This is CS50x
CS50’s Introduction to Computer Science
OpenCourseWare
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Lecture 0
!"Welcome
!"What is computer science?
!"Representing numbers
!"Text
!"Images, video, sounds
!"Algorithms
!"Pseudocode
!"Scratch basics
!"Abstraction
!"Conditionals and more
!"Demos
Welcome
, " This year, we’re back in Sanders Theatre, and David took CS50 himself as a sophomore
years ago, but only because the professor at the time allowed him to take the course
pass/fail.
" It turns out that computer science was less about programming than about problem
solving. And though there may be frustration from feeling stuck or making mistakes, there
will also be a great sense of gratification and pride from getting something to work or
completing some task.
" In fact, David lost two points on his first assignment for not following all of the
instructions correctly:
" And while this code (written in a programming language as opposed to a language
like English) looks cryptic at first, it may only take weeks or months before we can
understand main programming concepts and even teach ourselves new languages.
" Importantly,
what ultimately matters in this course is not so much where you end up relative to
your classmates but where you end up relative to yourself when you began
" In fact, two-thirds of CS50 students have never taken a computer science course before.
What is computer science?
" Computer science is fundamentally problem solving, but we’ll need to be precise and
methodical.
" We can think of problem solving as the process of taking some input (a problem we want
to solve) and generate some output (the solution to our problem).
, " To begin doing that, we’ll need a way to represent inputs and outputs, so we can store
and work with information in a standardized way.
Representing numbers
" To count the number of people in a room, we might start by using our fingers, one at a
time. This system is called unary, where each digit represents a single value of one.
" To count to higher numbers, we might use ten digits, 0 through 9, with a system called
decimal.
" Computers use a simpler system called binary, with just two digits, 0 and 1.
" For example, in binary this would be 0:
0 0 0
" And this would be 1:
0 0 1
" (We don’t need the leading zeroes, but we’ll include them to see the patterns more
easily.)
" Since there is no digit for 2, we’ll need to change another digit to represent the next
number:
0 1 0
" Then we’ll “add 1” to represent 3:
0 1 1
" And continue the pattern for 4 …:
1 0 0
, " … 5 …:
1 0 1
" … 6 …:
1 1 0
" … and 7:
1 1 1
" Each binary digit is also called a bit.
" Since computers run on electricity, which can be turned on or off, we can simply represent
a bit by turning some switch on or off to represent a 0 or 1.
" Inside modern computers, there are billions of tiny switches called transistors that can be
turned on and off to represent different values.
" And the pattern to count in binary with multiple bits is the same as the pattern in decimal
with multiple digits.
" For example, we know the following number in decimal represents one hundred and
twenty-three.
1 2 3
" The 3 is in the ones place, the 2 is in the tens place, and the 1 is in the hundreds
place.
" So 123 is 100×1 + 10×2 + 1×3 = 100 + 20 + 3 = 123 .
" Each place for a digit represents a power of ten, since there are ten possible digits
for each place. The rightmost place is for 100, the middle one 101, and the leftmost
place 102:
102 101 100
1 2 3
" In binary, with just two digits, we have powers of two for each place value:
22 21 20
# # #
" This is equivalent to:
4 2 1
# # #
CS50’s Introduction to Computer Science
OpenCourseWare
Donate ! (https://cs50.harvard.edu/donate)
David J. Malan (https://cs.harvard.edu/malan/)
! (https://www.facebook.com/dmalan) " (https://github.com/dmalan) #
(https://www.instagram.com/davidjmalan/) $ (https://www.linkedin.com/in/malan/)
% (https://orcid.org/0000-0001-5338-2522) & (https://www.quora.com/profile
/David-J-Malan) ' (https://www.reddit.com/user/davidjmalan) (
(https://www.tiktok.com/@davidjmalan) ) (https://twitter.com/davidjmalan)
Lecture 0
!"Welcome
!"What is computer science?
!"Representing numbers
!"Text
!"Images, video, sounds
!"Algorithms
!"Pseudocode
!"Scratch basics
!"Abstraction
!"Conditionals and more
!"Demos
Welcome
, " This year, we’re back in Sanders Theatre, and David took CS50 himself as a sophomore
years ago, but only because the professor at the time allowed him to take the course
pass/fail.
" It turns out that computer science was less about programming than about problem
solving. And though there may be frustration from feeling stuck or making mistakes, there
will also be a great sense of gratification and pride from getting something to work or
completing some task.
" In fact, David lost two points on his first assignment for not following all of the
instructions correctly:
" And while this code (written in a programming language as opposed to a language
like English) looks cryptic at first, it may only take weeks or months before we can
understand main programming concepts and even teach ourselves new languages.
" Importantly,
what ultimately matters in this course is not so much where you end up relative to
your classmates but where you end up relative to yourself when you began
" In fact, two-thirds of CS50 students have never taken a computer science course before.
What is computer science?
" Computer science is fundamentally problem solving, but we’ll need to be precise and
methodical.
" We can think of problem solving as the process of taking some input (a problem we want
to solve) and generate some output (the solution to our problem).
, " To begin doing that, we’ll need a way to represent inputs and outputs, so we can store
and work with information in a standardized way.
Representing numbers
" To count the number of people in a room, we might start by using our fingers, one at a
time. This system is called unary, where each digit represents a single value of one.
" To count to higher numbers, we might use ten digits, 0 through 9, with a system called
decimal.
" Computers use a simpler system called binary, with just two digits, 0 and 1.
" For example, in binary this would be 0:
0 0 0
" And this would be 1:
0 0 1
" (We don’t need the leading zeroes, but we’ll include them to see the patterns more
easily.)
" Since there is no digit for 2, we’ll need to change another digit to represent the next
number:
0 1 0
" Then we’ll “add 1” to represent 3:
0 1 1
" And continue the pattern for 4 …:
1 0 0
, " … 5 …:
1 0 1
" … 6 …:
1 1 0
" … and 7:
1 1 1
" Each binary digit is also called a bit.
" Since computers run on electricity, which can be turned on or off, we can simply represent
a bit by turning some switch on or off to represent a 0 or 1.
" Inside modern computers, there are billions of tiny switches called transistors that can be
turned on and off to represent different values.
" And the pattern to count in binary with multiple bits is the same as the pattern in decimal
with multiple digits.
" For example, we know the following number in decimal represents one hundred and
twenty-three.
1 2 3
" The 3 is in the ones place, the 2 is in the tens place, and the 1 is in the hundreds
place.
" So 123 is 100×1 + 10×2 + 1×3 = 100 + 20 + 3 = 123 .
" Each place for a digit represents a power of ten, since there are ten possible digits
for each place. The rightmost place is for 100, the middle one 101, and the leftmost
place 102:
102 101 100
1 2 3
" In binary, with just two digits, we have powers of two for each place value:
22 21 20
# # #
" This is equivalent to:
4 2 1
# # #
