[CS50x] Lecture 1.

Lecture 0 was about basic concepts of computational language and using scratch to understand the basic logic of it.

I have created some scratch projects and submitted one of them(which is not completed 100%) 

I am going to update it sometime soon. 

 

Lecture 1 has 2 labs and 2 assignments. (I have done 3 of them and 1 more to be submitted) 

It's so good to have such labs and assignments so that I can understand the theories along with the actual practices. (Actually, for me, the one who didn't study CS before, it's not easy at all.)

 

C 

what to consider for the quality 

Correctness 

Design: more efficient, well-written, w/o unnecessary repetition

Style: aesthetic, consistent indentation, affect how readable it is visually

 

CS50 IDE(Integrated Development Environment) - ide.cs50.io

Command Line Interface(CLI): prompt at which we need to enter text commands

- Scratch is a graphical user interface(GUI)

 

Compiling

the compiler takes source code(we write) as input and produces machine code as output 

- use "make" to access a compiler in CS50 IDE 

the compiler is the interpreter between computer and human 

the computer speaks with the patterns of 0 and 1 (binary)

 

functions, arguments, parameters

*Functions: small actions or verbs, programmed version of an algorithm

- can have side effects: visual side effect-> different form (printed to the screen)

- return values, variables 

*Arguments: the input to functions

*Library: a bunch of code already written 

- string is word, text, letter etc. 

string answer = get_string("What's your name?"); 

- right values to input left variable 

- datatype should be declared as well -> string 

printf("hello, %s" answer);
//printf: print also format code %s-> plug in some actual value (string type)

- use the return value(stored one) by using %s (format code) 

- put the following value into %s 

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

int main(void)
{
	string answer = get_string("What is your name?);
    printf("hello, %s", answer); 
}
#need to be re-compile

- Why do we have to re-compile it every single time? It's because C is an old language. 

- \n:  move the cursor to the next line by using \n. (escape sequence)

- do the right-hand side functions and save them to the left-hand-side variable. 

 

main, header files

int main(void)
{

}

- main: This is the same as  "when flag clicked"

- header file ends with .h : some other set of code like a library

 i.e. #include <stdio.h> : standard input output dot h. (famous c file)

 

Tools

- help50, styke50, check50 

- type this on the terminal: help50 make hello, syte50 hello.c, check50 cs50/problems/hello ->github username and pwd

- comment(//blah blah ~):  the purpose of the code to be presented

 

Command 

- ls: list of the contents 

- rm: remove 

- mv: move (rename the file: mv hello.c goodbye.c) 

- mkdir: make a directory

- mv hello.c lecture/ : move to the lecture directory(folder)

- cd lecture/: go inside of lecture directory 

- mv helloc.c ..: move the file to the folder up above(parent folder) 

- cd ..: go up to the parent folder 

- rmdir: remove directory 

- ..(double dot) parent directory , .(single dot) current directory

- source code: hello.c, machine code: hello(contains only 0 and 1) -> hello*(shown in the terminal when you check the ls) when you compile this file will be created automatically

 

+From the short Clip 

: can be used within the IDE or any UNIX-based system

: ls - list (read out all the files and folders from the current directory)

: cd- change directory ( cd directory or dot(.) or double dot(..) - go to the parent directory)

: pwd - present working directory ( ~/: tilde is home directory )

: cd enter -> go to tilde(~) 

: mkddir - make a directory 

: cp - copy ( cp <source><destination> : copy a file -> cp hello.txt hi.txt ) 

: copy whole directory -> cp -r pset0 pset3 

: rm - remove the file ( rm -f <file> : no undo(force to remove wo question) / rm -r <directory> : get rid of the directory including whole contents in it / rm -rf <directory> : get rid of the directory completely wo question and no undo

: mv - move location ( mv greddy.c greedy.c : rename the file / 

 

more to discover ... 

chmod 

Ln

touch

rmdir

man

diff

sudo

clear 

telnet  

 

DataType & variable 

Int 

- from "- 2 to the 31st power" to "2 to the 31st power -1" (because of zero)

***unsigned int( %u ): instead of storing negative values, storing only positive values : "0 to 2 to the 32nd power-1 "(0 to 4 billion)

 

 

 

 

 

 

 

 

Char: 0 to 127 characters 

Floating: 4 bytes memory, decimal parts, have a precision problem(we cannot so precise with limited digits(32 btis))

