Questions

Don't wait any longer to start homework 3

Tuples

list: sequence of arbitrary elements, elements can be added, removed, overwritten (mutable)

string: sequence of characters (single letters or symbols), elements are fixed (immutable)

tuple: sequence of arbitrary elements, elements are fixed (immutable)

a_list = ["a", "b", "c"]
a_str = "abc"
a_tuple = ("a", "b", "c")

tuples: can vs can't do

Why?

efficiency (technical reasons are beyond the scope of this course)

safety

for example, if you are representing locations in latitude and longitude, this data should always come in pairs

Scope

Mystery

x = 5
def square(x):
x = x * x
return x
y = 10
print(square(y))
print(y)
print(x)

what if we remove x=5

the scope of a variable refers to the part of a program where that variable exists

Practice

def cube(x):
x2 = x * x
x = x2 * x
return x
for num in range(4):
if x % 2 == 0:
x = cube(num)
print(x)
print(num)
print(x)
print(x2)

Aliasing

Mystery

vec = [5,5,5]
q = vec
q[1] = 7
print(vec)

vec = [5, 5, 5]
q = vec
vec = [1, 2, 3]
print(q)

def zero(vec):
    for i in range(len(vec)):
        vec[i] = 0
vec = [1,2,3]
print(zero(vec))

looking at these examples with pythontutor.com, we can see that the immediate value of a list variable in memory is an arrow to the actual list

when we assign a new variable to a list or give a list as input to a function, it's this arrow that is used

alias.py

name = "Aaron"
first_name = "Aaron"

print(id(name))
print(id(first_name))
print(id("Aaron"))

name = name + "!"
print(name, id(name))
print(first_name, id(first_name))

print("mutable example:")
ys = [1, 2, 3]
xs = [1, 2, 3]
print("ys", id(ys))
print("xs", id(xs))
ys.append(5)
print("ys", ys, id(ys))
print("xs", xs)
zs = ys
print("zs", id(zs))

def append10(nums):
    print("nums", id(nums))
    nums += [10]

append10(ys)
print("ys", ys, id(ys))

def add10(num):
    print("num", num, id(num))
    num += 10
    print("num", num, id(num))

a = 5
print("a", a, id(a))
add10(a)
print("a", a, id(a))