CS 111 f21 — `while` loops

1 Poll

What will be printed?¹

s = "that's good soup!"
acc = "mmm"
for i in range(1, len(s), 2):
    acc = acc + s[i]
print(acc)

What will be printed?²

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

What will be printed?³

def swap_first(xs, ys):
    t = xs[0]
    xs[0] = ys[0]
    ys[0] = t
v1 = [1, 2, 3]
v2 = ("a", "b", "c")
swap_first(v1, v2)
print(v1)

2 Return to the guessing game

Generate a random secret number, have the player try and guess it

2.1 Validate input

guess = input("Guess a number ")
while not guess.isdecimal() or int(guess) < 0 or int(guess) > 100:
    guess = input("Guess a number ")
guess = int(guess)

2.2 `while` loop

while CONDITION:
    BODY

Each time through the loop (including the first time), CONDITION is evaluated
if CONDITION is True, the loop goes through BODY again
if CONDITION is False, the program skips to the first unindented line after the loop
while True: — infinite loop???

2.3 Game loop

import random

def get_guess():
    guess = input("Guess a number ")
    while not guess.isdecimal() or int(guess) < 0 or int(guess) > 100:
        guess = input("Guess a number ")
    guess = int(guess)
    return guess


secret_num = random.randint(0, 100)
tries = 10
guess = get_guess()
while tries > 0 and guess != secret_num:
    tries -= 1
    diff = secret_num - guess
    diff = abs(diff)
    if guess != secret_num:
        if diff <= 5:
            print("lava")
        elif diff <= 10:
            print("warm")
        else:
            print("cold")
        guess = get_guess()
if guess == secret_num:
    print("YOU GOT IT!!!")
else:
    print("Better luck next time. The number was", secret_num)

3 Practice

poll: infinite while loop (no increment)
translate this for loop into a while loop:

total = 0
for i in range(10):
    total += i
print(total)

total = 0
i = 0
while i < 10:
    total += i
    i += 1
print(total)

4 Review

4.1 `is_valid_variable_name`

I'm trying to write a function that takes a string and returns whether that string would be a valid variable name. Remember that a variable name can only have numbers, letters, and underscores. In addition, a variable name cannot start with a number. Describe why each of the versions below will not return the correct result.⁴

def is_valid_variable_name1(name):
    if name[0].isdecimal():
        return False
    for c in name:
        if c.isalnum() or c == "_":
            return True
        else:
            return False

def is_valid_variable_name2(name):
    ok_characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_"
    return name[0] in ok_characters and name in ok_characters

def is_valid_variable_name3(name):
    result = True
    for c in name:
        result = c.isalnum()
        result = c == "_"
    return result and name[0].isdecimal()

4.2 List Mystery

What will be printed by the following code?⁵

v = [4, 5, 6, 7, 8, 9]
v[0] = v[-1] - v[-2]
print(v)
i = v[0] + 2
v[i] = v[i - 2] - v[i - 3]
print(v)
v[len(v) - 2] = i * 2
print(v)

Footnotes:

mmmhtsgo op

None because the function has no return.

print(vec)

would show [0, 0, 0]

There will be a TypeError because tuples are immutable and thus ys[0] = t causes an error when ys is a tuple. If we changed v2 to be a list, the program would work and ["a", 2, 3] would be printed.

⁴

is_valid_variable_name1: this will return after checking only the first character in the loop.

# fixed version:
def is_valid_variable_name1(name):
    if not (name[0].isalpha() or name[0] == "_"):
        return False
    for c in name:
        if not (c.isalnum() or c == "_"):
            return False
    return True

is_valid_variable_name2: name in ok_characters checks if the entire name appears in ok_characters, rather than checking whether each character of name is in ok_characters. Also fails to check that the first characters is not a number.

is_valid_variable_name3:

# fixed version:
def is_valid_variable_name3(name):
    result = True
    for c in name:
        result = result and (c.isalnum() or c == "_")
    return result and not name[0].isdecimal()

⁵

[1, 5, 6, 7, 8, 9]
[1, 5, 6, 4, 8, 9]
[1, 5, 6, 4, 6, 9]

CS 111 f21 — while loops