Double: 8 bytes memory, same as float but it can have 64 bits 

 

Void: this is not a data type, it's a type. The function can have a void return type(no output-i.e.printf) main doesn't take any arguments(it's also void) 

 

• For printf, too, there are different placeholders for each type:
  • %c for chars
  • %f for floats, doubles
  • %i for ints
  • %li for longs
  • %s for strings

 

*cs50.h to be included 

bool: Boolean data type, True or False(In C, this is not a standard default)

String: word, collections of characters, long or short series of characters  

 

*Creating variable : declare datatype and variable name -> int number; , char letter; you can also declare multiple variable(if they are same data type -> int height, weight; )

*declaration->assignment. / initialization(declare and assignment at the same time)  

: int number; -> number = 17; / int number = 17;

 

Operator 

Methematical operations

+ for addition

- for subtraction

* for multiplication

/ for division

% for remainder

Boolean operators 

- 0 is false, non-zero is true 

*Logical operators

- And(&&): true only when both are true. 

- OR( || ): False only when both are false 

- NOT( ! ) : opposite 

*Relational operators 

- less than , less than or equal to , greater than, greater than or equal to 

- Equality (==), inequality (!=) ; double equal for this case ! 

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

int main(void)
{
    int x = get_int("x: ");

    int y = get_int("y: ");

    printf("%i\n", x + y);
    //put number in i(i dont know what this is)
}

- if you input not an integer, it's going to be ignored until you input the integer value.

- If you input 4 billion or 3 billion, it's going to be ignored again. (int only use 32 bits total - roughly as high as 4 billion including negative and zero; 2 billion to the negative and positive directions) 

- what if we use "long(64 bits)" instead of int. ↓

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

int main(void)
{
    long x = get_long("x: ");

    long y = get_long("y: ");

    printf("%li\n", x + y);
	//%li for long
}

- it works with 4 billion 

 

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

int main(void)
{
    // Get numbers from user
    int x = get_int("x: ");
    int y = get_int("y: ");

    // Divide x by y
    float z = x / y;
    printf("%f\n", z);
}

- logically correct. But the result is given back the integer divide by integer-> lose all of the decimal point values 

- The math is already done from the right-hand side, so even though z is float, the result is already int 

- you can change the whole code, but you can also do casting.

- cast: the data type to another 

float z = (float) x / (float) y;

-after casting x and y, you will find the result is also float type. Indeed working. 

 

Conditional statements

if, ifelse, else - use boolean expression to make a decision 

 

switch: depends on what user types 

- use "break" or you will fall through each case

 

 

 

 

 

 

 

 

 

? : - ternary operator (simulate if - else) 

- if expr is true, first value , if its not later value. 

  int x = (expr) ? 5 : 6; 

 

 

 

 

 

 

 

 

 

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

int main(void)
{
    // Prompt user for x
    int x = get_int("x: ");

    // Prompt user for y
    int y = get_int("y: ");

    // Compare x and y
    if (x < y)
    {
        printf("x is less than y\n");
    }
    else if (x > y)
    {
        printf("x is greater than y\n");
    }
    else
    {
        printf("x is equal to y\n");
    }
}

- condition 

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

int main(void)
{
    char c = get_char("Do you agree? ");

    // Check whether agreed **Y and N to be quoted
    if (c == 'Y' || c == 'y')
    {
        printf("Agreed.\n");
    }
    else if (c == 'N' || c == 'n')
    {
        printf("Not agreed.\n");
    }
}

 

Loops( forever in the scratch ) - 3 majors

while(true): over and over and forever. (infinite loop) true is always true. until we find a break or kill the programming run 

while(boolean expression): until the boolean expression becomes false code inside of curly brace will last. 

do { } while(boolean expression): boolean expression is true, go back to do. at least do 1 time no matter it's true or not

for loop: for(int i=0; i<10; i++) {body}

-> for( 1 variable declare ; 2 boolean expression ; 4 finally counter variable is incremented ;) : { 3 body executed} 

-> 1 2 3 4 2 3 4 2 3 4 ... eventually i =10 then, stop the loop 

** Use a while loop when we want a loop to repeat an unknown number of times(might not run at all): keep updating or moving things no matter how long the user plays the game 

