More on Lists

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We learned a bit about lists in our last lecture. This time, we will see more on how useful these list things can be!

Creating a list

Let’s start off by generating a list of numbers. The range function should be able to do that:

>>> l = range(20)
>>> print(l)
range(0,20)

Hmmm, that did not work like I expected it to! In fact range is a special function Python calls an iterable. We will not study this kind of function in this class, but to generate the list, All I need to do is alter the way I try to build it:

>>> l = list(range(10))
>>> print(l)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Now we are getting somewhere!

Here is an interesting way to build up a list where all the elements are the same:

>>> z = [0]*10
>>> print(z)
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

I wonder what happens if I add two lists:

>>> s = l + z
>>> print(s)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

As we saw with strings, a list added to a second list gives a new list with the elements of both in the order specified.

Another way to do the same thing is this:

s = l
s.extend(z)
>>> print(s)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

This approach does not work (I am still scratching my head over this):

>>> s = l.extend(z)
>>> print(s)
None

I guess since the extend function does an in-place change, it returns no value.

List slices

Suppose you wanted the sequence from 5 to 15. We can slice off that segment of this list by using the Python slice notation:

>>> l = list(range(20)
>>> s = l[5:16]
>>> print(s)
[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

There is that funny problem again. The slice range notation stops one short of the last value!

We can do other kinds of slices:

>>> l
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
>>> l[:16]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
>>> l[5:]
[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

If you leave off the first number in the slice rance, the initial number is zero. If you leave off the last number, the end point is the last item in the list. What do you think you get if you leave off both numbers? (Trick question!)

l[:]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

Makes sense - right?

Lists can hold different kinds of things

Suppose we had a group of people and we wanted to store information about each one. We might do this:

person1 = ['Sarah', 'Fenway', 'f123456', 99]
person2 = ['Richard', 'Penn', 'p234567', 89]
person3 = ['Scott', 'Lauder', 'l54321l', 87]

What would happen if we constructed a list of people in this group?

students = [person1, person2, person3]

Here is some code to print out data from this super list:

s1 = students[1]
print(s1[0])
print(s1[1])
print(s1[2])
print(s1[3])

print(students[2][0])

Here is what we get when running this code:

python3 MultiList.py
[['Sarah', 'Fenway', 'f123456', 99], ['Richard', 'Penn', 'p234567', 89], ['Scott', 'Lauder', 'l54321l', 87]]
['Sarah', 'Fenway', 'f123456', 99]
['Richard', 'Penn', 'p234567', 89]
['Scott', 'Lauder', 'l54321l', 87]
Richard
Penn
p234567
89
Scott

That is not so bad, once you get used to remembering what each element in your list actually is. If it is another list, just tack on another set of square brackets to step inside that list!

Graphics using Lists

Let’s look at a program that plots the sine of X where X ranges over values from 0 to 360 degrees.

Here is the code that does that:

# SinePlot.py - draw a plot of the sin(x) from 0 to 360
from graphics import *
from math import sin, pi

def draw_axes(this_win):
    ''' Draw axes for the sine plot'''
    x1 = 10
    y1 = 125
    x2 = 370
    y2 = y1
    Line(Point(x1,y2),Point(x2,y2)).draw(this_win)
    x1 = 10
    x2 = x1
    y1 = 25
    y2 = 225
    Line(Point(x1,y1),Point(x2,y2)).draw(this_win)

def gen_data():
    ''' 
        generate the sine(x) for angles from 0 to 360
        return a list of the values
    '''
    vals = []
    for angle in range(361):
        x = angle * pi / 180
        sine = sin(x)
        vals.append(sine)
    return vals

def plot_data(data, this_win):
    ''' plot the data on the window'''
    angle = 0
    x1 = 10
    y1 = data[0]* 100 + 125
    angle = 1
    while angle <= 360:
        x2 = angle + 10
        y2 = 125 - data[angle]*100
        Line(Point(x1,y1),Point(x2,y2)).draw(this_win)
        angle += 1
        x1 = x2
        y1 = y2
def main():
    print("Sine Plotter")

    # draw the axes on the window
    win = GraphWin("Sine", 380, 250)
    draw_axes(win)

    # generate the data to plot
    data = gen_data()
    #print(data)

    # Plot the data on the window
    plot_data(data,win)

    win.getMouse()

main()

Here is what I got when running this: