[CS50] Week 1 Review

CS50

[CS50] Week 1 Review

jeearchive 2024. 9. 10. 10:27

Week 1

: learning about how to write good code in the programming language C

Lecture Contents

1. Machine Code

-Source Code : human readable
-Machine Code : machine readable, ones and zeros

We should convert source code to machine code using a compiler. A compiler is a very special piece of software.

Source Code ->

Compiler

-> Machine Code

2. Hello World

*VS Code*
We will be using three commands to write, compile, and run our first program.

code hello.c
make hello
./hello

1) code hello.c : creates a file and allows us to type instructions for this program.
2) make hello : compiles the file from our instructions in C and creates an executable file called hello.
3) ./hello : runs the program called hello.

then, write code as follows :

#include <stdio.h>

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

1) printf : a function that can output a line of text.
2) \n : creates a new line after the words hello, world.
3) #include <stdio.h> : tells the compile that you want to use the capabilities of a library called stdio.h, a header file. This allows you, among many other things, to utilize the printf function.

3. Functions

<In Scratch>
: Used say block to display any text on the screen
<In C>
: function printf

printf ("hello, world\n");

4. Variables

<In Scratch>
: we had the ability to ask the user “What’s your name?” and say “hello” with that name appended to it.
<In C>

#include <stdio.h>

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

1) get_string (function) : to get a string from the user
2) %s : a placeholder called a format code that tells the printf function to prepare to receive a string.
3) answer (variable) : is of type string and can hold any string within it.
**There are many data types such as int, bool, char, etc.

-> Notice that numerous errors appear when running make hello because string and get_string are not recognized by the compiler.

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

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

Notice that #include <cs50.h> has been added to the top of your code.
-> Now works!

There are many format codes :

%c
%f
%i
%li
%s

%s is used for string variables. %i is used for int or integer variables.

5. Conditionals

*In C, you can assign a value to an int or integer as follows:

int counter = 0;

*You can add one to counter as follows:

counter = counter + 1;

also as:

counter = counter ++;

Let's use this new knowledge to write some conditionals!
For example, you might want to do one thing if x is greater than y / something else if that condition is not met.

<In Scratch>
: If ~ then, If ~ else ~
<In C>

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

int main(void)
{
   int x = get_int("What's x? ");
   int y = get_int("What's y? ");
   
   if (x < y)
   {
      printf("x is less than y\n");
   }
}

Flow charts are a way by which you can examine how a computer program functions. Such charts can be used to examine the efficiency of our code :

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

int main(void)
{
   int x = get_int("What's x? ");
   int y = get_int("What's 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");
   }
}

-> You can see the efficiency of our code design decisions.

**string vs char
string : a series of characters
char : a single character

Using char in conditionals :

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

int main(void)
{
   // Prompt user to agree
   char c = get_char("Do you agree? ");
   
   // Check whether agreed
   if (c == 'Y' || c == 'y');
   {
      printf("Agreed.\n");
   }
   else if (c == 'N' || c == 'n');
   {
      printf("Not agreed.\n");
   }
}

1) == : something is equal to something else
2) || : or

6. Loops

How to make computer say 'meow' three times?

1) simply copy + paste

#include <stdio.h>

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

=> But we have an opportunity for better design!

2) while loop

#include <stdio.h>

int main(void)
{
     int i = 0;
     while (i < 3)
     {
          printf("meow\n");
          i++;
     }
}

we can even loop forever using the following code:

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

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

3) for loop + create my own function

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

=> void main (void)
void : the function does not return any values.
(void) : no values are being provided to the function.

#include <stdio.h>

void meow(void);

int main(void)
{
      for(int i = 0; i < 3; i++)
      {
           meow();
      }
 }
 
 void meow(void)
 {
     printf("meow\n");
 }

=> Notice that meow function is defined at the bottom of the code & the prototype of the function is provided at the top of the code as void meow(void);

#include <stdio.h>

void meow(int n);

int main(void)
{
     meow(3);
}

// Meow some number of times
void meow(int n)
{
     for (int i = 0, i < n; i++)
     {
          printf("meow\n");
     }
}

=> This is a better design because if you want to change the number of times saying 'meow', you can just change the number of 'n'.

7. Operators and Abstraction

How to implement a calculator in C.

