Module 2

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Module 2

• Some Simple Programs

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What is a C Program?

• It is a collection of one or more functions
• Functions act as organizing groups, each doing a
  smaller task and working together to produce the
  desired result
• A C program must have a function called main()
• Execution always begins with the first statement
  in main()
• All other functions are subprograms and are
  executed only when called

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Implication for writing software?

• The natural way to organize your code is to have
  the main() function serve as an outline
• Keep functions relatively small (one to two
  screen fulls)
• Another term for function is methodI will use
  the term function, mainly, since it is consistent
  with our text

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The Smallest C Program
// This is a commentit is ignored by the
compiler! int main( ) //function
header //braces tell beginning or
ending return 0 //our first statement!
//everything inside and including
the //braces is the function body
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What is in a Function Header?
type of returned value
parameter listlist of inputs to function
int main ( )
name of the function
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So what does this program do?
// This is a commentit is ignored by the
compiler! int main( ) //function
header //braces tell beginning or
ending return 0 //our first statement! //e
verything inside and including the //braces is
the function body
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What Happens to the Comments?

• The compiler works in several stages, including
• Preprocessor
• Syntax recognition
• Semantics
• Optimizer
• One job of the preprocessor is to remove comments
• The preprocessor also notes which libraries to
  load and takes care of defined macros

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General Structure of C Programs
include ltstdio.hgt include myheaderfile.h defi
ne PI 3.1415927 using namespace std const type
identifier type identifier function function
function
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Lets Start With Identifiers

• An identifier is a name you give to a data object
  (variable or constant) or function
• C is case sensitive
• Use meaningful identifiers!

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What does this function do?
int myfunction( int t, int v ) int t p t
v return p
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How about this one?
// Function calculate_pressure calculates
pressure from // inputs temperature and volume
and returns the value // of pressure. int
calculate_pressure( int temperature, int volume
) int pressure pressure temperature
volume return pressure
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Rules for Identifiers

• All elements of C follow rigid rules
• Remember, the computer is stupid!
• Identifiers must start with a letter or
  underscore, which is followed by zero or more
  letters, digits, or underscores
• Spaces not allowed they are delimiters
• Oddball characters not allowed

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Data Types
simple
structured
address
integral enum floating
array struct class union
pointer reference
char short long int
float double long double
unsigned
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Standard Data Types for C/C

• Integral Types
• Represent integerspositive and negative values
  without fractions
• Declared as int, short, or long eg 4578,
  -4578,0
• Floating Types
• Real numbers with a decimal point
• Declared as float or double eg 95.33, 95.0, 0.0
• Characters
• Represent single characters
• Declared as char eg B, d, ?,

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Putting Identifiers with declarations yields

• A variable is a location in memory which we refer
  to by the identifier it is given we can store
  data of the appropriate type here!
• Declaring a variable means specifying both its
  name and its data type
• Examples

int ageOfDog float taxRateY2K
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What a Variable Declaration Does

• A variable declaration tells the compiler to set
  aside enough memory for the item to be stored
• Different variables require different amounts of
  memory
• For example, the float might require four bytes
  and the integer two bytes

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Note on Declarations

• You can put more than one on a single line
• This makes adding comments harder, so dont abuse
  this privilege

int money, monie, moonie