CS61A: Lecture 3

Control

Print and None

• Pure functions just returns a value. None-pure functions have a side effect.
• Print is a none-pure function that always returns None, but also displays a given value.
• Practice: Implement a function h(x) that first prints, then returns, the value of f(x)

def f(x):
    return print(x+1)

def h(x):
    print(f(x))
    return f(x)

def h2(x):
    y = f(x)
    print(y)
    return y

h(2)

3
None
3

h2(2)

3
None

def f(x):
    return square(x + square(y+1))

def square(z):
    y = z * z
    return y

x, y, z = 1,2,3
    
print(f(3))

25

Multiple Environments

• Start from the earliest local frame, and trace back up in the environment until we reach the global frame.
  • A function that is defined within another function can access local parameters of the parent function.

Control

Conditional Statements.

• COnditional statements contain statements that may or may not be evaluated.

Iteration

While Statements:

• Contains stateemnts that are repeated as long as the condition is true.
  • Must eventually become a false for the statement to end (or return/break or some statement to stop iteration)
  • Entire body is executed.

Prime Factorization

• Every positive integer n has a set of prime factors: primes whose product is n

...
8 = 2 * 2 * 2
9 = 3 * 3
10 = 2 * 5
11 = 11
12 = 2 * 2 * 3
...

• My code:

def prime_factorize(n):
    k = n
    factors = []
    curr = 2
    while (k%curr==0):
        factors.append(curr)
        k//=curr
    curr = 3
    while (curr<=n):
        while (k%curr==0):
            factors.append(curr)
            k//=curr
        curr = curr+2
    return factors

prime_factorize(1298237392)

[2, 2, 2, 2, 23, 3527819]