CSC 107 - Programming for Science

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CSC 107 -Programming for Science

• Lecture 19
• Functions

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Question of the Day

• A landscaper plants 5 rows of 4 trees each, but
  only uses 10 trees. How is this possible?

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Todays Goal

• Todays lecture discusses writing using
  functions
• How to declare them, use them, trace them
• After todays lecture, you should be ready to
  write programs that include functions

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Functions

• Already written programs using functions
• Programs consist of at least one function
• main is a programmer-defined function
• Already know it can use pre-defined functions
• Can also use other programmer-defined functions

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Why We Use Functions

• Simplify code
• Parcel out each task to a function
• Compute commonly-used math function
• Each function can return a single value
• Perform input output
• Change arguments values (call-by-reference)

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Function Definitions

• Functions must be defined to use
• Otherwise, get Undefined Symbol error during
  compilation
• Functions defined outside of main
• All definitions have the same form
• return_type function_name(parameters)
  declarations statements

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Return Type

• Each function must declare a return type
• Can be any data type or void
• void used when not returning data
• Trying to return value will cause an error
• Otherwise, must return value of that type
• return expression ends function execution and
  returns result of expression
• Statement is required before function ends

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Function Body

• Begin by declaring variables it may use
• Already been doing this in main
• Variables created every time function called
• Variables destroyed when function finished
• Variables values not normally known at start of
  function
• No different than how main works
• Not changed by function having been called before
• Not changed by another function using variables
  of same name

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Function Parameters

• Parameters are another form of variable
• Declared in header between parentheses
• Functions can have 0 or more parameters
• 0 parameters noted using void keywordint
  main(void) ... int yourMommaWearsCombatBoots(v
  oid) ...double three(void) return 3.0
• Otherwise, function must list all parameters

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Function Parameters

• Each parameter must have type and name
• Just like the variables that they actually
  arevoid printInt(int printMe) ... int
  isLeapYear(int year) ... char dayOfWeek(int
  month, int day, int year) ... double
  nthRoot(double val, int n) ...
• Parameters also live for duration of call
• Values copied into them at function start
• Changing parameters value does NOT change other
  variables values

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Example Functions

• int fact(int n) int fact 1while(ngt1)
  fact factn n--// Could instead write
  return factreturn(fact)
• void printLine(FILE fout, int numStars) for
  (int i 0 i lt numStars i) fprintf(fout,
  )fprintf(fout, \n)

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Calling A Function

• Any function can call any other function
• We have already done this a lot from
  mainprintf, scanf, getchar, sqrt, fabs,
• No different calling programmer-written functions
• Call must include values (of proper type) for
  each parameter
• Function must be declared before the call
• Via include statements e.g. library functions
• Via defining it above any function calling it
• Via a function prototype

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Function Prototypes

• Function name, return type, parameters
• Line ends with a semicolon
• Similar to variable declaration
• Allows function calls to compile
• But get Unknown Symbol error if there is no
  function definition
• Usually listed at top of a file
• Just after include and define statements

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Your Turn

• Get into groups and complete daily activity

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For Next Lecture

• Continue programming assignment 2
• Hope that Prof. Hertz can revisit the 20th
  century and gets his electricity back