JF résumé notes social

CS50 Section #2

On Booleans

A quick note, don't do this:

if (bool == true) { // Do this }

Instead, do this:

if (bool) { // Do this }

Much better.

On Style

Sorting and Searching!

Linear search => O(n)

Binary search => O(log n)

Bubble sort, insertion sort, selection sort => O(n^2)

Merge Sort => O(n log n)

For descriptions of above algorithms

Pointers and Memory

Memory in a computer consists of a long "array" of possible locations, each denoted by an "address" (eg 0x53). These addresses are virtually mapped to physical memory (RAM 🐑). Each address corresponds to one byte of memory.

But... things can be multiple bytes. An int might take up 4 bytes. Referring to an int then refers to four adresses in memory.

Let's say we have some function, like

void add_one(int x) { x = x+1; }

When we call add_one(x), it won't actually change our original x in the memory. It will instead make a copy, and change that.

If we want to find the location in memory of our original x, we can do int *p = &x. This is a pointer to an address in the memory.

When we dereference a pointer *p = 3, this will go to the memory location referenced in p and change it to 3.

So, we could rewrite our function to be:

void add_one(int *p) { *p++; }

Here, we pass a pointer to a location in memory, which we then dereference (get the value of) and add one to.

Arrays with Pointers


char arr[4]; arr[0] = 1; arr[1] = 2; arr[2] = 3; arr[3] = 4;

We can do:

char *p = arr; *p = 3;

Which makes arr = {3, 2, 3, 4}.

We can also do:

*(p+1) = 5;

Which makes arr = {3, 5, 3, 4}.

*(p+i) is then equivalent to the arr[i] syntax.

Just a note, pointer arithmetic is actually done with respect to sizeof the type of the array, so *(p+1) in an int arr[n] will increment the pointer by addresses.

Dynamic Memory

int size = get_int(); char arr[size]; This is dynamic memory because we don't actually know the size of the array until the user inputs it.

So, rather than this, we could do:

int size = get_int(); char *arr = malloc(sizeof(char) * size); note that sizeof(char) is always 1

Which allocates the memory appropriately.


malloc(size_in_bytes) allocates the correct number of bytes.

If malloc() fails, it returns NULL--you can't dereference NULL. So we should always check:

if (arr == NULL) { return 1; }


C doesn't implement garbage collection or anything like that. If you malloc something, you must also free it. Otherwise, it's a memory leak.

We can do this by calling:


But only do it once, otherwise C has no idea what to do.


string in cs50.h literally just replaces string with char * (since a string is an array of chars which is a pointer to the first element).

A cool way to iterate through a string

Rather than doing:

char *s = "Hello World"; for (int i = 0; i < strlen(*s); i++) { printf("%c", s[i]); }

We can do:

char *s = "Hello World"; while (*s != '\0') { printf("%c", *s); s++; }