Reading Code

Warmup Exercises

Exercise 40 (Ice cream flavors)

Make a list of ice cream flavors, then print a menu with a number next to each flavor.

Flavors: Banana, Chocolate, Lemon, Pistachio, Raspberry, Strawberry, Vanilla

Exercise 41 (Ice cream combinations)

Print every possible sorbet duos from the same list of flavors. Don’t print recipes with twice the same flavor (no “Chocolate Chocolate”), and don’t print twice the same recipe (if you print “Vanilla Chocolate”, don’t print “Chocolate Vanilla”, or vice-versa).

Flavors: Banana, Chocolate, Lemon, Pistachio, Raspberry, Strawberry, Vanilla

Intro

We spend most of our time learning to write code, but just as important is knowing how to read and understand code. So for this exercise we’re going to practice reading code.

We’ll step through a small code sample, similar to how we did in in our Program Flow lesson. For each line that we walk through, you’ll see a section that looks like this. (Labels in blue are just for this example.)

line 5

variables

level	1
name	Yoshi

↑ Variables

Listing 95 steps to evaluate line

>>> print("Welcome", name)
>>> print("Welcome", "Yoshi")

↑ Steps to evaluate

output

↑ Output

level = 1
name = "Yoshi"

print("Starting level", level)
print("Welcome", name)

↑ Code sample

---

Each breakdown has 5 sections:

• code sample (on the left) – shows the code with the current line highlighted

• variables (1st on the right) – shows the variables that exist at the time when the line is evaluated

• steps to evaluate (2nd on the right) – shows the step by step process of evaluating the code

• output (3rd on the right) – shows the output of the code, with the current line highlighted

The above example is a breakdown of line 5.

Reading a while loop

Let’s walk through a simple while loop.

line 1

variables

none

Listing 96 steps to evaluate line

>>> total = 0

 

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

line 2

variables

total	0

Listing 97 steps to evaluate line

>>> i = 0

 

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 0, line 3

variables

total	0
i	0

Listing 98 steps to evaluate line

>>> i < 3
>>> 0 < 3  # True, so go to line 4

 

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 0, line 4

variables

total	0
i	0

Listing 99 steps to evaluate line

>>> print("loop", i)
>>> print("loop", 0)

loop 0

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 0, line 5

variables

total	0
i	0

Listing 100 steps to evaluate line

>>> total = total + i
>>> total = 0 + 0
>>> total = 0

loop 0

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 0, line 6

variables

total	0
i	0

Listing 101 steps to evaluate line

>>> i = i + 1
>>> i = 0 + 1
>>> i = 1
>>> # end of loop, go to line 3

loop 0

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 1, line 3

variables

total	0
i	1

Listing 102 steps to evaluate line

>>> i < 3
>>> 1 < 3  # True, so go to line 4
>>> True  # True, so go to line 4

loop 0

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 1, line 4

variables

total	0
i	1

Listing 103 steps to evaluate line

>>> print("loop", i)
>>> print("loop", 1)

loop 0

loop 1

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 1, line 5

variables

total	0
i	1

Listing 104 steps to evaluate line

>>> total = total + i
>>> total = 0 + 1
>>> total = 1

loop 0

loop 1

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 1, line 6

variables

total	1
i	1

Listing 105 steps to evaluate line

>>> i = i + 1
>>> i = 1 + 1
>>> i = 2
>>> # end of loop, go to line 3

loop 0

loop 1

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 2, line 3

variables

total	1
i	2

Listing 106 steps to evaluate line

>>> i < 3
>>> 2 < 3  # True, go to line 4
>>> True  # True, go to line 4

loop 0

loop 1

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 2, line 4

variables

total	1
i	2

Listing 107 steps to evaluate line

>>> print("loop", i)
>>> print("loop", 2)

loop 0

loop 1

loop 2

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 2, line 5

variables

total	1
i	2

Listing 108 steps to evaluate line

>>> total = total + i
>>> total = 1 + 2
>>> total = 3

loop 0

loop 1

loop 2

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 2, line 6

variables

total	3
i	2

Listing 109 steps to evaluate line

>>> i = i + 1
>>> i = 2 + 1
>>> i = 3
>>> # end of loop, go to line 3

loop 0

loop 1

loop 2

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

loop 3, line 3

variables

total	3
i	3

Listing 110 steps to evaluate line

>>> i < 3
>>> 3 < 3
>>> False  # False, so go to line 7

loop 0

loop 1

loop 2

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

line 7

variables

total	3
i	3

Listing 111 steps to evaluate line

>>> print("The total is:", total)
>>> print("The total is:", 3)

loop 0

loop 1

loop 2

The total is: 3

output

total = 0
i = 0
while i < 3:
    print("loop", i)
    total = total + i
    i = i + 1
print("The total is:", total)

---

Exercises

Exercise 42

In the following code sample, what is the value of name on line 12?

characters = [
    "mario",
    "luigi",
    "yoshi",
    "bowser"
]

i = 0

while i < len(characters):
    name = characters[i].upper()
    print(i+1, name)
    i += 1