Indefinite Looping with while
Truthiness and Falsiness A value is truthy if
when it is cast to a boolean, it casts to True. A value is
Falsy if when it is cast to a boolean, it casts to False.
Note that if statements will act on truthiness and falsiness.
Here are examples of truthy and falsy expressions.
>>> bool(0)
False
>>> bool(314)
True
>>> bool("")
False
>>> bool("weqrijterwwer")
True
>>> bool(None)
False
>>> bool([])
False
>>> bool([1,2,3])
True
Question What's +=? It's a compound
assignment operator. See these examples.
>>> x = 5
>>> x = x + 2
>>> x
7
>>> x += 2
>>> x
9
>>> x *= 2
>>> x
18
>>> #compound assignment operator
>>> x -= 1
>>> x
17
Afraid of commitment? So far, our looping has been walking through a collection such as a list, dictionary, or range object. Now we will look at a construct that can loop indefinitely.
Interchangeability The while loop is all you
really need. However as you will see, the for loop is less
error-prone.
A while statement is a boss statement. It looks like this.
while predicate:
block
The predicate is something that is truthy or falsy. If the predicate is truthy, the block runs. If not, you are out of the loop. Think of this as a "sticky if."
Let's write a function to see if a numerical list is in order.
def is_in_order(x):
"""prec: x is a homogeneous list of sortable items
postc: returns True if the list is in ascending order"""
k = 0
while k < len(x) - 1:
if x[k] > x[k+1]:
return False
k += 1
return True
def is_in_order_for(x):
for k in range(0, len(x) - 1):
if x[k] > x[k + 1]:
return False
return True
Let's write the Bozo Sort! Here is the algorithm
- Check to see if the list is sorted in order
- If it is, you are done.
- If not, shuffle and repeat.
def bozo(x):
while not is_in_order(x):
shuffle(x)
x = [5,2,8,9, 3, -2, -9, 6, 4]
bozo(x)
print(x)
Can you change each for to a while?
Get your name in lights by pasting a solution into chat!
def print_list(x):
for k in x:
print(k)
x = ["a", 1, True]
print_list(x)
Drake
def print_list(x):
a = 0
while a < len(x):
print(x[a])
a += 1
x = [1,2,3,4,5]
print_list(x)
Morrison appealing to truthiness.
def print_list(x):
while x:
print(x[0])
x = x[1:]
x = [1,2,3,4,5]
print_list(x)
def count_down(n):
for k in reversed(range(n + 1)):
print k
print("Blastoff!")
count_down(10)
Mitchell
def count_down_while(n):
k = n
while k >= 0:
print(k)
k -= 1
print("Blastoff!")
def harvest_caps(s):
out = ""
for k in s:
if k.isupper():
out += k
return out
print(harvest_caps("AEioU"))
def harvest_caps_while(s):
out = ""
k = 0
while k < len(s):
if s[k].isupper():
out += s[k]
k += 1
return out
Hanging and Spewing Uh oh,
while gone wild. Hanging is when your
program stops for "no good reason." It's frozen.
Spewing occurs when your program spews text to the screen withut surcease. In this case use control-c(Unix) or control-z(windoze).
Mini Case Study: A Number Guessing Game The computer picks a random integer 1-100 and it tells you if your guess is too high, too low, or if it is correct.
from random import randint
secret_number = randint(1,100)
guess = 0
while guess != secret_number:
guess = input("Enter a guess: ")
guess = int(guess)
if guess < secret_number:
print(f"Your guess {guess} is too small.")
elif guess > secret_number:
print(f"Your guess {guess} is too big.")
##what is true here
print(f"Your guess {guess} is correct!")
Kersey
from random import randint
secret_number = randint(1,100)
guess = 0
while guess != secret_number:
guess = input("Enter a guess: ")
guess = int(guess)
if guess < secret_number:
print(f"Your guess {guess} is too small.")
elif guess > secret_number:
print(f"Your guess {guess} is too big.")
