Lecture 3 – Expressions and Data Types

DSC 10, Fall 2022



Lots of programming – follow along in the notebook by clicking the "Expressions and Data Types" link on the course website.

What is code? What are Jupyter Notebooks? 💻

What is code?

Why Python?

Jupyter Notebooks 📓

Aside: lecture slides

This button!


Python as a calculator

Arithmetic operations

Operation Operator Example Value
Addition + 2 + 3 5
Subtraction - 2 - 3 -1
Multiplication * 2 * 3 6
Division / 7 / 3 2.66667
Remainder % 7 % 3 1
Exponentiation ** 2 ** 0.5 1.41421

Python uses the typical order of operations – PEMDAS (BEDMAS?)


In the cell below, replace the ellipses with an expression that's equivalent to

$$(19 + 6 \cdot 3) - 15 \cdot \left(\sqrt{100} \cdot \frac{1}{30}\right) \cdot \frac{3}{5} + \frac{4^2}{2^3} + \left( 6 - \frac{2}{3} \right) \cdot 12 $$

Try to use parentheses only when necessary.



Below, we compute the number of seconds in a year.

If we want to use the above value later in our notebook to find, say, the number of seconds in 12 years, we'd have to copy-and-paste the expression. This is inconvenient, and prone to introducing problems.

It would be great if we could store the initial value and refer to it later on!

Variables and assignment statements

$$ \overbrace{\texttt{myvariable}}^{\text{name}} = \overbrace{\texttt{2 + 3}}^{\text{any expression}} $$

Note that before we use it in an assignment statement, more_than_1 has no meaning.

After using it in an assignment statement, we can ask Python for its value.

Anytime we use more_than_1 in an expression, 10 is substituted for it.

Note that the above expression did not change the value of more_than_1, because we did not re-assign more_than_1!

Naming variables

The following assignment statements are valid, but use poor variable names 😕.

The following assignment statements are valid, and use good variable names ✅.

The following "assignment statements" are invalid ❌.

Assignment statements are not mathematical equations!

A variable's value is set at the time of assignment

Assignment statements are not promises – the value of a variable can change!

Note that even after changing uc, we did not change sd, so it is still the same as before.

A helpful analogy

Concept Check ✅ – Answer at cc.dsc10.com

Assume you have run the following three lines of code:

side_length = 5
area = side_length ** 2
side_length = side_length + 2

What are the values of side_length and area after execution?

A. side_length = 5, area = 25

B. side_length = 5, area = 49

C. side_length = 7, area = 25

D. side_length = 7, area = 49

E. None of the above

Aside: hit tab to autocomplete a set name

Call expressions 📞

Algebraic functions

$$f(x, y) = 2x^2 + 3xy - 1$$ $$f\left(\frac{1+2}{3+4}, (-1)^{15}\right)$$

Call expressions

Some functions can take a variable number of arguments

Use the ? after a function to see the documentation for a function

Or, use the help function, e.g. help(max).

Example: round

Nested evaluation

We can nest many function calls to evaluate sophisticated expressions.

...how did that work?

Import statements

Example: import math

sqrt, log, pow, etc.

It also has constants built-in!

Concept Check ✅ – Answer at cc.dsc10.com

Assume you have run the following statements:

x = 3
y = -2

Which of these examples results in an error?

A. abs(x, y)

B. math.pow(x, abs(y))

C. round(x, max(abs(y ** 2)))

D. math.pow(x, math.pow(y, x))

E. More than one of the above

Data types

What's the difference? 🧐

To us, 2.0 and 2 are the same number, $2$. But to Python, these appear to be different! 😱

Data types

Two numeric data types: int and float



The pitfalls of float

Type coercion between int and float

Aside: Jupyter memory model

Our notebook still remembers all of the variables we defined earlier in the lecture.

Summary, next time


Next time

We'll learn about strings, a data type in Python designed to store text. We'll also learn how to store many pieces of information in a single value, using arrays.

Note: We will introduce some code in labs and homeworks as well. Not everything will be in lecture. You will learn by doing!