Week 0
Scratch
Welcome!
mindset
π Welcome to CS50!
This class is about more than computer programming!
The practical skills you will learn may impact your life well beyond computer science.
This class is about problem-solving in a way that is exceedingly empowering!
journey
Your Learning Journey
| Aspect | What to Expect |
|---|---|
| Challenge | "Drinking from the firehose" of knowledge |
| Growth | Advance "you" from "where you are today" |
| Accomplishment | Amazed at what you'll accomplish in weeks |
The standard: Your personal growth matters more than any imagined benchmark!
system
You're Not Alone!
First CS class? Don't be scared! Most peers are beginners too!
Support Network:
- Teaching Fellows
- Course Assistants
- Peer Community
- All here to help you!
Key Advice: Give yourself the time you need to learn. Everyone learns differently. If something doesn't work at first, you will grow with time!
empowerment
Problem-Solving Empowerment
You will likely take the problem solving that you learn here, which will likely be instantly applicable to your work beyond this course and even your career as a whole!
This course focuses on advancing you from where you are today rather than hitting some imagined standard.
growth
Growth Mindset
The most important opening consideration: Give the time you need to learn through this course.
Everyone learns differently. If something does not work out well at the start, know that with time you will grow and grow in your skill.
Progress over perfection is the goal!
Community!
global
π Join Our Community
You are part of:
- Harvard College
- Harvard Extension School
- edX.org worldwide learners
Virtual & In-Person Events:
- CS50 Puzzle Day
- CS50 Fair
campus life
Harvard Campus Life
Available to on-campus students:
- CS50 Lunches
- CS50 Hackathon
Community Support:
- Collaborative learning environment
- Peer support networks
- Teaching staff guidance
together
We're In This Together
Remember: Most students are experiencing their first computer science class - just like you!
Support System:
β
Teaching Fellows
β
Course Assistants
β
Peer Community
β
All here to help you succeed!
You're not alone in this learning journey!
support network
Learning Support Network
Don't be scared if this is your first computer science class! For most of your peers, this is their first computer science class, too!
Available Help:
- Teaching fellows
- Course assistants
- Your peer community
All are here to help you throughout the course!
events
π CS50 Events
Join us (whether in person or virtually) at:
CS50 Puzzle Day - Collaborative problem solving
CS50 Fair - Showcase your projects
On-campus students can also attend CS50 Lunches and CS50 Hackathon for additional community building and learning opportunities.
Computer Science and Problem Solving
core concept
The Essence of CS
Computer programming = Problem solving
counting systems
From Unary to Binary
Unary (Base-1):
- Counting one finger at a time
- Simple attendance tracking
Binary (Base-2):
- Computers use this!
- Bits: 0 or 1 (off/on)
- Millions of transistors = on/off switches
binary visual
π‘ Binary with 3 Light Bulbs
| 4 | 2 | 1 | Decimal |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 |
| 0 | 1 | 0 | 2 |
| 0 | 1 | 1 | 3 |
| 1 | 0 | 0 | 4 |
| 1 | 1 | 1 | 7 |
Binary Places: 2Β² 2ΒΉ 2β° = 4 2 1
Inside devices: Millions of "light bulbs" (transistors)!
black box
Input β [ Black Box ] β Output
What happens inside the "black box" is our focus!
ASCII
text mapping
ASCII: Letters β Numbers
Just as numbers are binary patterns of ones and zeros, letters are represented using ones and zeros, too!
Since there is an overlap between the ones and zeros that represent numbers and letters, the ASCII standard was created to map specific letters to specific numbers.
For example, the letter A was decided to map to the number 65. 01000001 represents the number 65 in binary.
example
How Messages Work
Text Message: "HI!"
Binary Numbers: 72, 73, 33
ASCII Mapping: H=72, I=73, !=33
Visualization for 65 (A):
128: 0 / 64: 1 / 32: 0 / 16: 0 / 8: 0 / 4: 0 / 2: 0 / 1: 1
benefits
The Power of Standards
ASCII enables:
- Universal character mapping
- Consistent text representation
- Reliable communication between systems
Without ASCII: Every computer might use different number-to-letter mappings!
Standards allow computers worldwide to agree on character representations.
Unicode
beyond ascii
Unicode: Global Text
Why needed? ASCII limited to 255 characters
Unicode expands:
- International characters: δΈζ, EspaΓ±ol, FranΓ§ais
- Special symbols
- Emoji! π
Supports thousands of characters beyond ASCII limits.
emoji world
The Emoji Revolution
Examples:
π π π π π π
π π π π π π π
π π π π π π π π π€ π€ π π€ π
Note: Display varies by device manufacturer
Unicode includes emoji that you probably use every day!
features
Unicode Technical Features
Key Aspects:
- Expanded character set beyond ASCII
- Supports thousands of characters
- Includes special characters and emoji
- Standardized binary patterns
Display Note: Each device manufacturer may display emoji slightly differently
More features are continually added to represent new characters and emoji!
RGB
color system
π¨ RGB Color Representation
RGB = Red, Green, Blue
Zeros and ones can be used to represent color!
Each color: 0-255 value (one byte)
Three bytes combine to create colors
Example: 72, 73, 33 as RGB = light yellow shade
rgb basics
How Images Work
Each pixel requires three bytes:
- Red value (0-255)
- Green value (0-255)
- Blue value (0-255)
Combination creates the color you see
Digital images = Grid of RGB values
Videos = Many images stored together
Music = Bytes representing sound patterns
Algorithms
What are Algorithms?
Definition: Step-by-step instructions to solve a problem
Example: Finding a name in a phone book
Different approaches = Different algorithms
Each has different efficiency characteristics!
search comparison 1
Algorithm 1: Linear Search
- Read page 1, then 2, then 3...
- Big-O: O(n)
search comparison 2
Algorithm 2: Two Pages at Once
- Search two pages simultaneously
- Big-O: O(n/2)
search comparison 3
Algorithm 3: Binary Search
- Start middle, eliminate half each time
- Big-O: O(logβn)
efficiency
Algorithm Efficiency
Big-O Notation measures how algorithm speed scales with input size
| Algorithm | Speed | Efficiency |
|---|---|---|
| Linear | O(n) | Slowest |
| Two Pages | O(n/2) | 2x Faster |
| Binary | O(logβn) | Most Efficient |
Key Insight: Doubling problem size barely affects binary search time!
CLICK HERE!
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