##what is true here
print(f"Your guess {guess} is correct!")
Mitchell
from random import randint
secret_number = randint(1,100)
guess = 0
while guess != secret_number:
guess = input("Enter a guess: ")
guess = int(guess)
if guess < secret_number:
print(f"Your guess {guess} is too small.")
elif guess > secret_number:
print(f"Your guess {guess} is too big.")
##what is true here
print(f"Your guess {guess} is correct!")
Here it is getting played.
unix> python guessing_game.py Enter a guess: 50 Your guess 50 is too small. Enter a guess: 75 Your guess 75 is too big. Enter a guess: 63 Your guess 63 is correct!
Can you tell the user how many guesses he made?
Mini Case Study: Nagging a Dolt This program asks a user to enter an integer. It nags the user until a parseable integer is entered.
def is_legit_integer(x):
k = 0
if x[k] == "+" or x[k] == "-":
k += 1
while k < len(x):
if not x[k].isdigit():
return False
k += 1
return True
number = "x"
while not is_legit_integer(number):
number = input("Enter an integer")
print(f"{number} is an integer")
Here is a smart person.
unix> python nag.py Enter an integer56
Here is the dolt
(base) MAC:Thu Mar 04:11:22:0304B> python nag.py Enter an integercows Enter an integerhorses Enter an integercrowley Enter an integerdunderhead Enter an integer3432 3432 is an integer
Please sign on.
(base) MAC:Thu Mar 04:11:23:0304B> python nag.py Enter an integer-5 -5 is an integer (base) MAC:Thu Mar 04:11:23:0304B> python nag.py Enter an integer+14 +14 is an integer
Mini Case Study: Roll a fair pair of dice until you get doubles Print out the rolls
from random import randint
def roll_dice():
return (randint(1,6), randint(1,6))
x = (0, 100)
count = 0
while x[0] != x[1]:
x = roll_dice()
count += 1
print(f"You rolled doubles of {x[0]} after {count} rolls.")
from random import randint
def roll_dice():
return (randint(1, 6), randint(1, 6))
roll_val = (0, 1)
k = 0
while roll_val[1] != roll_val[0]:
k += 1
roll_val = roll_dice()
print(f"You rolled a {roll_val[0]} and a {roll_val[1]}")
print(f"Congratulations! You rolled double {roll_val[0]}s! It took you {k} rolls")
(base) MAC:Thu Mar 04:11:32:0304B> python moore.py (1, 6) (2, 4) (1, 2) (4, 4) You rolled doubles of 4 after 4 rolls. (base) MAC:Thu Mar 04:11:32:0304B> python moore.py (2, 4) (3, 4) (2, 1) (2, 4) (3, 6) (2, 3) (2, 5) (6, 3) (2, 4) (1, 3) (5, 6) (5, 3) (5, 3) (4, 2) (6, 3) (1, 3) (1, 5) (4, 1) (3, 6) (6, 5) (5, 4) (3, 2) (3, 6) (6, 5) (4, 3) (5, 5) You rolled doubles of 5 after 26 rolls. (base) MAC:Thu Mar 04:11:32:0304B> python moore.py (2, 2) You rolled doubles of 2 after 1 rolls. (base) MAC:Thu Mar 04:11:32:0304B> python moore.py (5, 2) (6, 5) (3, 1) (5, 1) (1, 3) (2, 1) (3, 4) (3, 6) (3, 6) (6, 3) (5, 5) You rolled doubles of 5 after 11 rolls. (base) MAC:Thu Mar 04:11:32:0304B> python moore.py (5, 3) (4, 2) (1, 2) (5, 2) (4, 2) (5, 2) (1, 5) (1, 4) (6, 1) (4, 1) (6, 5) (1, 3) (3, 1) (1, 3) (3, 3) You rolled doubles of 3 after 15 rolls.
Mini Case Study: Flip a fair coin until N heads occur N is a command line argument.