Lecture 15

Functions

What is a function?:

a function is a group of code that takes in some arguments (inputs) and returns a value (output)

Why is a function important?:

you can reuse code
- you can reuse the code inside the function
- you can reuse functions
you can update a function and everywhere that calls the function will get the updated code
you can give a group of code a name to express what the group of code is trying to do
- using functions with expressive names makes your code easier to read than using comments

When is a good time to utilize functions?:

whenever you find yourself copy and pasting code
whenever you find yourself wanting to add a comment to explain what a section of code is doing
- comments are notes to yourself and other programmers within the code
- like Post-It notes on a word document
- example usage:
  - // single line comment
  - ```
  /* multi-line comment: line 1
  multi-line comment: line2 */
```

How to utilize functions?:

define a function
- your_function is the function name
- double your_function tells the compiler that the function is going to return a value with type double
- your_function (int first_argument, double second_argument) is the argument list; the list of arguments (and the type of each argument) that the functions expects as inputs
call a function
- your_function(first_argument, second_argument) is calls the function
- your_function(first_argument, second_argument) are the arguments passed to the function
function prototype:
- a function can be declared first with a function prototype, called, and then defined
- when building a program using multiple .c files, function prototypes will make your life easier (and the program might not compile without it)
- the function prototype tells compiler the name of the function (e.g. your_function, the inputs of the function (arguments and argument type, e.g. int first_argument & double second_argument), and the output of the function (the type of the value, e.g. double your_function)
arguments are values or variables that is passed to the function
read more at http://www.mycplus.com/tutorials/c-programming-tutorials/functions/
read more at http://en.wikibooks.org/wiki/C_Programming/Procedures_and_functions; it's a long read but contains a lot of useful information
read more at http://www.tutorialspoint.com/ansi_c/c_using_functions.htm; it's a long read but contains a lot of useful information

An Example Program that Uses a Function

An example program (functions.c) that uses a function:

source code:

output:

Hi from main
-Hi from simple_function
-You passed in an argument with the value of 1
Bye from main
-Hi from simple_function
-You passed in an argument with the value of 2

things to note:
- simple_function is used in multiple places
- simple_function produced different outputs based on its inputs
- simple_function returns a value that can be used by the program later on
- simple_function's returned value does not always need to be used
- main is shorter and less cluttered; compare for yourself
- changes can be made in one place later on if things change

Scope

What is scope?:

scope refers to how and when an identifier (e.g. variables and functions) is accessible
read more at http://icecube.wisc.edu/~dglo/c_class/scope.html
read more at http://www.tutorialspoint.com/cprogramming/c_scope_rules.htm

Why is scope important?:

you and the program has a predefined way to mitigate naming collisions (i.e. calling a variable whose name is used to define variables in multiple places)
isolates variables and functions that should not be accessible, reducing the chance of unknowingly referring to a variable or function in some distant part of the program

Example Program to Illustrate the Effects of Scope

scope.c:

output:

[Start of Main] variable: 20 
[Start of function1] variable: 30 
[Start of function1] passed_in_variable: 20 
[End of function1] variable: 35 
[End of function1] passed_in_variable: 25 
[End of Main] variable: 20

Things to Note:

the function uses the value of the arguments; the actual variable is unaltered
to alter the actual variable, pass pointers to the function; more on this later

Another Example Program to Illustrate the Effects of Scope

global_scope.c:

output:

[In main] variable: 20 
[In function] variable: 10

Things to Note:

the more "local" variable "overrides" the more "global" variable
if there's no local variable the C language will search in outer function (main is also considered a function) until it reaches the global layer
use of global variables should be avoided; other functions might change the variable value unexpected and will cause bugs that are hard to debug
if you use global variables, try to store only values that is set at the beginning of the program and never modified afterwards (there's always exceptions to guidelines)

Write Your First Function

write a function that:

takes in an integer, a decimal, a character as arguments
print out their values
return the decimal number as the output of the function

Assignment (1/2)

Be sure to review the Pointers and Addresses lecture for next class

Homework Problem 15.1, at the end of this problem you should submit to Laulima:

a file named UHusername_15_1.c
- zhaol_15_1.c
fill in the add function in add.c so it returns the value of the sum of the two inputs:

Test Case 1:

Please enter in the first number:
[user enters 3]
Please enter in the second number:
[user enters 6]
The sum of your two numbers is: 9

Pass By Value

What is pass by value?:

pass by value is how you've been using functions

Why is pass by value important?:

as you've seen, passing by value to a function leaves the original variable unaltered
you could store the value returned by the function or choose not to use it or a function might not return a value

Pass By Value Example

pass_by_value.c:

What is happening when you pass an argument by value to the function:

Pass By Value Workarounds

How have we been updating the variables?:

variable = add_one(variable); will effectively update variable

How can we update multiple variables?:

write 2 functions and pass each variable separately, e.g. something like this:
pass_by_value_workaround.c:
or pass both variables by reference to the function

Pass By Reference

What is pass by reference?:

pass a reference (aka pointer) to a function, not just the value
- http://www.yukselgunal.org/c/pointers_in_func_args.pdf
- http://stackoverflow.com/questions/3829167/returning-multiple-values-from-a-function

Why is pass by reference important?:

it allows the function to update variable(s) directly

Pass By Reference Example

pass_by_reference.c:

What is happening when you pass an argument by reference to the function:

Pass By Value vs Pass By Reference

Let us summarize the main differences between pass by value and pass by reference:

Main Differences
	Pass By Value	Pass By Reference
calling the function	`function(main_variable);` (pass in the variable value)	`function(&main_variable);` (pass in the variable address)
defining the function	`function(int function_variable);` (initialize an `int` variable)	`function(int *function_variable_pointer)` (initialize a pointer to an `int`)
within the function	`function_variable++;` (process as a variable)	`(*function_variable_pointer)++;` (process as a pointer)
effect on variable	changes in the function will not affect the original variable	changes in the function will take affect on the original variable
analogous to	Person A passing information to Person B the information passed along is that the most awesome students at UH Manoa are in EE160 when Person B takes the information and says "EE160 students rocks", the students of EE160 will not be affected	Person A passing information to Person B the information passed along is that the most awesome students at UH Manoa are located in MSB114 when Person B goes to MSB114 and says "EE160 students rocks", the students of EE160 will be affected

Let us visualize the differences between pass by value and pass by reference:

Update Your First Function

write a function that:

takes in an integer address, a decimal, and an address to a character as arguments
modify the value at the integer address to be one more than its current value
modify the value at the character address to be 'a'
print out their values
return the decimal number as the output of the function

Assignment (2/2)

Homework Problem 15.2, at the end of this problem you should submit to Laulima:

a file named UHusername_15_2.c
- zhaol_15_2.c
fill in the add_now function in add_now.c so it increments both variables by 1:

Test Case 1:

Please enter in the first number:
[user enters 4]
Please enter in the second number:
[user enters 8]
The first number is now: 5
The second number is now: 9