Where we are: Lecture 3, part 3 of 3

Why loops?

for i in range(1, 6):
    print(f"Message {i} checked")

Message 1 checked
Message 2 checked
Message 3 checked
Message 4 checked
Message 5 checked

Anatomy of a for loop

inbox = ["Hi", "Win cash", "Bye"]
for msg in inbox:
    print(msg)

Hi
Win cash
Bye

Tracing a for loop, round by round

Each round, msg takes the next item. The body runs once per round.

inbox = ["Hi", "Win cash", "Bye"]
for msg in inbox:
    print(len(msg), msg)

2 Hi
8 Win cash
3 Bye

range(): a sequence of numbers

list() is only here so we can see the numbers. In a for loop you write range(...) directly.

Predict the range

for i in range(2, 11, 3):
    print(i)

Answer: Predict the range

for i in range(2, 11, 3):
    print(i)

2
5
8

The accumulator: adding up as you go

Start with 0 before the loop, then add each item inside the loop.

nums = [4, 7, 1, 8]
total = 0
for n in nums:
    total = total + n
print(total)

20

Watch it run: a for loop

Counting: an if inside a loop

The count goes up only in rounds where the condition is True.

count = 0
for i in range(1, 8):
    if i % 2 == 0:
        count = count + 1
print(count)

3

Anatomy of a while loop

n = 3
while n > 0:
    print(n)
    n = n - 1
print("Go!")

3
2
1
Go!

Tracing the while loop

The condition is checked before each round. The loop ends the first time it is False.

n = 3
while n > 0:
    print(n)
    n = n - 1
print("Go!")

3
2
1
Go!

Watch it run: a while loop

for or while?

Infinite loops, and how to stop one

Without n = n - 1, n stays 3 and n > 0 is always True. In Colab, click the stop square next to the cell, or Runtime > Interrupt execution.

n = 3
while n > 0:
    print(n)
    # forgot: n = n - 1   ->  prints 3 forever

break: stop the loop early

inbox = ["Hi", "See you", "WIN cash", "Bye"]
for msg in inbox:
    if "win" in msg.lower():
        print("First spam:", msg)
        break
    print("OK:", msg)

OK: Hi
OK: See you
First spam: WIN cash

continue: skip to the next round

for msg in ["Hi", "", "Bye", ""]:
    if msg == "":
        continue
    print("Checking:", msg)

Checking: Hi
Checking: Bye

enumerate(): position and item together

inbox = ["Hi", "Win cash", "Bye"]
for i, msg in enumerate(inbox):
    print(i, msg)

0 Hi
1 Win cash
2 Bye

Nested loops: a loop inside a loop

spam_words = ["free", "win"]
inbox = ["Win cash", "Class at 10",
         "Free gift"]
for msg in inbox:
    found = False
    for word in spam_words:
        if word in msg.lower():
            found = True
    print(found, msg)

True Win cash
False Class at 10
True Free gift

Tracing the nested loop

Outer loop: one message at a time. Inner loop: every word for that message.

for msg in inbox:
    found = False
    for word in spam_words:
        if word in msg.lower():
            found = True
    print(found, msg)

Watch it run: nested loops

Build it: a rule-based spam filter

Loops + decisions + a list of suspicious words: the filter from Lecture 1's demo, in real code.

spam_words = ["free", "win", "prize", "cash"]
inbox = ["Win a free phone",
         "Class moved to room 305",
         "Claim your cash prize",
         "See you at the library"]
spam_count = 0
for msg in inbox:
    is_spam = False
    for word in spam_words:
        if word in msg.lower():
            is_spam = True
    if is_spam:
        spam_count = spam_count + 1
        print("SPAM  ", msg)
    else:
        print("OK    ", msg)
print(f"Flagged {spam_count} of {len(inbox)}")

SPAM   Win a free phone
OK     Class moved to room 305
SPAM   Claim your cash prize
OK     See you at the library
Flagged 2 of 4

Where rules break

Same rules, two trickier messages. Predict the labels before you run it.

tricky = ["Claim your FR33 g1ft now",
          "Free lunch after the seminar"]
for msg in tricky:
    is_spam = False
    for word in spam_words:
        if word in msg.lower():
            is_spam = True
    print(is_spam, msg)

False Claim your FR33 g1ft now
True Free lunch after the seminar

Common mistakes with loops

Read the last line of an error first. It names the problem.

for i in range(5)
    print(i)

SyntaxError: expected ':'

for i in range(5):
print(i)

IndentationError: expected an indented block after 'for' statement on line 1

for x in 5:
    print(x)

TypeError: 'int' object is not iterable

for i in range(3):
    totl = totl + i

NameError: name 'totl' is not defined

Solutions 1 and 2

range(1, N + 1) includes N. end=" " prints on one line instead of a new line each time.

N = 20
for i in range(1, N + 1):
    if i % 2 == 0:
        print(i, end=" ")
print()
for i in range(1, N + 1):
    if i % 5 != 0:
        print(i, end=" ")

2 4 6 8 10 12 14 16 18 20 
1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 

Solution 3: brackets matter

and is evaluated before or. Without brackets, 15 would wrongly be printed.

N = 20
for i in range(1, N + 1):
    if (i % 3 == 0 or i % 4 == 0) and i % 5 != 0:
        print(i, end=" ")

3 4 6 8 9 12 16 18 

Solution 4: primes with a nested loop

A prime has no divisor between 2 and itself. break stops checking as soon as one divisor is found.

N = 20
for n in range(2, N + 1):
    is_prime = True
    for d in range(2, n):
        if n % d == 0:
            is_prime = False
            break
    if is_prime:
        print(n, end=" ")

2 3 5 7 11 13 17 19 

Solutions 5 and 6

Second largest: skip the maximum, then track the largest of the rest. Digits: take the last digit, then drop it.

nums = [4, 9, 2, 9, 7]
largest = max(nums)
second = None
for x in nums:
    if x != largest:
        if second == None or x > second:
            second = x
print(second)

n = 4721
total = 0
while n > 0:
    total = total + n % 10
    n = n // 10
print(total)

7
14

Today's toolkit

Homework 3.3

Due before Lecture 4.1 · [date]