CS61A Lab 1: Functions

Week 1, Spring 2012

Try to get as much done as possible, but don’t panic if you don’t finish.

Exercise 1: Unix/Emacs Tutorial

Do the Unix/Emacs Tutorial which can be found here: Unix/Emacs Tutorial

Exercise 2: Command-line - Python Expressions

At your terminal, type in python as follows:

	star[200] ~ # python

# this is a comment 

# do this column second 

3 

from math import sqrt, exp 

2 + 3 

exp(1) 

5 + 6 + 7 

sqrt(144) 

-16 - -16 

pi 

3 * 4 + 1 

from math import pi

3 * (4 + 1) 

pi 

from operator import mul, add 

pi * 3 

3 * 4 

print(pi) 

mul(3, 4) 

print(4) 

mul(3, add(4, 1)) 

print(add(9, 1)) 

2 ** 3 

print(print(2)) 

pow(2, 3) 

pow(pow(2, 3), abs(-2)) 

Exercise 3: Defining functions at the command-line

Recall the structure of defining a function:

def <name>(<formal parameters>):
    return <return expression>

At your Python prompt “>>>” type the following:

>>>def cube(n):
...    return n * n * n
...   
>>>

Be sure to indent the return statement correctly. Then call the function cube, with some numerical argument.

Exercise 4: Writing functions in Emacs

Now we will use Emacs, our code editor. Create a new file sumofsquares.py and type in the program below.

Don’t copy and paste the following code directly. Retype it! Copying and pasting can introduce weird characters or unusual quotes, and they tend to confuse Python. (Poor Python!) Note the indentation.

# put your name here
# put your TA name here

def square(x):
    return x * x

def sum_of_squares(a, b):
    return square(a) + square(b)

Now load this program into the Python interpreter and test your function by calling sum_of_squares with some numerical argument at the prompt.

Exercise 5: Defining your own function

Finally, we will have you write your own function! In the emacs editor, create a distance.py file that contains the following program:

# put your name here
# put your TA name here
 
from math import sqrt, pow

def distance(x1, y1, x2, y2):
    # write your code for distance here

distance should take in two sets of x-y coordinates (x1, y1) and (x2, y2) [parameters shown above] and should compute the Euclidean distance between the two points. Use the following formula:

Load this program into the Python interpreter, and test the function by calling distance at the python prompt “>>>” with the appropriate arguments.

>>> distance(1, 1, 1, 2)
1.0
>>> distance(1, 3, 1, 1)
2.0
>>> distance(1, 2, 3, 4)
2.8284271247461903

Now let’s edit this program to get the distance between two 3-dimensional coordinates. Your distance function should now take in 6 arguments, and compute the following:

Reload the program and again test your function does the right thing!

>>> distance(1, 1, 1, 1, 2, 1)
1.0
>>> distance(2, 3, 5, 5, 8, 3)
6.164414002968976		

Exercise 6 - ucb.py Features

For this course, there are a couple features that end up being very useful in writing code - the staff have provided these in a file called ucb.py (which will be provided with every project). You will need to import the features of the ucb.py file into your Python files though - to do so, you will want to include a statement at the top of each file importing the features you will want to use. For now, we are going to go over the "main" feature, which allows you to easily test your functions:

	from ucb import main 

Section I: main

An entry point of a program is the place where execution starts happening. It's usually very convenient to be able to demarcate an entry point in a Python file for testing purposes. Say we have the following file cube.py:

def cube(x):
    return x * x * x 

print("Should be 1:", cube(1))
print("Should be 8:", cube(2))
print("Should be 27:", cube(3))
	

star [123] ~ # python cube.py
Should be 1: 1
Should be 8: 8
Should be 27: 27

One problem with this file is that the tests aren't cleanly arranged - it'd be much better if we had a test function that performed these tests:

def cube(x):
    return x * x * x

def run_tests():
    print("Should be 1:", cube(1))
    print("Should be 8:", cube(2))
    print("Should be 27:", cube(3))

However, now, if I run the file, nothing happens:

star [123] ~ # python cube.py
star [124] ~ # 

This is because, to Python, all we've done is define two functions: cube, and run_tests. We want Python to actually do something when we type in 'python cube.py', however. So, we specify an entry point with the @main annotation:

def cube(x):
    return x * x * x

def run_tests():
    print("Should be 1:", cube(1))
    print("Should be 8:", cube(2))
    print("Should be 27:", cube(3))
	
@main
def main():
    print("Starting.")
    run_tests()
    print("Ending.")

star [123] ~ # python cube.py
Starting.
Should be 1: 1
Should be 8: 8
Should be 27: 27
Ending.