[CS50] Week 2 Review

 

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Week 2

:  Deeper look about C

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Lecture Contents

 

 

 

 

 

1. Compiling 

 

-Encryption : the act of hiding plain text from prying eyes (암호화)

-Decrypting : the act of taking an encrypted piece of text and returning it to a human-readable form (해독)

 

We learned about a compiler last week:

source code

-> compiler ->

machine code

- VS Code utilizes a compiler called clang or c language

Ex) If you were to type make hello, it runs a command that executes clang

 

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

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

 

In the terminal window, type : 

clang -o hello hello.c -lcs50

=> Enables the compiler to access cs50 library.

 

 

Major steps of compiling :

 

ⓐ preprocessing 

: where the header files (#) in your code are effectively copied and pasted into your file. 

The code from cs50.h is copied into your program. This step can be visualized as follows:

string get_string(string prompt);
int printf(string format, ...);

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

 

 

ⓑ compiling

: where your program is converted into assembly code. This step can be visualized as follows:

 

 

ⓒ assembling

: where the compiler converts your assembly code into machine code. This step can be visualized as follows:

 

ⓓ linking

: where code from your included libraries are concerted also into machine code and combined with your code.

This step can be visualized as follows:

 

 

 

 

 

2. Debugging

 

: to address the bugs

 

Let's purposely insert a bug within your code:

#include <stdio.h>

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

=> 4 bricks appear instead of the intended 3. 

#

#

#

#

 

 

ⓐ printf : a very useful way of debugging your code.

#include <stdio.h>

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

=> i is 0, i is 1, i is 2, i is 3

You might realize that your code needs to be corrected as follows: (<= is replaced with <)

for(int i = 0; i < 3; i++)

 

 

This code can be further improved as follows:

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

void print_column(int height);

int main(void)
{ 
   int h = get_int("Height: ");
   print_column(h);
}

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

=> This sill results in a bug. (n+1 bricks instead of the intended n)

 

 

ⓑ debugger : a software tool created by programmers to help track down bugs in code.

  ▷ First, set a breakpoint by clicking to the left of a line of your code, just to the left of the line number.

      When clicked, you will see a red dot appearing.

  ▷ Second, run debug50 ./buggy(the name of your file).  It will seem like this:

=> Click the step over button and notice how the value of h increases.

It will show how your code is running step by step. 

 

 

ⓒ Rubber duck debugging 

 

 

 

 

 

3. Arrays

 

Each data type requires a certain amount of system resources:

◎ bool : 1 byte

◎ int : 4 bytes

◎ long : 8 bytes

◎ float : 4 bytes

◎ double : 8 bytes

◎ char : 1 byte

◎ string : ? bytes

 

#include <stdio.h>

int main(void)
{
   int score1 = 72;
   int score2 = 73;
   int score3 = 33;
   
   printf("Average: %f\n", (score1+score2+score3) / 3.0);
}

=> Notice of using %f and 3.0 so that the cauculation is rendered as a floating point value in the end.

 

These variables are stored in memory like:

* 1 block = 1 digit

 

 

Using Arrays

▶ int scores[3] : a way of telling the compiler to provide you 3 back-to-back places in memory of size int to store 3 scores. 

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

int main(void)
{
   //Get scores
   int scores[3];
   scores[0] = get_int("Score: ");
   scores[1] = get_int("Score: ");
   scores[2] = get_int("Score: ");
   
   //Print average
   printf("Average: %f\n", (scores[0] + scores[1] + scores[2]) / 3.0);
}

-> We can still improve this code as follows:

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

int main(void)
{
   int scores[3];
   for (int i = 0; i < 3; i++)
   {
      scores[i] = get_int("Score: ");
   }
   
   printf("Average: %f\n", (scores[0] + scores[1] + scores[2]) / 3.0);
}

-> We can simplify or abstract away the calculation of the average as follows:

 

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

const int N =3;

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

int main(void)
{
   //Get scores
   int scores[N];
   for (int i = 0; i < N; i++)
   {
      scores[i] = get_int("Score: ");
   }
   printf("Average: %f\n", average(N, scores));
   
float average(int length, int array[])
{
   //Calculate average
   int sum = 0;
   for (int i = 0; i < length; i++)
   {
      sum += array[i];
   }
   return sum / (float) length;

- A new function called 'average' is declared. 

- Const (constant value) of N is declared. 

- 'Average' function takes int array[] : the compiler passes an array to this function.

 

 

 

 

 

4. Strings

 

A string is an array or characters (char).