1) simple way

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

int main(void)
{
     // Prompt user for x
     int x = get_int("x : ");
     
     // Prompt user for y
     int y = get_int("y : ");
     
     // Perform addition
     printf("%i\n", x + y);
}

%i : be ready to print the value of x + y, which is int

Mathematical operators included in C :
+ (addition)
- (subtraction)
* (multiplication)
/ (division)
% (for remainder)

2) add function

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

int add(int a, int b);

int main(void)
{
   int x = get_int("x: ");
   
   int y = get_int("y: ");
   
   int z = add(x, y);
   printf("%i\n", z);
}

int add(int a, int b)
{
     int c = a + b;
     return c;
}

=> However, do we actually need variable z & c?

It can be further improved as follows:

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

int add(int a, int b);

int main(void)
{
   int x = get_int("x: ");
   
   int y = get_int("y: ");
   
   printf("%i\n", add(x,y));
}

int add(int a, int b)
{
   return a + b;
}

=> We can put add function in printf function.

3) truncation : truncates a double value after the decimal point and gives the integer part as the result.

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

int main(void)
{
   long x = get_long("x: ");
   
   long y = get_long("y: ");
   
   printf("%li\n", x + y);
}

8. Linux and the Command Line

- Linux : an operating system that is accesible via the command line in the terminal window in VS Code.

cd : for changing our current directory
cp : for copying files and directories
ls : for listing files in a directory
mkdir : for making a directory
mv : for moving (renaming) files and directories
rm : for removing (deleting) files
rmdir : for removing (deleting) directories

9. Mario

1) How to represent these four horizontal blocks?

#include <stdio.h>

int main(void)
{
   for (int i = 0; i < 4; i++)
   {
        printf("?");
   }
   printf("\n");
}

2) How to represent these three vertical blocks?

#include <stdio.h>

int main(void)
{
   for (int i = 0, i < 3; i++)
   {
      printf("#\n");
   }
}

3) How to make a three-by-three group of blocks?

#include <stdio.h>

int main(void)
{
   for (int i = 0; i < 3; i++)
   {
      for (int j = 0; j < 3; j++)
      {
         printf("#");
      }
      printf("\n");
   }
}

=> ###
###
###
Notice that one loop is inside another.

4) What if we want to ensure the number of bloks to be constant = unchangeable?

#include <stdio.h>

int main(void)
{
   const int n = 3;
   for (int i = 0; i < n; i++)
   {
      for (int j = 0; j < n; j++)
      {
         printf("#");
      }
      printf("\n");
   }
}

=> now n is a constant, which means it can never be changed.

5) How to prompt the user for the size of the grid?

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

int main(void)
{
   int n = get_int("Size: ");
   
   for (int i = 0; i < n; i++)
   {
      for (int j = 0; j < n; j++)
      {
         printf("#");
      }
      printf("\n");
   }
}

=> get_int

★ We can protect our program form bad behavior by checking to make sure the user's input satisfies our needs.

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

int main(void)
{
   int n;
   do
   {
      n = get_int("Size: ");
   }
   while (n < 1);
   
   for (int i = 0; i < n; i++)
   {
      for (int j = 0; j < n; j++)
      {
         printf("#");
      }
      printf("\n");
   }
}

=> Now the user will be continuously prompted for the size until the user's input is 1 or greater.

10. Comments

-where you leave explanatory remarks to yourself and others.
-each comment is a few words or more, providing the reader an opportunity to understand what is happening in a specific block of code.
-form : // comment

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

int main(void)
{
   //Prompt user for positive integer
   int n;
   do
   {
      n = get_int("Size: ");
   }
   while (n < 1);
   
   // Print a n-by-n grid of bricks
   for (int i = 0; i < n; i++)
   {
      for (int j = 0; j < n; j++)
      {
         printf("#");
      }
      printf("\n");
   }
}

11. Types

-Types refer to the possible data that can be stored within a variable.
Ex) char : a single character like 'a' or '2'.

-Types are very important because each type has specific limits.
Ex) the highest value of int : 4294967295

bool : a Boolean expression of either true or false
char : a single character like 'a' or '2'
double : a floating-point value with more digits than a float
float : a floating-point value, or real number with a decimal value
int : integers up to a certain size, or number of bits
long : integers with more vits, so they can count higher than an int
string : a string of characters