** do-while loop when you don't know how many times to repeat, but run at least one; prompting the user for input 

** for a loop when you know a discrete number of times, though you might not know the number 

while(true)    //to make infinite loop input True 
{
	printf("hello, world\n");
}

int i = 0; 
while(i<50)                     //counting up to number 50
{
	printf("hello, world\n");   //keep printing this 
	i++;                        //i = i +1; , i += 1; 
}

int i = 1;                       //count through 50
while (i <= 50)
{
    printf("hello, world\n");
    i++;
}

//this is not conventional 

- There are various ways to solve the problems. But it's highly recommended you use the conventional ways. 

for (int i = 0; i < 50; i++)
{
    printf("hello, world\n");
}
//inside of the parenthesis in for clause, you can put more then boolean and also more than 1 line)

- The most convenient ways to conduct. 

 

Abstraction: problem-solving principle 

- better design: flexibility to reuse the function in multiple places 

#include <stdio.h>

int main(void)
{
	printf("meow\");
    printf("meow\");
    printf("meow\");
}

- this is not a good design

#include <stdio.h>

int main(void)
{
    for (int i = 0; i < 3; i++)  //initialize the values, check the condition , add the number
    {
        printf("meow\n");
    }
}

#include <stdio.h>

void meow(void) {                 //create own custom functions "meow" 
  printf("meow\n");
}

int main(void)
{
    for (int i = 0; i < 3; i++)
    {
        meow();					 //call this function
    }
}

#include <stdio.h>

void meow(void);             //meow function is declared at the end it's because it's customed functions

int main(void)
{
    for (int i = 0; i < 3; i++)
    {
        meow();            //but if you don't declare this function before this command, it will cause an error
    } 
}

//Ususally put the custom functions at the bottom of code 
//Because main function rathen being at the top

void meow(void)
{
    printf("meow\n");
}

- give the hint on the top, it's called prototype,(not declare it yet), to make sure that code is compiled successfully. 

#include <stdio.h>

void meow(int n);  //prototype

int main(void)
{
    meow(3);
}

void meow(int n)
{
    for (int i = 0; i < n; i++)
    {
        printf("meow\n");
    }
}

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

int get_positive_int(void);   //prototype : will be declared later 

int main(void)                //main function
{
    int i = get_positive_int();   //user will input the data 
    printf("%i\n", i);
}

// Prompt user for positive integer
int get_positive_int(void)       //customed function, no input: void 
{
    int n;
    do
    {
        n = get_int("Positive Integer: ");    //scope of variable: inside of the curly brace (check below comment
    }
    while (n < 1);            // not negative integer, until it returns positive value, repeatedly do "do" 
    return n;
}

- if you declared the variable type inside of the curly brace, you cannot use it outside of the curly brace. so assign it earlier

 

Mario

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

int main(void)
{
    for (int i = 0; i < 3; i++)
    {
        for (int j = 0; j < 3; j++)
        {
            printf("#");
        }
        printf("\n");   //every single time after printing J
    }
}

 

Memory, imprecision, and overflow

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

int main(void)
{
    float x = get_float("x: ");
    float y = get_float("y: ");

    printf("%.50f\n", x / y);    
    //to print more than significant digit->%.50f: 
    //the number of decimal places displayed
}

-> floating-point imprecision: not enough bits to store all possible values, computer store the closest values it can

- Integer overflow: 2 billion int + 2 billion ints? (over 32 bits) , date data: 19 Jan 2038 already reaches 4 billion 

 

 

 

 

 

 

 

 

 

command line 

: can be used within the IDE or any UNIX-based system 

: ls - list (read out all the files and folders from the current directory)

: cd- change directory ( cd directory or dot(.) or double dot(..) - go to the parent directory)

: pwd - present working directory ( ~/: tilde is home directory )

: cd enter -> go to tilde(~) 

: mkddir - make a directory 

: cp - copy ( cp <source><destination> : copy a file -> cp hello.txt hi.txt ) 

: copy whole directory -> cp -r pset0 pset3 

: rm - remove the file ( rm -f <file> : no undo(force to remove wo question) / rm -r <directory> : get rid of the directory including whole contents in it / rm -rf <directory> : get rid of the directory completely wo question and no undo

: mv - move location ( mv greddy.c greedy.c : rename the file / 

 

more to discover ... 

chmod 

Ln

touch

rmdir

man

diff

sudo

clear 

telnet