[PPT] - Variables and Expressions Craig Zilles (Computer Science) PowerPoint Presentation

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Variables and Expressions

Craig Zilles (Computer Science) February 3, 2020 https://go.illinois.edu/cs105sp20

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To Today

1. Objects, literals
2. Types and representation
Integers
Strings, Unicode, and Escaping
Reals, floating point
3. Identifiers, assignment, immutability
4. Expressions and Operator Precedence
Division, Floor division, and modulo operator
5. Modules and Importing
6. Excel: relative and absolute references
Moving formulas between cells

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Yo You are in good company (m (muddy uddy pt pts.)

"I find it difficult to memorize all the different

functions and commands and how to write them

correctly. "
"Will we have to know the encoded values of each

character in order to code?"

"I think having to remember the Unicode and escape

sequences was pretty difficult and confusing"

"The mass amounts of vocabulary was very confusing

and should be gone over."

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The The Pytho hon n interpr preter is yo your friend!

Use it to play around with the language

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Al All data a in Python is stored in an an ob

bject
Every object has:
A type
A value
Literals are textual descriptions, read by Python to make
bjects
“hi there”
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-203.5974

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We We’ve now met three ty types:

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
f 1’s and 0’s
The type says how to interpret the 1’s and 0’s

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Ho How in ints ar are stored

Integers are usually used for counting things

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int 1 000110101 int 2 000101011 110101001

Type Number

f chunks

Chunks (Store the number in binary)

Small number Large number

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Ho How st strings s are e stored ed

8 String 6 001000011 001010011

Type Number of characters Characters (Stored using Unicode encoding)

000100000 000110001 000110000 000110101 ‘CS 105’ C S 1 5

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Whi Which o ch of t the he f followi wing ng ar are c cons nside dered ‘ d ‘whi whitespace pace’?

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

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Whi Which o ch of t the he f followi wing ng ar are c cons nside dered ‘ d ‘whi whitespace pace’?

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

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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

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Ho How st strings s are e stored ed

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?

11 String 6 001000011 001010011

Type Number of characters Characters (Stored using Unicode encoding)

000100000 000110001 000110000 000110101 ‘CS 105’ C S 1 5

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Esc Escapi ping ng

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

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Ho How man any visible char arac acters ar are printed?

print('\\n\t\\t')

A) 1 B) 2 C) 3 D) 4 E) 5

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Nu Number ers s beyond integ eger ers

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

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Ho How fl floats s are e stored ed

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

15 Float

3.76341

+22

Type Mantissa Exponent

Can specify in scientific notation

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Wh Which isn’t a valid Python literal

A) 1.000001 B) 1E-7 C) 1,097 D) -3.00 E) '\'\'\'' # Consists only of single quotes and slashes

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We We’ve now met three ty 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()

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Va Variables

Useful to give names to objects
Names in Python have to follow certain rules:
Begin with letter or underscore
Contains only letters, numbers, or underscores
Avoid reserved words (also known as key words)
Python recommended style: Snake Case
lower_case_words_separated_by_underscores

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Wh Which are legal Pyt ython names?

1. ______
2. 12monkeys
3. m0nk3yM4n
A. 1 and 2
B. 1 and 3
C. 2 only
D. 2 and 3
E. 1, 2, and 3

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As Assig ignment

Variable names are bound to values with assignment

statements

Structure: variable_name = expression
What happens:
1. expression evaluated to compute a value
2. variable_name is bound to the value

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Wh What is the value of y af after this code ex executes

x = 2 y = x + 3 x = 5

A) 2 B) 3 C) 5 D) 8 E) 10

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Wh What is the value of y af after this code ex executes

x = 7 y = x x = x + 2

A) 2 B) 5 C) 7 D) 9 E) None of the above

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Im Immutability ility

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 object
If object is immutable, updating one variable

doesn’t affect the others

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Ga Garba bage e Collec ection

If an object isn’t bound to any variable, the Python

interpreter gets rid of it.

Frees up space for new objects

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Lo Logistics cs

Lab next week: Excel + Spatial skills development
Bring plug-in headphones if you can
Homework on PrairieLearn:
More points available than you need
Report any problems with “Report an error in this question”
Code reading exercises using natural language processing
Exam 0 next week: Sign up on

https://cbtf.engr.illinois.edu/sched/

Meant to get comfortable in the CBTF
Drawn almost completely from pre-lecture 1 & 2 and HW 1 & 2

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Lo Logistics, cs, c cont.

Need help?
Sending me email is probably not the answer
Questions about course content?
Search Piazza (first) or ask question on Piazza (second)
Office Hours: Wohlers Hall Computer Lab
CS 105 Tutoring (TBA soon)
Gies Tutoring
Questions about course policies, grading -> Piazza
Questions/documents about DRES, excused absences?
cs105admin@Illinois.edu

(our course administrator)

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Expr Expressi ssions ns

Any Python code fragment that produces a value
Can include:
Literals
Variables
Operators
Functions
Right-hand side of assignment can be arbitrary

expression

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Or Order of Op Operations

Parentheses

() highest precedence

Exponentiation

**

(unary) Positive, negative

+x, -x

Multiplication, Division, Modulo

*, /, %

Addition, Subtraction

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

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Wh What is the value of this expression?

3 ** 2

A) -9 B) -8 C) 8 D) 9 E) None of the above

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Go Good d Styl yle e with h Expr Expressi essions ns

Put a single space between every variable, operator, and

number

this_is + a_readable – expression
Break up complicated expressions
vs
Be generous with parentheses

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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

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Wh What would happen…

if I type the following into a brand new Python

interpreter: x + y A) nothing B) SyntaxError C) NameError D) ValueError E) TypeError

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Ex Expression

n 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

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Wh What (if any) kind of error is in this pr program?

value = input("Input your fave number!\n") print("Your new fave number is", value + 1)

A) No error B) SyntaxError C) NameError D) ValueError E) TypeError

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Di Divisi sion, Fl Floor Di Divisi sion, 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).

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Fl Floor divi visi sion an and modulo exam ample

dollars = product_cost_in_pennies // 100 cents = product_cost_in_pennies % 100

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Mo Modul dules

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

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Im Impor

rtin

ting mod

dule

les

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

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Ex Excel: Re Relative and Absolute Re 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

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Ne Next Week eek's s rea eading

We've introduced 3 types: int, float, string
These are individual things
It is useful to be able to group things together
Python has special types for these collections
Each has its own use
Lists are ordered sequences of things (e.g., play list)
Dictionaries map one thing to another
E.g., a gradebook maps a person to their grade
Sets are un-ordered collections (w/o duplicates)
E.g., a guest list
Intended to be first exposure; we'll see them again

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Se Sequence ces

Strings and Lists are both kinds of sequences
You can reach into both and pull things out: "indexing"
Python starts counting at zero
Lists are mutable! You can change them
There are functions for doing that

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