Welcome back
How was lab? Was anything confusing?
Notes for week 1: https://tinyurl.com/4ypzsmzu
Important note: do not read ahead
x = [41, 2, 89, 50, 12, 13]
x
if x[0] % 2 == 0:
print(x[0])
if x[1] % 2 == 0:
print(x[1])
if x[2] % 2 == 0:
print(x[2])
if x[0] % 2 == 0:
print(x[0])
if x[1] % 2 == 0:
print(x[1])
if x[2] % 2 == 0:
print(x[2])
if x[3] % 2 == 0:
print(x[3])
if x[4] % 2 == 0:
print(x[4])
if x[5] % 2 == 0:
print(x[5])
x = [41, 2, 89, 50, 12, 13]
x
x = [41, 2, 89]
x
index = 3
if x[index] % 2 == 0:
print(x[index])
x = [41, 2, 89, 50, 12, 13]
x
index = 0
while index < len(x):
# How do I tell if x[index] is even?
if x[index] % 2 == 0:
print(x[index])
index += 1
print("Loop finished!")
Review
Last time, we covered:
• While loops
• Basic functions
While loop syntax
The syntax for while loops is:
while <condition>:
<expression>
As with if statements, the <condition> can be anything that evaluates to a boolean. The
<expression> can be complex.
a = 1
b = 1
num = 0
while num < 4:
print(a)
c = a + b
a = b
b = c
num += 1
print(a, b, c, num)
print()
num = 0
while num < 10:
print(num)
if num % 2 == 0:
print(num)
num += 1
print(num)
print()
IMPORTANT: THE WHILE LOOP MUST
TERMINATE
# WARNING
# WARNING
# WARNING
# DO NOT RUN THIS
while True:
print('hi')
# WARNING
# WARNING
# WARNING
# DO NOT RUN THIS
num = 0
while num < 10:
print(num)
2 ** 8
summ = 0
index = 1
summ += index * index
index += 1
summ += index * index
index += 1
summ += index * index
index += 1
summ += index * index
index += 1
summ += index * index
index += 1
summ
i = 1
summ = 0
while i <= 5: # i = 5
summ += i**2 # summ = 55
i += 1 # i = 5
print(summ)
# I want to add (and print) the first few numbers
# until their sum exceeds 100.
# but nothing is being printed
total = 0
i = 1
while total <= 100:
total += i
i += 1
print(i)
print("Done!")
Indentation
Indentation is critical. It tells Python what code is associated with which Python syntax we've
seen so far (if, while, functions).
# A tab is four spaces
a = 1
b = 1
if a == 0: # Level 0
print(a) # Level 1
print("hi") # Level 1
if b == 0: # Level 0
print(b) # Level 1
print(b) # Level 0
# Do not use one, two, or three spaces.
# Always use four spaces for an indent
a = 0
b = 1
if a == 0:
print(a) # <-- RED IS BAD
print("hi")
print(b)
# You can "nest" expressions
num = 0 # Level 0 = zero tabs
while num < 5: # Level 0
if num % 2 == 0: # Level 1 = one tab
print(num) # Level 2 = two tabs
num += 1 # Level 1
print("Done") # Level 0
# What's the difference?
num = 0
while num < 5:
if num % 2 == 0:
print(num)
num += 1
print("Done")
How Python runs
a = 0
a
a = 0
a + 1
a
a = 0
a + 1
(a + 1) * 2
a
num = 0
while num < 5:
if num % 2 == 0:
print(num)
num += 1
Printing
You can print anywhere you want. It can help you understand code
sum_of_nums = 0
n = 5
index = 1
while index <= n:
sum_of_nums += index
index += 1
sum_of_nums
15
sum_of_nums = 0
n = 5
index = 1
while index <= n:
print("Start of while loop")
print(sum_of_nums)
print(index)
sum_of_nums += index
index += 1
print("End of loop")
print(sum_of_nums)
print(index)
print()
sum_of_nums
Start of while loop
0
1
End of loop
1
2
Start of while loop
1
2
End of loop
3
3
Start of while loop
3
3
End of loop
6
4
Start of while loop
6
4
End of loop
10
5
Start of while loop
10
5
End of loop
15
6
15
Conditions
x = 1
x == 1
True
x = 1
y = (x == 1)
y
True
if True: # True/False, comparison, variable with a boolean
print("Hi")
if x == 1:
print("hi")
if y:
print("Hi")
Hi
hi
Hi
Syntax for functions
The general syntax for functions is:
def function_name(<arguments>):
<body>
return <expression>
There are several things to note about the syntax (we will go through examples):
• You can have zero, one, or more arguments.
