CS 105: VARIABLES AND EXPRESSIONS Max Fowler (Computer Science) - - PowerPoint PPT Presentation

CS 105: VARIABLES AND EXPRESSIONS

Max Fowler (Computer Science) https://pages.github-dev.cs.illinois.edu/cs-105/web/

June 14, 2020

Video Series Two Topics

 Objects, Literals  Types and Representation  Identifiers, Assignment, Immutability  Expressions and Operator Precedence, Module Imports  Excel Referencing and Moving Formulas Between Cells

Objects and Literals

All data in Python is stored in an object

 Objects have:

a type a value

 Literals are textual descriptions, read by Python to make an

• bject

"Hello there!" is a -> 5 is a -> 23.32 is a ->

5 int

string literal integer literal floating point (float) literal

Three types we've met

 Integers: whole numbers of arbitrary precision  Strings: e.g., our string literals like “Hello CS 105!”  Floating point numbers: approximations of real numbers  Types are important, because it specifies how to store data

 Computers represent everything as a finite number of 1’s and 0’s  The type says how to interpret the 1’s and 0’s

Video Question – Objects have a WHAT and a WHAT?

Types and Representations

How ints are stored

 Integers are usually used for counting things

How strings are stored

Which of the following are considered ‘whitespace’?

A) Spaces B) Tabs C) Newlines D) Spaces and Tabs E) Spaces, Tabs, and Newlines

Which of the following are considered ‘whitespace’?

A) Spaces B) Tabs C) Newlines D) Spaces and Tabs E) Spaces, Tabs, and Newlines

In computer programming, whitespace is any character or series of characters that represent horizontal or vertical space in typography. When rendered, a whitespace character does not correspond to a visible mark, but typically does occupy an area on a page. --Wikipedia

How strings are stored

 Unicode can encode pretty much any character

 Including many things that aren’t on your computer keyboard  How do we tell Python we want to use those characters?

 Can specify the Unicode codepoint: e.g., 0394 is the Greek delta (Δ)  How do we distinguish a codepoint from a number?

Escaping

 Treat slash (\) as a special character  \ means that the following characters should be interpreted differently

 \u followed by a number is a code point

 '\u0394’ is the Greek delta (Δ)

 \” and \’ are quote characters that don’t end a string  \t encodes a tab  \n encodes a new line  \\ encodes a slash

Numbers beyond integers

 Integers only represent whole numbers  Sometimes you need to represent numbers between

integers

 Often when measuring things (lengths, speeds, etc.)

 Real numbers:

 Mathematically, there are an infinite number of numbers between each

integer

 On computers, we can’t represent an infinite number of possible

numbers with a finite number of bits

 Can only approximate real numbers

How floats are stored

 Like scientific notation: 6.02 x 1023

 mantissa x 10exponent

 Fixed-size mantissa: finite precision

 Normally hidden by python  format(0.1, '.17f')

 Fixed-size exponent: limited range

 100.1 ** 200

Can specify in scientific notation

We’ve now met three types:

 Integers: whole numbers of arbitrary precision  Strings: e.g., our string literals like “Hello CS 105!”  Floating point numbers: approximations of real numbers

 You can ask a value what its type is using:

type(expression)

 You can convert between them with str() and

int() and float()

Video Question – How many visible characters are printed with print('\\n\t\\t')?

Identifiers, Assignments, Immutability

What's a variable?

 A variable is effectively a name for an object  Names in Python have rules…

 Begin with letter or underscore  Contain only letters, numbers, underscores  While not a rule, AVOID reserved words (key words)

 Python recommends Snake Case

 snek_case_uses_underscores

Danger Noodle is not the only case

art by @allison_horst

Assignment?

Variables names are bound to values with

assignment statements

Structure: variable_name = expression How does this work?

First, expression is evaluated to a value Second, variable_name is bound to the value

Immutability

 strings, ints, and floats are all immutable Once an object has been created, it can’t be

changed

New ones must be made instead  Multiple variables can be bound to the same

• bject

If object is immutable, updating one variable

doesn’t affect the others

Video Question – What is the value of y after this code executes?

x = 2 y = x + 3 x = 5

 2  3  5  8

Expressions and Operator Precedence, Modules

Expressions

 Any Python code fragment that produces a value  Can include:

 Literals  Variables  Operators  Functions

 Right-hand side of assignment can be arbitrary expression

Order of Operations

 Parentheses

() highest precedence

 Exponentiation

**

 (unary) Positive, negative

+x, -x

 Multiplication, Division, Modulo

*, /, %

 Addition, Subtraction

+, - lowest precedence Left-to-right within a precedence level

Order of operations (full gory details)

highest lowest

Good style with expressions

 Put a single space between every variable, operator, and number

 this_is + a_readable – expression

 Be generous with parentheses – almost no such thing as too much  Break up complicated expressions

total = num_machines * (cost_per_machine * (1 + tax_rate) + shipping rate)

machine_cost = num_machines * cost_per_machine machine_cost_with_tax = machine_cost * (1 + tax_rate) shipping_cost = num_machines * shipping_rate total = machine_cost_with_tax + shipping_cost

Expression types

 Result type generally depends on types of values in

expression:

 an_integer + another_integer -> an integer  a_float + another_float -> a float  a_string + another_string -> a string

 If you mix ints and floats, ints will be promoted to floats:

 3.0 + 7 -> 3.0 + 7.0 -> 10.0

 Generally can’t mix strings with either ints or floats

Division, Floor Division, and Modulo

 Division operator (/) gives best approximation to true result

 always results in a float

 Floor Division (//) rounds down to closest whole number

 Uses normal type rules for result

 Modulo operator (%) performs a division and returns a

remainder

 Uses normal type rules for result

 For any numbers x and y, the following equality holds:

y = (y // x) * x + (y % x).

Floor division and modulo example

dollars = product_cost_in_pennies // 100 cents = product_cost_in_pennies % 100

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Modules

 Very few real computer programs are written from scratch

 Too inefficient

 Frequently use previously written code

 Libraries  Python functions you previously wrote

 We call both of these modules

32

Importing modules

 import command puts module in your program’s namespace  Access functions and variables in module with qualified name:

math.sin(7.3)

 Access documentation with help() and tab completion

33

Video Question – What is the value the following expression?

• 3 ** 2

 -9  -8  8  9

Excel – cell referencing, formula dragging

Excel: Relative and Absolute References

 Every cell in Excel has a name: e.g., C7 (column C, row 7)  When written in an Excel expression, this is a relative reference

 If moved/copied to another cell, it will change proportionally

 If you move = 2 * C7 down two rows, it will become = 2 * C9

 You can make absolute references by adding $ before row and/or

column

 $C$7 moved anywhere stays $C$7  $C7 moved two down and two to the right becomes $C9  C$7 moved two down and two to the right becomes E$7

Video Question – If a relative reference is drug down in Excel, what changes?