Java borrowed much of C’s syntax, so a lot of it will be familiar

same control flow

if, for, while, switch

{ } denote code blocks

; at the end of each statement

two primary differences

operations in C are more similar to what the computer is actually doing (compared to Java)

this is what people mean when they say C is a “lower-level” language

end result: you have to take care of more things yourself (e.g., memory)

procedural programming

no objects, all computation takes place inside functions

data types

integer

char (1 byte), int (4 bytes)

char is like Java’s byte, usually used to hold ASCII characters

int can be short or long

short int = 2 bytes

long int = 8 bytes

char and int can be signed (default) or unsigned

int ranges from -2 billion to +2 billion

unsigned int ranges from 0 to 4 billion

exercise: what would char and unsigned char range from?

-128 to 127, 0 to 255

floating point

float (4 bytes), double (8 bytes), long double (16 bytes)

arrays and structs (more later)

physical vs virtual

machines have a finite amount of memory available to running processes

processes don’t get to manage this themselves

operating system provides illusion that every process has its own identical chunk of memory

OS takes care of linking this virtual memory up with the physical memory

prevents processes from messing up each other’s memory

each cell of memory has an address

in virtual memory, addresses always go from 0 to 2^64 (on a 64-bit system)

exercise: how many bits do we need to represent an address

an address will take 64 bits

each piece of data in a C program (or any program) is stored at a specific location in memory

we say this location is the “address” of that data

this was true in Java, but in C, we can work directly with these addresses

a pointer is simply the address of some data

draw result of int a = 5;

we can have variables that hold the address of some data

draw result of int *ptr = &a;

ptr is the location of a

*ptr is the value of a (called dereferencing)

what is the result of int b = *ptr;

b is now 5

how about a = a + *ptr;

a is now 10

pointers can also be used to modify the data they point to

*ptr = 8; sets the value of a to 8

pointers can be reassigned like any variable

exercise: difference between ptr = &b; and ptr* = b;

first set ptr to contain the location of b

second changes the value of what ptr points to to be the value of b

exercise: does ptr = b; make any sense?

ptr now points to the memory location 5

will result in undefined behavior if ptr is dereferenced (probably segmentation fault)

swap example

write a function to swap two integers