• You need the : after the function
• The body can be complex!
• You technically don't need a return statement at the end, but we will almost alway have a
return at the end
def add_two_numbers(x, y):
# x = 5
# y = 15
s = x + y # s = 20
return s
add_two_numbers(5, 15)
20
def add_two_numbers(x, y):
# x = 5
# y = 15
return x
add_two_numbers(5, 15)
def add_two_numbers(x, y)
return x + y
add_two_numbers(10, 20)
Cell In[25], line 1
def add_two_numbers(x, y)
^
SyntaxError: expected ':'
def add_two_numbers(x, y):
x + y
add_two_numbers(10, 20)
In mathematics, the Fibonacci sequence is a sequence in which each number is the sum of the
two preceding ones. Numbers that are part of the Fibonacci sequence are known as Fibonacci
numbers, commonly denoted F n .
F 0=0
F 1=1
In particular: F n=F n− 1+ Fn − 2
The sequence commonly starts from 0 and 1, although some authors start the sequence from 1
and 1 or sometimes (as did Fibonacci) from 1 and 2. Starting from 0 and 1, the first few values in
the sequence are:
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144.
a = 0 # F_0
b = 1 # F_1
num = 0
c = b + a # F_2 = F_1 + F_0
a = b # F_1
b = c # F_2
num += 1 # 1
c = b + a # F_3 = F_2 + F_1
a = b # F_2
b = c # F_3
num += 1 # 2
c = b + a # F_4 = F_3 + F_2
a = b # F_3
b = c
num += 1 # 3
# F_4 = F_3 + F_2
def fibonacci(nth):
# nth = 8
a = 0
b = 1
num = 0
while num < nth - 1:
c = a + b
a = b
b = c
num += 1
return a
fibonacci(8)
13
fibonacci(15)
377
New kind of loop: for loop
In addition to while loops, there is another kind of loop called a for loop. Like while loops, they
allow for repeated computation. The syntax and how they work (semantics) are different though.
As usual we will do examples, the syntax, and go in depth.
for x in [1, 2, 3]:
print(x)
1
2
3
Basic syntax for for loops
The basic syntax for for loops is
for <variable> in <iterable expression>:
<expression> / <body>
Don't worry too much about what an <iterable expression> is for the time being. For this
lecture, it will always be a list, string, dictionary, or range. We will cover range later.
As with ifs and whiles, the <expression> can be complex.
x = [41, 2, 89, 50, 12, 13]
for y in x: # y = x[2]
print(y)
41
2
89
50
12
13
x = [41, 2, 89, 50, 12, 13]
y = x[0]
print(y)
y = x[1]
print(y)
y = x[2]
print(y)
y = x[3]
print(y)
y = x[4]
print(y)
y = x[5]
print(y)
41
2
89
50
12
13
x = [41, 2, 89, 50, 12, 13]
for y in x: # Level 0
if y % 2 == 0: # Level 1
print(y) # Level 2
2
50
12
summ = 0
x = [1, 2, 3, 4, 5, 6]
y = x[0]
summ += y * y
y = x[1]
summ += y * y
y = x[2]
summ += y * y
y = x[3]
summ += y * y
y = x[4]
summ += y * y
y = x[5]
summ += y * y
summ
91
x = [1, 2, 3, 4, 5, 6]
sum_of_squares = 0
for y in x:
sum_of_squares += y * y
sum_of_squares
91
x = "hello"
for y in x: # y = x[1]
print(y)
h
e
l
l
o
x = {
1: 2, # x indexed 0
3: 4, # x indexed 1
}
for y in x: # y = x indexed 1
print(y)
1
3
x = {
1: 2,
3: 4,
}
for y in x:
print(y, x[y])
1 2
3 4
Expressions can be complex!
We've seen condtionals within a for loop. You can also have loops inside loops!
for x in [1, 2]: # Level 0
for y in [4, 5]: # Level 1
print(x, y) # Level 2
for x in [1, 2, 3]:
y = 0
while y < x:
print(y)
y += 1
print()
Special function: range
When using for loops, we will sometimes use a special function called range. Don't worry too
much about what it returns for the time being - we will stick to using range in for loops.
