CS50 2019

 

Lecture 0 - Computational Thinking, Scratch

 

input -> [ black box ] -> output

 

How to represent input/output information?

• represent number
• represent characters/letters:
  • ASCII (8 bits)

A:65(01000001) 

• • Unicode(a superset of ASCII, 8/16/24/32 bits)

　　　　　　emoji 😂 ：128514（11111011000000010）

 

• represent colors
  • RGB: every dot in an image is stored using three value

　　　　　　a shade of yellow: 72 73 33

　　　　　　every dot in an image is used three value

• • • animated GIF/videos

multiple images in the same file, one image fly past after another to create an illusion of moving

• represent music

　　　　　　maybe the note, the duration, the value

 

input -> [ algorithms ] -> output

The algorithm is a step-by-step process for solving a problem.

 

• functions: verbs or actions to tell computers what to do
• conditions
• Boolean expressions: one or zero questions
• loops
• variables
• threads
• events

 

Lecture 1 - C

20200610

source code(e.g. C) -> [ compiler ] -> machine code (01)

#include <stdio.h>

int main(void)
{
    printf("hello world\n");
}

 

run the code:

in the Terminal:

$ clang hello.c
$ ./a.out

or assign a name to the compiled file

$ clang -o hello hello.c
$ ./hello

-o: output

 

List

a list of all the files in the current folder/directory

$ ls
a.out*  hello*  hello.c

*: machine code

without*: source code

 

Remove

$ rm a.out
rm: remove regular file 'a.out'? y

y: yes

 

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    string anwser = get_string("what's your name?\n");
    printf("hello,%s\n", anwser);
}

 

Terminal:

$ clang -o string string.c -lcs50
$ ./string
what's your name?
David
hello,David

Or

$ make string
clang -fsanitize=signed-integer-overflow -fsanitize=undefined -ggdb3 -O0 -std=c11 -Wall -Werror -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-unused-variable -Wshadow    string.c  -lcrypt -lcs50 -lm -o string
$ ./string
what's your name?
David
hello,David

 

 

    int counter = 0;
    counter = counter+1;
　　 counter += 1;

 

    if(x<y)
    {
        printf("x is less than y\n");
    }
    else
    {
        printf("x is not less than y\n");
    }

; only after an action.

 

    int i = 0;
    while(i<50)
    {
        printf("hello,world!\n");
        i++ ;
    }

 

Datatype	Prompt Function	Placeholder
bool
char	get_char	%c
double(decimal with more digits)	get_double	%f
float(decimal)	get_float	%f
int(<4 billion)	get_int	%i
long(more than 4 billion)	get_long	%li
string	get_string	%s

 

 

 

 

 

 

 

 

 

 

#include <cs50.h>
#include <stdio.h>

void cough(void);//prototype

int main(void)
{
    cough();
}

void cough(void)
    {
        printf("cough!");
    }

 

Lecture 2 - Arrays

20200615

 

Compile:

source code

⬇️

preprocessing

compiling

assembling

linking

⬇️

machine code

 

Bug

syntactically error

logical error

 

Byte

bool	1 byte
char	1
int	4
float	4
long	8
double	8
string	?

 

 

 

 

 

 

 

 

 

#include <stdio.h>

int main(void)
{
    char c1 = 'H';
    char c2 = 'I';
    char c3 = '!';
    printf("%i %i %i \n", c1,c2,c3);
}

/*
$ ./hi
72 73 33 
*/

 

    int n1 = 72;
    printf("%c\n", n1); //H

 

Array

#include <stdio.h>
#include <cs50.h>

//global variable(outside of functions)
const int N = 3; //avoid accidentally change;

int main(void)
{
    int score[N];
    score[0] = 72;
    score[1] = 73;
    score[2] = 33;
    printf("Average: %i\n", (score[0]+score[1]+score[2]) / 3); 
}

 

averages

#include <stdio.h>
#include <cs50.h>

float average(int length, int array[]);

int main(void)
{
    int n = get_int("Number of scores:");
    int scores[n];
    for (int i = 0; i < n; i++)
    {
        scores[i] = get_int("Socre %i:", i+1);
    }
    printf("Average: %f\n",average(n,scores));
}

float average(int length, int array[])
{
    int sum = 0;
    for (int i = 0; i < length; i++)
    {
        sum += array[i];
    }
    return (float)sum / (float)length;
}

 

In C， the array doesn't remember its length.

 

\0 terminating character of a string;

 

command line arguement;

 

int main(int argc, string argv[])

 

plaintext & key --> [ ] --> ciphertext

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