-Begins with the first character and ends with a special character called a NUL character:

In decimal :

 

 

▶ Hi!

#include <stdio.h>

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

-> When we replace %c with %i :

printf("%i%i%i\n", c1, c2, c3);

=> 72  73  33

 

 

Using arrays

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

int main(void)
{
   string s = "HI!";
   printf("%i %i %i %i\n", s[0], s[1], s[2], s[3]);
}

=> 72  73  33  0(NUL)

 

 

▶ Hi! BYE!

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

int main (void)
{
   string s = "HI!";
   string t = "BYE!";
   
   printf("%s\n", s);
   printf("%s\n", t);
}

=> We can improve this code as follows using array:

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

int main(void)
{
   string words[2];
   
   words[0] = "HI!";
   words[1] = "BYE!";
   
   printf("%s\n", words[0]);
   printf("%s\n", words[1]);
}

 

 

 

 

 

5. String Length

 

 

: How do we discover the length of an array?

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

int main(void)
{
   //Prompt for user's name
   string name = get_string("Name: ");
   
   //Count number of characters up until '\0' (aka NUL)
   int n = 0;
   while (name[n] != '\0')
   {
      n++;
   }
   printf("%i\n", n);
}

=> This can be improved as follows:

 

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

int string_length(string s);

int main(void)
{
   //Prompt for user's name
   string name = get_string("Name: ");
   int length = string_length(name);
   printf("%i\n", length);
}

int string_length(string s)
{
   //Count number of characters up until '\0'
   int n = 0;
   while (s[n] != '\0')
   {
      n++;
   }
   return n;
}

 

 

Other programmers have created code in the string.h library to find the length of a string.

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

int main(void)
{
   string name = get_string("Name: ");
   int length = strlen(name);
   printf("%i\n", length);
}

→ Notice that it uses a function strlen.

 

 

 

: How do we convert all lowercase characters to uppercase ones?

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

int main(void)
{
    string s = get_string("Before: ");
    printf("After: ");
    for(int i = 0, n = strlen(s); i < n; i++)
    {
        if(s[i] >= 'a' && s[i] <= 'z')
        {
            printf("%c", s[i] - 32);
        }
        else
        {
            printf("%c" , s[i]);
        }
    }
    printf("\n");
}

→ If the character is lowercase, it subtracts the value 32 from it to convert it to uppercase, according to this:

ASCII values chart

 

Now, we can use ctype.h library :

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

int main(void)
{
     string s = get_string("Before: ");
     printf("After: ");
     for (int i = 0, n = strlen(s); i < n; i++)
     {
          if(islower(s[i]))
          {
               printf("%c", toupper(s[i]));
          }
          else
          {
               printf("%c", s[i]);
          }
     }
     printf("\n");
}

→ toupper function is used to convert each character to uppercase.

 

Because the toupper function automatically knows to uppercase only lowercase characters, your code can be simplified as follows:

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

int main(void)
{
     string s = get_string("Before: ");
     printf("After: ");
     for (int i = 0, n = strlen(s); i < n; i++)
     {
          printf("%c", toupper(s[i]));
     }
     printf("\n");
}

 

 

 

 

 

6. Command-Line Arguments

 

 

-Command-line arguments are those arguments that are passed to your program at the command line.

Ex) All those statements you typed after clang = command-line arguments

 

greet.c

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

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

→ It says 'hello' to the user. However, it would be nice to be able to take arguments before the program even runs.

 

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

int main(int argc, string argv[])
{
     if (argc == 2)
     {
        printf("hello, %s\n", argv[1]);
     }
     else 
     {
        printf("hello, world\n");
     }
}

 

☆ Notice that this program knows 

1) argc    : the number of command line arguments

2) argv[] : the array of the characters passed as arguments at the command line

 

☆ argc == 2 / argv[1]

: './greet Jeeyoon \0'

         0          1         2

   argv[0]  argv[1]  argv[2]

 

 

 

 

 

7. Exit Status

 

 

When a program ends, a special exit code is provided to the computer.

When exits without error: a status code of 0

When an error occurs: a status code of 1

 

status.c

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

int main(int argc, string argv[])
{
     if (argc != 2)
     {
          printf("Missing command-line argument\n");
          return 1;
     }
     printf("hello, %s\n", argv[1]);
     return 0;
}

 

 

 

 

 

8. Cryptography (암호화)

 

 

 

-Cryptography : the art of ciphering and deciphering a message

*key : a special argument passed to the cipher along with the plaintext

 

 

 

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