CSCI 230

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Department of Computer and Information
Science,School of Science, IUPUI
CSCI 230
Functions
Declarations
Dale Roberts, Lecturer IUPUI droberts_at_cs.iupui.edu
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Functions Support Decomposition

• Functions
• Category Programmers Duration Size of Code
• Trivial 1 1-2 weeks lt 500 lines (student
  homework)
• Small 1 - 3 Few Weeks 500 -2000 lines (term
  projects)
• Medium 2 - 5 Few Months 2000 -10000
  (research project)
• Large 5-25 1 - 3 years 10,000-100,000(current
  applications)
• Very Large 25-100 3 - 5 years 100,000 - 1M
  (real-time operations)
• Extremely Large gt 100 gt 5 years gt1M (advanced
  military work)
• Divide and conquer
• Large programs cannot be monolithic
• Construct a program from smaller pieces or
  components. These smaller pieces are called
  modules
• Each piece is more manageable than the original
  program

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Program Modules in C

• Functions
• Modules in C
• Programs combine user-defined functions with
  library functions
• C standard library has a wide variety of
  functions
• Math function, I/O, string function, such as
  printf(), scanf()
• Function calls
• Invoking functions
• Provide function name and arguments (data)
• Function performs operations or manipulations
• Function returns results
• Write function once and call it many times
• Function call analogy
• Boss asks worker to complete task
• Worker gets information, does task, returns
  result
• Information hiding boss does not know details
• Also called Encapsulation

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Math Library Functions

• Math library functions
• perform common mathematical calculations
• include ltmath.hgt
• gcc compiler requires lm parameter
• Format for calling functions
• FunctionName( argument )
• If multiple arguments, use comma-separated list
• printf( ".2f", sqrt( 900.0 ) )
• Calls function sqrt, which returns the square
  root of its argument
• All math functions return data type double
• Arguments may be constants, variables, or
  expressions

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Functions

• Functions
• Modularize a program
• All variables declared inside functions are local
  variables
• Known only in function defined
• Parameters
• Communicate information between functions
• Local variables
• Benefits of functions
• Divide and conquer
• Manageable program development
• Software reusability
• Use existing functions as building blocks for new
  programs
• Abstraction - hide internal details (library
  functions)
• Avoid code repetition

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Function Definitions

• Function definition format
• return-value-type function-name( parameter-list
  ) declarations and statements
• Function-name any valid identifier
• Return-value-type data type of the result
  (default int)
• void indicates that the function returns
  nothing
• An unspecified return-value-type is always
  assumed by the compiler to be int
• Parameter-list comma separated list, declares
  parameters
• A type must be listed explicitly for each
  parameter, unless the parameter is of type int
• Declarations and statements function body
  (block)
• Variables can be declared inside blocks (can be
  nested)
• Functions can not be defined inside other
  functions
• Returning control
• If nothing returned
• return
• or, until reaches right brace
• If something returned
• return expression

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Function Prototypes

• Function prototype
• Function name
• Parameters what the function takes in
• Return type data type function returns (default
  int)
• Used to validate functions
• Prototype only needed if function definition
  comes after use in program
• The function with the prototype
• int maximum( int, int, int )
• Takes in 3 ints
• Returns an int
• Promotion rules and conversions
• Converting to lower types can lead to errors

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1 / Fig. 5.4 fig05_04.c
2 Finding the maximum of three integers /
3 include ltstdio.hgt
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5 int maximum( int, int, int ) / function
prototype /

• Function prototype
• (3 parameters)
• 2. Input values
• 3. Call function
• Function definition

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7 int main()
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9 int a, b, c
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11 printf( "Enter three integers " )
12 scanf( "ddd", a, b, c )
13 printf( "Maximum is d\n", maximum( a, b,
c ) )
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15 return 0
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18 / Function maximum definition /
19 int maximum( int x, int y, int z )
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21 int max x
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23 if ( y gt max )
24 max y
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26 if ( z gt max )
27 max z
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29 return max
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Program Output
Enter three integers 22 85 17 Maximum is 85
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Examples
plus2(int x,y) int sum sum x
y return sum

• float plus1(float x,y)
• float sum
• sum x y
• return sum

• The minimal function, called a null function, is
  dummy()
• Again, the return statement can be
• return expression Example return aa
• return (expression) Example return ((a gt 0)? a
  -a))
• return
• In conclusion, to call a function from another
  function, you should
• Declare the type of the called function, if not a
  int function
• Use format of function_name(actual parameters)

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Acknowledgements

• Some examples were obtained from the course
  textbook.