QMCS 130: Today

1
QMCS 130 Todays Class

• Exam Status
• Functions and C
• Using Functions
• Defining Functions
• Problem Solving with Decomposition
• Shelf Example (Lab 7)
• Lab 11

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Functions and C

• All C programs are functions
• main is a function and we always start from
  there
• We call functions to do outside work
• Math library, I/O library (printf, scanf)
• Functions make the work easier
• We can break programs into smaller pieces
  easier to write
• If a function works, we can reuse it without
  touching it
• Each time we touch/change something, we might
  break it

3
Using Functions

• What are examples of functions we use?
• How do we use them?
• What does main mean?

4
Using Functions

• Example functions printf, scanf
• Some in class have used pow and ceil
• Most functions take a list of parameters
• Also called arguments
• Tailors the result to specific input parameters
• Most functions return a value
• pow(x,y) returns the result of x raised to the
  yth power
• main returns an int, usually zero (its generally
  ignored)
• main as a function
• Declared an int function and usually returns a
  0
• No argument list (void)

5
Defining Functions

• Its safest to declare functions BEFORE main
• Not always essential, but it generally works best
• Why? Because then the compiler knows the
  function before it has to use the function
• Whats in the declaration?
• Consider main

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Defining Functions

• Header
• Data type returned int
• Functions name main
• Parameter list in parentheses
• A list of variable declarations, separated by
  commas
• Actual contents are in curly braces
• Just like main
• Contents are invisible to other functions,
• EXCEPT through passing arguments and results
• Always do a return to match the header

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Some Tricks with Variables

• What does it mean to pass arguments ?
• What if we re-use variable names?

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What does it mean to pass arguments ?

• Make a copy of the value of a variable
• The function uses a copy of the value
• We can change the value in the function, but it
  remains unchanged in the main

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What if we use the same variable names?

• We can define int i as often as we want in
  different functions
• Each is its own int i
• If we change one, it doesnt affect the others

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Problem Solving with Decomposition

• How do you solve a really big problem?
• How can we divide up a programming problem?

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Decomposing a Problem

• Make a list of the things we must do
• Identify inputs
• Identify outputs
• Identify calculations to yield those outputs
• Each step above might make a good function
• Or parts of those steps might make good functions
• If a job is isolated from other data and likely
  to be used more than once, then it should
  probably be a function

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Decomposition and Functions

• The tricky part is in variables
• Its easy to send data in (using the parameter
  list)
• It can be hard to get data out
• Good decomposition
• Minimize the number of variables and results
  passed back and forth between main and the
  functions
• Sometimes we just have to figure out what the
  right functions are to put in their own
  separate places
• To I/O or not to I/O a major decision for a
  function
• Dont do I/O unless thats the point of the
  function
• Pass results out via arguments instead more
  generally useful

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Decomposition should make things easier

• Shorter functions
• Fewer variables in individual functions
• Fewer loops in individual functions
• If a function is simpler, then
• Its easier to read and understand
• its easier to debug

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Decomposition Hassle Factors

• Extra typing for separate headers and such
• Requires thought and planning
• If you get a parameter list wrong, you may have
  to fix things in several different places
• Good design breaking it into the right pieces

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Shelf Example (Lab 7)

• Lets analyze the problem
• Lets identify some procedures
• Putting some procedures in Lab 7

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Looking at Lab 11

• Break it into the necessary pieces
• Function Inputs? Outputs?
• Header?

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