CSci 1100 Lab 3 — Functions

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Checkpoint 1: Using existing functions
First, we will experiment with using functions and debugging code. Create a new directory in your
dropbox for Lab 3, and copy from Piazza the program called lab3 check1.py. Run this code and
see that it gives an error message.
Let’s look closely what the code does: it creates a function that takes four values: x1,y1,x2,y2,
and returns a floating point value that represents the Euclidean distance between two points with
Cartesian coordinates (x1,y1) and (x2,y2). The program starts from an initial point and then
reads the next (x,y) coordinates of an object. It computes the distance between these two and
outputs the result.
The code contains many bugs. Often, the program aborts after the first error and you need to fix
it and rerun the program. So, we would like you to go through this process with this program. In
fact, even if you can see a bug in the program, we recommend that you do not fix it until you see
the error it produces first.
Each time you run the program, read the error message. Sometimes it is easy to interpret and
sometimes not. But very often it will tell you the line number and the exact location. Learn to
read these messages and interpret them. You will learn a lot from fixing mistakes, so do not let
others tell you what each error means. If you are having trouble figuring them out, read the relevant
course notes and think. If you figure out it yourself, you will learn so much more.
Please take the time to figure out all the bugs. Also, format the output nicely so that it looks like
this:
The next x value ==> 12
The next y value ==> 12
The line has moved from (10,10) to (12,12)
Total length traveled is: 2.83
After you fixed al the bugs, show the corrected program to a TA or a mentor. They will ask you
about different errors and discuss strategies for figuring out what they mean.
To complete Checkpoint 1: Show the TA the working version of the program. Be prepared to
discuss with them debugging methods and ask for advice.
Checkpoint 2: Restructuring code
In this section, we will restructure code with the help of a function. First start by downloading the program called lab3 check2.py and save it in your lab3 folder. Create a copy of it
lab3 check2v2.py by saving it a second time with the new name. You will be modifying this
copy and comparing its output to the original.
Run the original program and make sure you understand what it does. You can see that this
program contains repetitive code. If you found a bug in one computation, you would have to fix it
in 4 places. Instead, you can put all this computation into a function improving both readibility
and maintainability.
Write a function that takes as input all the necessary data for the repeated code, does the computation and prints the results. Your function definition must come before any other code in the
program. What should the arguments for this function be?
Now, rewrite the rest of the code by simply calling this function four times. Check and make sure
it provides the same output as the original program.
To complete Checkpoint 2: Show the TA your refactored program. Make sure that your
program follows the required structure: function first, followed by function calls.
Checkpoint 3: Prey and Predator Population
Now that we have experimented with functions, you will write a full program that defines functions
and then uses them.
Suppose you are trying to understand how the population of two types of animals will evolve over
time. You have bunnies and foxes. Unfortunately, foxes like to eat bunnies. So, even though the
bunny population grows every year when baby bunnies are born, it also goes down because some
of them are eaten. Would the fox population keep increasing in size until there are no bunnies left?
Not so. The fox population is directly linked to the bunny population, their main food source.
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What will happen over time if you start with a given population of bunnies and foxes? Who will
win out? Will they balance each other? This is what you will investigate in this part.
Suppose bpop, fpop are the population of bunnies and foxes currently. Then, next year’s population of bunnies (bpop next) and foxes (fpop next) are given by:
bpop next = (10*bpop)/(1+0.1*bpop) – 0.05*bpop*fpop
fpop next = 0.4 * fpop + 0.02 * fpop * bpop
What will be the population of bunnies and foxes next year? Your program should first read the
population of bunnies and foxes using input, and then compute and print the next years’ population
for both based on this current value. Its output initially should look like this:
Number of bunnies ==> 100
100
Number of foxes ==> 10
10
Year 1: 100 10
Year 2: 40 24
Let’s think about how to solve this problem. Start by creating a new file in the Wing IDE to work
with. You will need variables to store the current population of bunnies and foxes. You need one
function for computing the population of bunnies for the next year and another function for the
population of foxes. Now, call these functions once to find the population of both animals in the
next year and print.
Remember two crucial things:
ˆ Population is an integer, not a float. You never see half bunnies running around.
ˆ The second one is a bit tricky. The population of animals cannot be negative (something
worse than extinct?). Don’t listen to people around you who are telling you to use an if
statement. You are forbidden to use an if statement. Think about how to use the
max() function to make sure that your function does not return a negative number.
Now, for the final challenge, use Year 2 values to find Year 3 populations of bunnies and foxes. Do
not create new variables, just use the same variables you currently have. Once you figure this out,
you can repeat this part 2 more times to find the population in Year 5. When completed, your
program should look like this:
Number of bunnies ==> 100
100
Number of foxes ==> 10
10
Year 1: 100 10
Year 2: 40 24
Year 3: 32 28
Year 4: 31 29
Year 5: 30 29
In the next few weeks, we will see that we can use loops to make this calculation much simpler.
Loops will allow you avoid repeatedly typing (or copying-and-pasting) the same code and make
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your programs shorter, clearer, more general, and less likely to contain errors. For now, let’s use
this lab to learn how to use the same variables again and again in computations.
To complete Checkpoint 3: show a TA or a mentor the completed solution and illustrate its
use with the values given above.
Make sure your program follows the structure we outlined earlier in this lab: function definitions
first, then your variables bpop, fpop are assigned their initial value, and finally the code to compute
and print the year-by-year population.
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