C and Functions

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C and Functions

• A first look

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Function

• A function can have many input variables but only
  one (well defined) output variable.
• y 3x 1
• z xx xw ww
• y sqrt (x)
• y Arcsin(x)
• sometimes call many-one.

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Concept of functions in C
mysterious function
input1
output1
input2
input3
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Example pow
pow
double x
double x y
double y
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or
pow x to the y power
double
double
double
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Example scanf
scanf
int
format string

• one or more variables

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Example main
main
void
void
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How is this represented in C?

• double pow (double x, double y)
• void main (void)
• int scanf( string, variable1, variable2,..)
• the key to is the data types

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comments

• C only works with functions.
• main is a function.
• A function can use other functions.
• Other names for functions
• procedures (Pascal)
• subroutines (Fortran, Basic)
• methods (Java)
• include this brings in function libraries

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Functions are useful and fun!
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Why functions?

• A functions is a reusable block of code that can
  be shared.
• A function is a box that has input (called
  parameters) and at most one well defined output.
• The data type of the parameters and output must
  be known to use a function properly.
• You can use functions to breakup a program into
  smaller, easier to code modules.

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A function prototype

• A function prototype tells you about the data
  type of the input and output.

Input Output
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Function Prototypes

• Basic prototype
• outputDataType functionName ( inputDatatype1
  dummyVariable1, )
• examples
• double pow(double a, double b)
• void main (void)
• int product ( int a, int b)
• Note prototypes are terminated with

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Write prototype statements for the following
functions

• Display only your name on the screen
• void myNamePrint (void)
• Find the area of a circle given the radius
• double circleArea (double radius)
• Find the first non-zero digit of a student_id.
• int firstDigit (long id)
• Display the largest of 3 integers
• void printBiggest ( int a, int b, int b)

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Find the error in the following prototypes

• int funOne ( double x, float y)
• no ending
• void funTwo (void, void)
• Can not use void, void. Just use void.
• float funThree( float x, y)
• each dummy variable must have its own data type.

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Corrected prototypes

• int funOne ( double x, float y)
• void funTwo (void)
• float funThree( float a, float x)
• Proper C prototype is different. Though C and
  java uses dummy variables, C just uses the only
  data type in the prototype. However all work
  correctly.
• int funOne(double, double)
• float funThree (float, float)

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A function definition or how to build your own

• int product (int x, int y)
• int t
• t x
• t t y
• return t

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Some comments

• A function consists of
• the header which is similar to the prototype
  statement with no
• The function body
• return
• variables or parameters are never defined (x and
  y in this case)
• other variable, like t, are called local
  variables.

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More comments

• local variables can not be seen outside the
  function in which they are defined
• in this example the parameters are passed by
  value and can NOT be changed.
• there can only be one function called main

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Another look at product

• // A function definition or how it works
• int product (int x, int y)
• int t
• t x
• t t y
• return t

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Find the error in the following

• void fun (int x, int y)
• int z
• z xx 2xy yy
• return z

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Your own function in a program

• A function consists of two parts.
• the function prototype
• the function definition
• Common location in source code
• include
• one or more function prototypes
• main
• one or more function definitions

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Using a function

• Use the defined functions as you would any other
  function (like y sqrt(x), y sin(x))

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Factorial Function

• include ltstdio.hgt
• int factorial (int)

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• void main (void)
• int a, c
• a 7
• c factorial (a)
• printf("\nd factorial is d", a, c)
• printf("\nd factorial is d",9,factorial(9))
• getchar()
• return

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• int factorial (int x)
• int t, j
• t 1
• for(j 1 jltx j)
• t tj
• return t

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Fini