Lecture 05

Classrooms and Classes

You are already familiar with how classrooms and classes work:

classrooms hold classes
classrooms are permanent fixtures
classrooms can hold different classes at different times

Now imagine all of University Of Hawaii's classrooms side by side like this:

Same Thing With Computer Memory

Classrooms are to addresses as classes are to values:

Addresses

What are addresses?

location of computer memory
it's an overly simplified explanation, but it's all you need to know for programming

Why are addresses important for programming?

the C programming language allows the programmer the flexibility to manage memory
having a high level of control over memory allows C programs to be more efficient and take up less memory
other program languages take care of some memory issues for programmers

Values

What are values?

data
could be real number, integer, or character
things you have been storing in your variables

Why are values important for programming?

programs ultimately have to interact with some sort of value
values are important to humans
processing values (data) are what makes programs important

Addresses vs Values

What are values and addresses?

values are things like:
- 1 (integer)
- 1.1 (real number)
- a (character)
addresses are things like:
- 0x7fff68db4100 (hexidecimal number)

Why are values and addresses needed?

Values are used by humans
- though a value like "1" is still kept in computer memory as some other type of machine readable concept (bit, byte, hexidecimal, etc)
Addresses are used by computers
- addresses are represented in this way because computers work with binary values (i.e. 0/1, true/false, or on/off)

Analogy of Addresses and Values

Analogy of Addresses and Values
Concept	Programming World	Real World
Address	0x7fff68db4100	MSB114, 2500 Campus Rd, Honolulu, HI 96822
Value	1.234	EE160 Class

Things to note:

addresses are physical locations that should be treated as not changing, like classroom MSB114
values are transient data that can be modified, like MSB114 can house EE160 now and then later house another class like MATH101

Reference and Dereference Operators

What are the reference and dereference operators?

reference operator &
- &value_variable: "address of" value_variable
- read more at http://www.c4learn.com/pointer-address-operator-in-c-programming.html
de-reference operator *
- *pointer_variable: "value pointed by" pointer_variable
- read more at http://wiki.answers.com/Q/Dereferencing_operator_in_c
pointers
- int *pointer_variable: declare pointer_variable as a pointer to an object of type int
- in programming, variables can store integers, real numbers, characters, or addresses
- you have mainly been working with variables that store integers, real numbers, and characters
- pointers are variables that store an address
- pointers are usually used in conjunction with the de-reference operator *
- pointers are just variables used in a specific way
- you will be using pointers later on in the course
- read more at http://www.physics.drexel.edu/courses/Comp_Phys/General/C_basics/#pointers (Section 6. Pointers)

Example of Pointers, Reference (Address Of) Operator, and De-Reference (Value Pointed By) Operator

Examine pointer.c:

output of pointer.c:

===integer===
                   1: integer, value of 'integer' variable
      0x7fff81d4c12c: &integer, address of 'integer' variable
                    : *integer is a syntax error

===integer_pointer===
      0x7fff81d4c12c: integer_pointer, value of 'integer_pointer' variable
                   1: *integer_pointer, value of the object that the 'integer_pointer' variable points to
      0x7fff81d4c120: &integer_pointer, address of 'integer_pointer' variable

Step through the first part of the code (setting the information):

In the end, you have something like this:

Step through last part of the code (getting the information):

Pointer and Detour Analogy:

Takeaway:

Different Ways of Getting Values and Addresses
	To Get The Value	To Get The Address
using the variable `integer`	`integer`	`&integer`
using the pointer `integer_pointer`	`*integer_pointer`	`integer_pointer`

the value of the variable (integer) is the same as the dereferenced pointer (*integer_pointer)
the value of the pointer (integer_pointer) is the same as the address of the variable (&integer)

Things to note:

the pointer conversion specifier is %p
the * and & can be stacked like *(&integer)
the most likely pointer use cases are:
- initialize pointer: int *integer_pointer
- set to the address of a variable integer_pointer = &integer
- get the value of the variable that the pointer is pointing to *integer_pointer

Assignment

Homework Problem 5.1, at the end of this problem you should submit to Laulima:

a file named UHusername_05_1.c
example:
- zhaol_05_1.c
Write a program that takes in an integer with the scanf function and prints the integer out with a printf function
References:
- http://forum.codecall.net/topic/37537-how-to-use-scanf-in-c/ (just reading the first post is fine)
- http://en.wikipedia.org/wiki/Scanf_format_string
- http://www.cplusplus.com/reference/cstdio/scanf/
- https://docs.google.com/drawings/d/1eF9WWOYhbmuyrOaAhyeAZcC4cguEdlk4b_DOh7aEEY4/pub?w=2486&h=2110 (be aware of this "scanf phenomenon" for future assignments
Please use the following program template:

test case1:

Please enter an integer:
[user enters 5]
integer: 5

Homework Problem 5.2, at the end of this problem you should submit to Laulima:

a file named UHusername_05_2.c
example:
- zhaol_05_2.c
Write a program (based off of homework problem 5.1) that takes in an integer, decimal, and character and then prints them out
the program shall display the decimal value out to 5 decimal places by using the printf conversion specifier
the program shall accept only the first 6 digits of the integer entered by using the scanf conversion specifier

test case1:

Please enter a character, a decimal, and an integer:
[user enters b 1.23456 123456]
character: b
decimal: 1.23456
integer: 123456

test case2:

Please enter a character, a decimal, and an integer:
[user enters z]
[user enters 12345.6]
[user enters 123456789]
character: z
decimal: 12345.60000
integer: 123456