CS50 Week 2: Arrays

On arrays, strings (or are they?) and functions.

Written by Eva Dee on (about a 6 minute read).

Replacing make with clang

  • When you run make hello.c you get a file called a.out (aka assembly output).
  • To give it a name yourself: clang -o hello hello.c
  • To include custom libraries, you have to include them as a header file, but also tell clangwhere to find it: clang -o hello hello.c -lcs50 (link in the cs50 library)

What is compiling?

Compiling actually involves four steps:

  1. preprocessing - evaluating libraries and imports, basically looking for everything that has a # symbol. The header files include all the prototypes.
  2. compiling - translating your code into assembly code
  3. assembling - translating assembly code into binary
  4. linking - linking all the different files into one

Debugging

Syntactic errors vs logical errors.

  1. using printf
  2. use debug50 (a debugger built on the industry standard's tool called GDB - the GBU debugger)
    • make sure to set up breakpoints before running the debugger!
  3. rubber duck debugging

Memory

  • think of RAM (random-access memory) as a grid, 1 byte taking one space inside the grid.
  • RAM is volatile; it requires electricity to continue powering it
  • when you run a program in C, it is running in your RAM
  • think of the computer's memory as a grid of bytes
// this is inside CS50 IDE
bool   1 byte (bit wasteful, could be just a bit instead)
char   1 byte
double 8 bytes
float  4 bytes
int    4 bytes
long   8 bytes
string ? bytes

Arrays

  • An array is a sequence of values stored in memory back to back to back (aka contiguous).
  • Arrays are used to hold values of the same data type.
  • Arrays can be multidimensional.
  • Array values are passed by reference.

With an array declaration, you need to specify three things:

  • type
  • name
  • size
  • e.g. int scores[3];

And this is how you pass an array as an input:

// you use square brackets, but no size
// arrays in C do not know how long they are
// so you'll need to pass in length separately
float average(int length, int array[])

What is a (damn, you) segmentation fault ?

  • When you work with data outside of your declared array. For example, you have an array with size 10, but you are accessing the element at position 11.
  • Random access is one of the most powerful yet one of the scariest features in C. You can touch, look at, change any memory you want.

🤔 What is the difference between conversion, casting, coercion?

  • Conversion: either implicitly or explicitly changing a value from one data type to another, e.g. a 16-bit integer to a 32-bit integer.
  • Coercion: an implicit conversion.
  • Casting: an explicit type conversion int() or Number()

String

  • a string is an array of characters! (there is no 🥄!)

How do we know when the string ends?

  • \0 null (terminating) character where you basically "waste" one byte to indicate where the string ends.
  • So a string with length 3, actually takes up 4 bytes.
  • And how do you loop over the string's characters (for how long)?:

// since every string end is padded with the NULL char
for (int i = 0; str[i] != '\0'; i++) {}

// can be replaced with strlen
// make sure to include the <string.h> header
for (int i = 0, len = strlen(str); i < len; i++) {}

Food for thought 🤔

  • The whole point of programming ultimately is to abstract those lower-level implementation details.

Command-line arguments

  • argc: argument count. The program's name is by default, the first argument
  • argv: argument vector (vector is another word for array), is an array of strings. The first element argv[0] is always the name of the program. The last element is always stored at argv[argv-1]
  • By default, main returns 0.
int main(int argc, string argv[])
{
  printf("hello, %s\n", argv[1]);
}
else
{
  printf("hello, world\n");
}

Exit status

  • 0 in computer speak typically means all ok 👍: return 0
  • you can return a custom value (in case of errors or any results you want to keep track of): e.g. return 1
  • to check the exit status of a program you just ran: echo $?

Functions

  • synonyms: procedures, methods, subroutines There are three parts in a function declaration:
  1. return type
  2. name
  3. the argument list (comma-separated, with a type and a name)

🤔 What is the difference between function declaration vs function definition?

float sum_two (int a, int b);

// the definition (aka the actual recipe)
float sum_two (int a, int b)
{
  int result =  a + b;
  return result;
}
  • Functions can have 0 (void) arguments or no output (also void).