lecture 0

Question 1

where bits are derived from?

Answer 1

bits are binary digits, 0 and 1, two possibilities

Question 2

the only physical input of computer is

Answer 2

electricity

Question 3

in computer science, what number do we start counting

Answer 3

Computer scientists and, in turn, computer programs,
typically start counting from zero, just because it makes sense
because when everything is off, you might as well call that zero.

But at least in the computing world now, we’ll generally, by convention,

Question 4

what is a transistor?

Answer 4

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. Transistors are one of the basic building blocks of modern electronics. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit.

Question 5

what decimal number represent capital letter “A”

Answer 5

65. In binary it is 01000001. This is ASCII, the American Standard Code for Information Interchange

Question 6

Nowadays, how many bits do computer use to represent a character?

Answer 6

8 bits, before it was 7

Question 7

what is a byte?

Answer 7

1 byte is just 8 bits, so I can say 1 byte can represent a character.

Question 8

how many characters ASCII can represent

Answer 8

2^8 = 256 characters

Question 9

acronyms

Answer 9

acronyms

Are the members of your after-school club tripping over the name Student Parachuting League & Aerial Team? Why not use each word’s first letter to make an acronym, and call the club SPLAT? There, that’s better.

What do NASA, sonar, and scuba have in common? They are all acronyms. Not only does each letter come from the first letter of another word, but each group of letters is easy to pronounce as its own word. Though its roots are classic (acro means “tip” and nym means “name”), acronym is fairly new as English words go; it has only been around since the early 1940s.

Definitions of acronym
noun a word formed from the initial letters of the several words in the name
see more

Question 10

what is a verb in programming language?

Answer 10

verbs are functions, a statement that gets the computer to do something.

Question 11

what are the types of code

Answer 11

source code input in compiler to have machine code (binary digits)

Question 12

terminal window , what is CLI is that similar to it

Answer 12

command line interface

Question 13

in C, what is the long form of stdio.h

Answer 13

standard input output dot h

So standard io.h is just a very popular file
that is used in C programs that gives you the ability
to get input and output from the user.

Question 14

what is the shorthand notation for Ls in C?

Answer 14

list

Question 15

what is the symbol of an asterisk in C?

Answer 15

*

And that’s just a visual cue that that file is executable.

Question 16

what is Rm in C?

Answer 16

remove in command line interface

Question 17

what is Mv in C?

Answer 17

Maybe now I want to change this program, and I don’t
want to write hello.c, but goodbye.c.
Well, let me close the tab up there.
And yes, I could go and Right click or Control click on it,
but, again, we don’t need to use the graphical interface.
Let me go ahead and instead do mv hello.c goodbye.c.
Mv is the move command.
And even though it would be nice if it’s called rename instead of move,
move just moves one file to another location or to another name.
So if I do mv hello.c goodbye.c.
Notice what happened at top left.
Now my same file is called goodbye.c.

Question 18

what is another name of folder?

Answer 18

In a moment we’re going to start writing more programs,
and so my account is going to get a little bit
messy with more and more files over the course of today.
So suppose you want to create a folder, otherwise known as a directory.
I’m going to go ahead and type mkdir for make directory.
And then the name of the directory I want to make, for instance, lecture.

Question 19

what is mkdir in C

Answer 19

So suppose you want to create a folder, otherwise known as a directory.
I’m going to go ahead and type mkdir for make directory.
And then the name of the directory I want to make, for instance, lecture.
You can call it anything you want, but if I’m in lecture, I’m writing code,
maybe I want to store all of today’s files in a lecture directory.

Question 20

what is cd in C?

Answer 20

Now, in the world of Macs and PCs we, obviously,
would just double click on a folder and voila we’re inside of it.
In a command line interface you need to be more deliberate.
So I’m going to do cd, for change directory,
then lecture, and then I’m going to go ahead and hit Enter.
And now notice, and now it might make more sense
why this whole time we’ve been seeing in blue this tilde lecture slash.
The tilde just means my so-called home directory.

