Syntax of Functions

• Functions are declared in a two step process
  • A function prototype is a declaration of the function without the body.
    • It specifies the function interface (return type, name, parameters)
    • It is declared at the beginning of the program (with global variables)
    • type identifier (parameter list);
  • A function definition is a declaration of the function that includes the function header and the function body or code.
    • it is declared anywhere in the code.

    • type identifier (parameter list) {
          local variable declarations
      
          code
      
          return statement
      } 

• The header includes
  • The return type for the function.
    • We have seen this
    • int main()
    • float pow(float base, float exponent)
    • The return statement must return something of this type.
    • void is a type which means nothing is returned.
    • return; matches this type
    • Some types may not be returned, but we have not encountered any of these yet.
  • The identifier
    • This is the name of the function.
    • Any identifier will do but the conventions are
      • Start with an upper case letter
      • CamelCase
      • Have a meaning, describe what the function does.
  • Formal Parameter list
    • A formal parameter consists of three pieces
      • The type (int, float, ...)
      • variability
        • blank if the parameter can not be changed in the function
        • & if the parameter can be changed in the function
      • an identifier
    • if no parameters are needed the word void can be inserted.
    • If there are multiple parameters, they are separated by a comma.
  • Some examples:
    • A function to change a string to all upper case.
      • It will not return anything, it will change the parameter, so the return type is void
      • Name: ChangeToUpper
      • It will need a single parameter
        • The type is string
        • We want to change it, so an &
        • we will call it phrase.
      • void ChangeToUpper( string & phrase);
    • A function which takes a planet name and returns if it is a valid planet or not would take a string input and produce a boolean output
      • the type will be bool (true the name is valid, false it is not)
      • The name will be ValidName
      • There is a single parameter
        • a string
        • it will not change in the function, so no &
        • call it name
      • bool ValidName(string name);
    • A function which plays a guessing game as the guesser would pass no information
      • There is no return value, so void type
      • Name: PlayAsGuesser
      • There are no parameters
      • void PlayAsGuesser()
      • or void PlayAsGuesser(void)
• The function definition
  • Contains the function header
  • Plus a code block implementing the function
  • This includes
    • Local variable declarations
    • code
    • a return statement.
  • Examples

    • void ChangeToUpper( string & phrase) {
          int i;
      
          for(i=0; i < phrase.size(); i++) {
              phrase[i] = toupper(phrase[i]);
          }
      
          return;
      } 

    • bool ValidName(string name) {
          bool isValid = false;
      
          ChangeToUppper(name);
         
          // i don't like this, but I want it to fit on the screen 
          if (("MERCURY" == name) or ("VENUS" == name) or ("EARTH") == name) 
              or ("MOON" == name) or ("MARS" == name ) or ("JUPITER" == name)
      	or ("SATURN" == name) or ("NEPTUNE" == name ) or ("URANUS" ==  name)
      	or ("PLUTO" == name)) {
      	     isValid = true;
          }
      
          return isValid;
      } 

    • void PlayAsGuesser() {
           int low = LOW_VALUE,
               high = HIGH_VALUE;
         
           bool found = false;
           int guess;
           int result;
      
           while (not found and low <= high)  {
              guess = NextGuess(low, high);
      
              result = DoGuess(guess);
      
      	switch (result) {
      	   case RIGHT:
      	      found = true;
      	      break;
      	   case LOW:
      	      low = guess+1;
      	      break;
      	   case HIGH:
      	      high = guess-1;
      	      break;
      	}
      
           }
      
           PrintWin();
      
           return;
      }