from that list. So if they say they are thinking about 5 restaurants, your program should ask them for five names of restaurants, and then pick from among those five. If they say 2, it should only ask them for 2 names.

Here is a potential use of the program:

How many places are you thinking about for lunch? 4
Enter the name of restuarant 1: Jimmy Johns
Enter the name of restuarant 2: Chipotle
Enter the name of restuarant 3: Five Guys
Enter the name of restuarant 4: Noodles and co.
You should go to Noodles and co.

Note 2: Notice that I made the program a little more user friendly by printing out the words “You should go to” in addition to the final choice. It is always a good idea to try to make things easier for your users to use and understand.

Challenges

The following exercises are related to the ones above, but a little more uncertain. As you get further into programming, you get fewer and fewer directions for how to do things, and have to do more work to figure it out yourself. Try these exercises, and feel free to talk to your peers or your instructors as you think through how you want to make these work. You should do at least one of the Loops challenges, and also at least one of the Lists challenges.

Loops Challenge 0: Backwards Triangle

The triangle program you just created is up against the left side of the window. Create a copy of the triangle program and call it backwards.py. Modify that program so that the vertical edge of the triangle is on the right side of the triangle, like this:

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Be sure to test this program with different sizes.

Hint: You will probably need to add a second for loop to print out the leading spaces.

Hint 2: Think about indentation. If code is indented, then it happens inside that for loop – it happens over and over in the loop. If it isn’t indented, then it happens after the loop, only once.

Add the backward triangle progrm to git. Don’t forget to commit and push.

Loops Challenge 1: Pyramid

Create a copy of your backwards triangle program and call it pyramid.py. Modify the program to print out a pyramid structure where the height of the pyramid is what the user asks for, and the base is twice that size. So a pyramid of size 5 should look like:

    **
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  ******
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**********

Hint: A pyramid is a backwards triangle next to a forwards triangle

Be sure to test this program with different sizes.

Add the pyramid progrm to git. Don’t forget to commit and push.

Loops Extra Challenge 2: Pointy Pyramid

Can you write a program that creates a pointy pyramid? Note the odd number of stars in each row.

    *
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  *****
 *******
*********

Lists Challenge 1: Homework Chooser

A problem that many students have is choosing which class’s homework to do next. Write a homework chooser that asks the student which classes they currently need to do homework for, and then randomly chooses which order the student should do homework.

Hint: random.shuffle(list) will randomly re-order a list. Picture in your head putting each item in your list on a separate index card, and then shuffling the deck of index cards.

Lists Challenge 2: Infinite coin flipper

Make a copy of your coin program and call it infinite.py. Modify the program so that it keeps flipping coins as long as the user wants it to. It should flip a coin, and then ask the user if it should continue. Unless the user says no, it should go and flip another coin. Then ask the user again. It should keep doing that until the user says “no”.

As a bonus, get the infinite coin flipper to print out counts of how many heads and tails it flipped so far.

Don’t forget to add this program to your git repository.

Hint: while true: will loop forever. You can break out of a loop with the break command.

Heads
Should I continue? yes
You have flipped 1 coins
1 have come up Heads
Which is 100%
Tails
Should I continue? yes
You have flipped 2 coins
1 have come up Heads
Which is 50%
Heads
Should I continue? yes
You have flipped 3 coins
2 have come up Heads
Which is 66.66666666666666%
Heads
Should I continue? yes
You have flipped 4 coins
3 have come up Heads
Which is 75%
Tails
Should I continue? no

Lists Challenge 3: Unlimited Restaurant Options

Your restaurant choosing program asks the user to pre-specify how many options there are. I don’t know about you, but when I’m hungry and trying to decide where to go, I can’t figure out how many options there are. That’s just too much advance thinking.

Modify your program so that it doesn’t need to ask ahead-of-time for the number of restaurants. The user should be able to just keep entering restaurants until they type “done” or just hit enter or something. Then it will randomly choose one of the ones they entered and print it. The program should still accept as many restaurants as they want.

Here is an example output:

Enter the name of a restuarant: Culvers
Enter the name of a restuarant: Chipotle
Enter the name of a restuarant: Jimmy Johns
Enter the name of a restuarant: No Thai!
Enter the name of a restuarant: Potbellys
Enter the name of a restuarant:
You should go to No Thai!

Lists Challenge 4: Random 99 bottles

Two weeks ago, we wrote a program that sings the song “99 bottles”. The last exercise introduced a new way to sing the song – a change to the 3rd line.

Modify the 99 bottles program to randomly choose which version of the verse is sung, for all verses. When you do this, store the possible verses in a list, and then use random.choice() to choose which one to sing.