Week 2 #

Cryptography #

String #

a sequence of characters, in an array (a list of things right next to each other) in memory.

Sample with string.h:

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

int main(void)
{
    // ask user for input
    string s = get_string();

    // make sure get_string returned a string
    if (s != NULL)
    {
        // iterate over the characters in s one at a time
        for (int i = 0, n = strlen(s); i < n; i++)
        {
            // print i'th character in s
            printf("%c\n", s[i]);
        }
    }
}

Typecasting #

ASCII is a standrad for mapping characters to letters. Here are some sample ones:

A   B   C   D   E   F   G   H   I  ...
65  66  67  68  69  70  71  72  73  ...

a   b   c   d   e   f   g   h   i   ...
97  98  99  100 101 102 103 104 105 ...

We can experiment with this program:

#include <stdio.h>

int main(void)
{
    //  treat numbers like characters:
    for (int i = 65; i < 65 + 26; i++)
    {
        printf("%c is %i\n", (char) i, i);
    }

    //  we can also treat characters like numbers:
    for (char c = 'A'; c <= 'Z'; c++)
    {
        printf("%c is %i\n", c, c);
    }
}

toupper in <ctype.h>, implements:

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

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

String in Memory #

strlen:

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

int main(void)
{
    string s = get_string();
    int n = 0;
    while (s[n] != '\0') # \0 is end of the string, NOT space
    {
        n++;
    }
    printf("%i\n", n);
}

a string in C is just the location of the first character in memory, which are stored with a character at the end marking the end of a string, since there’s no predetermined length, so a string in memory really looks like:
```
------------------------------
| Z | a | m | y | l | a | \0 |
------------------------------
```
- And with \0, C indicates the end of our string.

We can represent more of our computer’s memory as a grid:

-----------------------------------
| Z | a | m | y  | l | a | \0 | A |
-----------------------------------
| n | d | i | \0 |   |   |    |   |
-----------------------------------
|   |   |   |    |   |   |    |   |
-----------------------------------
|   |   |   |    |   |   |    |   |
-----------------------------------

We can imagine each byte (each box in this grid) of memory as labeled from 0 to 31, since there are 32 bytes total. In the sample, Zamyla start with 0, and Andi start with 7.

Command-Line Arguments #

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

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

~/workspace/ $ ./argv0 hello
hello, hello

argc : argument count
argv : argument vector, a list of strings
argv[0] is always the name of the program itself.

`main`’s output #

main return a number 0 to indicate a program exists successfully. A non-zero number is used to present an error.:

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

We can use command $? to see the exit code in terminal, like this:

~/workspace/ $ ./exit
missing command-line argument
~/workspace/ $ echo $?
1

Refers #

http://docs.cs50.net/2016/fall/notes/2/week2.html