Statements?Functions

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Statements?Functions

• Big programs get complicated very fast. Small
  programs are simpler.
• Key CS idea Decomposition
• Break big complicated sets of instructions into
  smaller, simpler modules
• In C, these modules are functions
• function a separate named block of statements
  that you call by name in order to execute

a block of statements statement1
statement2 ...
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A function...

• ...is a "black box", a mini program with
• a simple set of inputs
• simple output
• a body of statements
• ...performs one well-defined task
• e.g. printf only prints to the screen

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Function essentials

• A function has
• a unique name
• a body
• some (optional) inputs (called arguments)
• one (optional) output (called return value)
• Just like variables, before a function can be
  used it must be
• declared ( specify name, arguments, return value
  type)
• defined ( write the body)

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Functions you know already

• main()
• the first (and last) function executed when the
  program runs
• printf(), scanf()
• You can invoke (call) other functions from within
  main()
• You can write your own functions which in turn
  may call other functions and so on.

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How to use existing functions

1. Use include directives to tell C where to find
   the declarations of these functionsinclude
   ltstdio.hgt / includeltXgt "search the usual
   places for file X"/include "coatrack.h"/
   include "X" "search current directory for
   file X"/
2. Call the functions by name in your
   programs.scanf("d",numhats)isFull
   hatCheck(numhats)

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How to write a new function

• Declare the function
• Write a function prototype that specifies
• the name of the function
• the type of its return value
• its list of arguments and their types
• Define the function
• Write the block of statements (body) of the
  function

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Declaring a function

• The syntax of a function declaration (formally
  called prototype) contains
• The type of the return value of the function
• if the function does not return anything, the
  type is void
• The name of the function
• same rules as for variable naming
• An argument list made up of the input variables
  names and types. This is enclosed in parentheses
• return_type function_name ( argument_list )
• Function prototypes are placed between the
  preprocessor directives and main().

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Defining a function

• The syntax of a function definition is
• return_type function_name ( argument_list )
• statements

similar to prototype but no semicolon
If the return type is not void, the function
must have a return statement which stops the
function and sets its output value
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Local variables

• Variables declared inside a function are
  meaningful ONLY within that function.
• This means that two different functions can use
  the same names for their local variables without
  conflict (just like two different people can have
  the same name)

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Global variables

• Variables declared outside of all functions
  (before main()) apply to ALL parts of a program.
• They can be used and modified in any function.
• When a global variable is modified, the new value
  is available anywhere in the program.
• Global variables should be avoided
• They make it difficult to split the program in
  separate pieces
• They make it difficult to debug the program
• They are almost never necessary.

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Examples of function prototypes

• double convert_F_to_C (double degreesF)
• function convert_F_to_C takes as argument a
  double variable named degreesF and returns a
  double value
• int gcd(int num1, int num2, int num3)
• function gcd takes three integers as arguments
  and returns an integer value.
• void make_tea ( double water, int tea_bags)
• This function does not return value. Its return
  type is void.

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Example without functions
includeltstdio.hgt int main() /Farenheit-to-Ce
lsius / double degF,degC, ratio printf("Ente
r degrees F ") scanf("lf", degF)
/ F-to-C conversion
/ ratio 5.0 / 9.0 / We will unify this
/ degC (degF-32)ratio / set of
statements to / /
make a new function. / printf("f degrees F
are f degrees C\n",degF,degC)return
0 What if we need this conversion step in
many different places in our program?
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Example with functions
includeltstdio.hgt double F_to_C (double
degreesF) int main() /Farenheit-to-Celsius
/ double degF,degC, ratio printf("Enter
degrees F ") scanf("lf", degF) degC
F_to_C (degF) printf("f degrees F are f
degrees C\n",degF,degC)return 0 double
F_to_C (double degreesF) const double ratio
5.0/9.0 return (degreesF-32)ratio
Declare the function (Function prototype)
Call the function when you need it.
Define the function.
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return statement

• A function returns a computed value back to its
  caller via a return statement.
• A function with a non-void return type must
  always have a return statement.
• Code that follows a return statement is never
  executed.

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return statement
includeltstdio.hgt double compute_average (int
num1, int num2) int main () double
average average compute_average(24,
68) return 0 double compute_average (int
num1, int num2) double average average
(num1 num2) / 2.0 return average
this becomes the value of that expression
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return statement
int main () printf("f\n", compute_average(24,
68) ) return 0 double compute_average (int
num1, int num2) double average average
(num1 num2) / 2.0 return average
A call to a function that returns a value
mayappear in a printf(). It is an expression!
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return statement
int main () print_info() return 0 void
print_info() printf("Name Vana
Doufexi\n") printf("Phone [phone]\n") re
turn / optional /