Lectures
At this week's double we will look dictionaries. Dictionaries are a convenient way to handle collections of things that you need to be able to access directly. We will also be looking at how to read and write files.
At the single lecture will will walk through the assignment from last week and talk about aliasing.
Computer exercises
Complete the the Dictionaries exercise. Then complete the Reading and Writing Files exercise.
Reading material
Chapter twelve and of How to think like a computer scientist and the following tutorial on file processing.
Weekly assignment
This weeks assignment will walk you through a number of list manipulations. All the functions you are asked to code up return a list. So the basic structure is looping over a list (or lists) with a for loop, and in doing so, appending to a new list that is finally returned by the function. Remember that apending to a list goes as follows:
L = [] L.append(x)and that you have to define a list before you can append to it — which makes sense :-).
There is one caveat that you might run into so here is a heads up: if you use the following construct:
lst = [2, 3, 4] for i in lst: i = i**2
you will not be changing the lst list. The reason is that the i variable in the loop holds copies of the elements in lst — not the elements themself. To change the input lst parameter you would need to do the following:
lst = [2, 3, 4] for i in range(len(lst)): lst[i] = lst[i]**2
Any way — this not a good way to implement it because this way you (maybe unintentionally) change the list you give as as argument to your function. This happens because list variables are references. To get arround this the first thing you need to do in each function is to define a new list, e.g. L = [], and then fill results into that as mentioned above.
The actual assignment
Write a function square(L) that takes a list of numbers and returns the list of those numbers squared.
Example usage:
print square([2, 5, 3, 6])
[4, 25, 9, 36]
Write a function even(L) that takes a list of numbers and returns a list of only those numbers that are even. Remember that you can test if a number is even using n % 2 == 0.
Example usage:
print even([2, 5, 3, 6])
[2, 6]
Write a function, differences(xs,ys) that takes two lists of equal length, xs and ys, and returns a list of the pairwise differences. One way to solve it is similar to the way you did the pairwiseDifferences function in last weeks exercise. An other way is to use the built in function zip(xs,ys) to get a list of all the matching pairs of xs and ys to iterate through (look at the documentation yourself).
Example usage:
print differences([2, 5, 3, 6], [1, 5, 6, 3])
[1, 0, -3, 3]
Write a function squaredDifferences(xs, ys) that takes two lists of equal length, xs and ys, and returns a list of the pairwise differences squared. You can use the differences(xs,ys) function to make it easier to write this function.
Example usage:
print squaredDifferences([2, 5, 3, 6], [1, 5, 6, 3])
[1, 0, 9, 9]
Handing in
To hand in the assignment put the code in a file named after your self and the week. If it was me it would be Kasper_Munch_week4.py. Attach it to an email with subject "Assignment" and send it to asand@cs.au.dk before 10am the following Wednesday.