for x in range(3): # [0, 1, 2]
print(x)
print()
for x in [0, 1, 2]:
print(x)
0
1
2
0
1
2
for x in range(10):
print(x)
0
1
2
3
4
5
6
7
8
9
for x in range(3, 7): # [3, 4, 5, 6]
print(x)
3
4
5
6
for x in range(6, 9): # [6, 7, 8]
print(x)
6
7
8
for x in range(3, 10, 2): # [3, 5, 7, 9]
print(x)
3
5
7
9
for x in range(10, 3, -1): # [10, 9, 8, 7, 6, 5, 4]
print(x)
10
9
8
7
6
5
4
for x in range(10, 3, -2): # [10, 8, 6, 4]
print(x)
10
8
6
4
for x in range(5.0):
print(x)
----------------------------------------------------------------------
-----
TypeError Traceback (most recent call
last)
Cell In[78], line 1
----> 1 for x in range(5.0):
2 print(x)
TypeError: 'float' object cannot be interpreted as an integer
One specific use of range
x = [47, 6, 9, 2]
for ind in range(len(x)):
print(ind, x[ind])
# print()
# for y in x:
# print(y)
0 47
1 6
2 9
3 2
x = [1, 2, 3, 4]
y = [5, 6, 7, 8]
mul_sum = 0
for ind in range(len(x)): # range(4) -> [0, 1, 2, 3]
mul_sum += x[ind] * y[ind]
mul_sum
for i in range(1, 3, -3):
print(i)
while loops vs for loops
Every while loop can be written as a for loop and vice versa. When to use which loop depends
on the situation at hand. For the time being, we will practice both ways.
x = [1, 2, 3, 4]
for y in x:
print(y * y)
print()
i = 0
while i < len(x):
print(x[i] * x[i])
i += 1
1
4
9
16
1
4
9
16
for num in range(10, 30, 7):
print(num)
print()
num = 10
while num < 30:
print(num)
num += 7
10
17
24
10
17
24
Advanced loop concepts: break
Sometimes we want to end a loop early. We can do this using the break keyword.
x = [1, 4, 7, 9, 11, 15]
# Compute the squared sum of the values of x until we reach 7
squared_sum = 0
for y in x: # y = x[2]
if y == 7:
break
squared_sum += y * y
squared_sum
17
# Fun exercise: do the same with a while loop!
# Breaks can come anywhere in the body of a for loop
y = [1, 2, 3]
for x in y: # x = y[0]
break
print(x)
print("hi")
hi
Advanced loop concepts: continue
Sometimes we want to skip the rest of the body of a loop. We can do this using the continue
keyword.
x = [1, 4, 7, 9, 11, 15]
# Compute the squared sum of the _even_ values of x
squared_sum = 0
for y in x: # y = x[3]
if y % 2 == 1:
continue
squared_sum += y * y
squared_sum
x = [1, 4, 7, 9, 11, 15]
# Compute the squared sum of the _odd_ values of x
squared_sum = 0
for y in x:
if y % 2 == 0:
continue
squared_sum += y * y
squared_sum
x = [1, 4, 7, 9, 11, 15]
# Compute the squared sum of the _odd_ values of x. Stop at 9
squared_sum = 0
for y in x: # y = x[3]
if y % 2 == 0:
continue
if y == 9:
break
squared_sum += y * y
squared_sum
Review redux: syntax for functions
The general syntax for functions is:
def function_name(<arguments>):
<body>
return <expression>
There are several things to note about the syntax (we will go through examples):
• You can have zero, one, or more arguments.
• You need the : after the function
• The body can be complex!
• You technically don't need a return statement at the end, but we will almost alway have a
return at the end
def fahrenheit_to_celsius(f):
# ???
return 0
def celsius_to_fahrenheit(f):
# ???
return 0
A more involved example
Let's say we had a meal with friends. We ordered multiple items.
Let's put the price of the items with a list:
• One shiro: 100 birr
• One tibbs: 200 birr
• Two firr-firr: 150 birr x 2
The list might look like [100, 200, 150, 150]
Let's create a function that computes the total for the bill.
def sum_bill(bill):
total = 0
for item in bill:
total += item
return total
sum_bill([100, 200, 150, 150])
sum_bill([100, 200, 200, 150, 150])
But wait! We forgot the tax. Let's say there's a 5% tax rate. We can reuse functions we've
created before. Let's try that now
def total_after_tax(bill):
sum_total = sum_bill(bill)
return sum_total * 1.05
total_after_tax([100, 200, 150, 150])
Functions can call functions!
As we've seen, functions can call functions. This will become critical later!
Warning: function naming
Functions cannot have the same name as a variable and vice versa. For this class, do not name a
function a variable you've already declared and vice versa.
More on function and variable names
There are specific reserved keywords in Python. Do not name functions and variables these
reserved keywords:
Types:
• int
• float
• str
• bool
• list
• dict
Syntax:
• if
• while
• for
• break
• continue
if = 1
