CS 111 w20 lecture 10 outline

1 Debugging word_check.py continued

2 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 do everything lists can do except item assignment
    • 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

3 Scope

3.1 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?

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

3.2.1 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)

Need to add initial value for x

4 Aliasing

4.1 Mysteries

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

4.2 Deep dive

name = "Aaron"
first_name = "Aaron"

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

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

print()
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))

print()
def append10(nums):
    print("nums", id(nums))
    nums.append(10)

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

print()
def addfun(s):
    print("s", s, id(s))
    s += "!"
    print("s", s, id(s))

course = "111"
print("course", course, id(course))
addfun(course)
print("course", course, id(course))