“Now I’m inside this lecture directory in my terminal window.”

Question 21

what is .. in C?

Answer 21

.. means moving certain file to the previous folder single dot : . refers to the current directory

Question 22

what is rmdir in C

Answer 22

remove directorly

Question 23

what is the difference of hello.c and hello

Answer 23

DAVID MALAN: Oh, it is though.
So recall that we have two things in the story.
We have source code, which is the C code I’ve been writing.
And then machine code, which is the zeros
and ones that the computer understands.
I have been writing all of my code in the file called hello.c.
But after I compile it with make, the make program
creates a new file called hello that technically contains only zeros
and ones.
And that is the machine code that I’m actually
running when I do dot slash hello.
So again, I can use rm and I can get rid of the hello program
just like I did before.
And now we’re back at the very beginning of the story
where we wrote this code from scratch.
If I now type make, and let me do this now, ls.
Notice I’ve only got one file.
Let me now do make hello.
I see that cryptic output, but if I type ls again, now I have two files.
And that’s because only the green one with the asterisk is executable.
That is the machine code that the compiler has created for me.
And I should say and disclaim, I’ve been telling a little bit of a white lie
today.
Make itself is not actually a compiler.
We’ll see next week exactly what make is doing.
But it’s making it easier for us to actually compile our code,
but more on that next time.

Question 24

what many bits and the possibility of integers in C or most modern system

Answer 24

So integers, again, are implemented in C as these things ints.
Ints only use, it turns out, 32 bits total.
And you would only know that by having been taught it
or looked it up for a particular computer system.
But they on CS50 IDE, and most modern systems, an integer is only 32 bits.

And the catch is that if you want to support both positive numbers
and negative numbers, you can represent 4 billion or so total possible values.
But if you want to go as far to the left and as far to the right on the number
line that I’m describing.
You can only really count as high as 2 billion in the positive direction
and negative 2 billion in the negative direction.

for number that is higher than 2 billion (positive or negative), we should use the data type long

Question 25

how compiler may confused about the bug

Answer 25

And I can’t emphasize this enough.
Sometimes– I got lucky here and I only have one mistake, apparently,
in the error messages– it is not going to be uncommon
for you to have two errors, 10 errors, in like two lines of code.
This is because sometimes when you have errors in your code,
the compiler sometimes just gets confused.
And if it gets sufficiently confused, it starts thinking everything
is an error in your actual code.
So the most important takeaway is that no matter
how many errors you seem to have, always scroll up to the top of the output
and address the first error first.

Question 26

in C, if:

int x = 1;
int y = 2;

float z = x + y;

what is the value of z?

Answer 26

the answer is 0, because when two int interact, their answer can be only int, the decimals are discarded.

Having z assign to be float is irrelevant. cuz the result of operation is already set to be int

so convert x and y to be float,

use method (float)x

Question 27

the logical sequence for for loop in C

Answer 27

But for now just realize this says, initialize i to zero.
Check the condition.
And if i is less than 50 print hello, world.
Then update i.

for ( int i = 0; i > 0; i++)

Question 28

Can we use double code for char in C?

Answer 28

And this is because C is very specific when it comes to its data types.
All this time I’ve been using double quotes for strings,
but it turns out in C you have to use single quotes when
you’re comparing individual characters.

Question 29

what is C99

Answer 29

Implicit declaration of function meow is invalid in C99.

C99 is referring to the 1999 version of C the language we’re using,

Question 30

in Javascript, what will console.log (0.2+0.4) result in?

Answer 30

0.6000000000000001, the reason is computer in binary digits, 在電腦的角度, 這些小數點是除不盡的. 所以衹要電腦是計算小數點, 都會有不精准的情況, 所以在計算價錢的時候要特別注意

Question 31

in Javascript, how do make the code become comment

Answer 31

highlight then press ctrl + /

Question 32

console.log(sentence.substring(sentence.length-4)) . What is the purpose of substring

Answer 32

the argument in substring is that what LAST number of index of string the computer will show