• What is a void function and how does it differ from a non-void function?
  
• What is a function prototype and what is its purpose?
• Where do you place a function prototype in your program?
• What is the syntax of a function prototype?
  
• How do you properly comment a function?
  
• How can you call one function inside of another function?

Void Functions

• Previously we looked at functions that returned one value
• Functions returning a value use a return statement

  return result;

• A function that returns no value is called a void function
• In C++, void functions are defined like functions that return a value
• However, the keyword void replaces the return type
• For example, what do you notice that is different about the following?

  void displayDegrees(double degreeFarenheit) {
      double degreeCelsius = 5.0 / 9 * (degreeFarenheit - 32);
      cout << degreeFarenheit
           << " degrees Fahrenheit is equivalent to "
           << degreeCelsius << " degrees Celsius." << endl;
      return;
  }
  

• There are only two differences between definitions for void functions and other functions:
  • void return type
  • return statement is optional and does not specify a value if used
• If no return type is specified, the function returns after executing the last statement
• Here is an example program using the void function shown above

Example Program With a void Function

#include <iostream>
using namespace std;

void displayDegrees(double degreeFarenheit) {
    double degreeCelsius = 5.0 / 9 * (degreeFarenheit - 32);
    cout << degreeFarenheit
         << " degrees Fahrenheit is equivalent to "
         << degreeCelsius << " degrees Celsius." << endl;
    return;
}

int main() {
    double fTemperature;

    cout << "Enter a temperature in Fahrenheit: ";
    cin >> fTemperature;
    displayDegrees(fTemperature); //Notice function call without assigning result to variable

    return 0;
}

When to Write void Functions

• When we use a non-void function, we are asking a question
• The function returns a value in response to our question

  cout << sqrt(9.0);
  

• When we use a void function, we are giving the computer a command

  displayDegrees(212);

• We do not expect or receive an answer

Common Errors With void Functions

• Note that we cannot call a void function from a cout statement
• For example, the following causes a compile error:

  cout << displayDegrees(fTemperature); // NO!

• The reason is that a void functions does not return a value and cout has nothing to print
• Similarly, we cannot call a void function in an assignment statement:

  double temp = displayDegrees(fTemperature); // NO!

• There is nothing to assign to the variable temp

Activity 17.1: Printing Squares (10 pts!)

• Remember our programs that used nested for loops to print out shapes.
  
• Let's write a similar program with a function that prints squares of different sizes for our user.
• Open up CodeBlocks and create a new C++ file called squares.cpp.
• Then, copy and paste the starter code into your file, save it and run it to make sure everything is working properly.

• Now, write a function that prints squares called printSquares(). Your function should take in an integer argument for the length of one side of the square and should return nothing.
• Call your function inside the while loop so that it will print out a square given the user input for the length of a side.
  
• Run the program again. Does it print out a square?
• When you are finished, upload your squares.cpp file to Catalyst.
• The output of your program should look identical to the sample output below (except user input will vary).

Activity 17.2: Prototypes and Comments (10 pts)

• Open up a text editor and create a new file named prototypes.txt.
• In the file, write the prototypes for the following functions.
• Then below each prototype, add an appropriate comment in the style shown above.

double areaTriangle(double base, double height) {
    double area = 0.5 * base * height;
    return area;
}

string myName(string firstName, char initial, string lastName) {

    string fullName = firstName + " " + initial + ". " + lastName;

    return fullName;

}

bool isLeapYear(int year) {

    if (year % 4 == 0) {

        return true;

    } else {

        return false;

    }

}

• When you are finished, submit your prototypes.txt to Catalyst.

• Functions may call other functions
• Within the body of one function, we can call another function call
• Functions can call other functions as often as needed
• We are already doing this when main() calls a function
• The following program calls a "helper" function to help calculate the BMI.
• Because calculating the BMI is a somewhat complicated process, it is helpful to create a second function to do part of the work for us.
• The heightToSquareInches function handles turning the height from feet and inches (such as 5'8") into inches squared (such as 68"²).
  

Example of Functions Calling Functions

#include <iostream>
using namespace std;

    
double calculateBMI(int heightinFeet, int heightInInches, double weight);
//Calculates a user's Body Mass Index

int heightToSquareInches(int feet, int inches);

//Converts height in feet and inches to height in inches squared

int main() {
    const double WEIGHT = 135.5;
    const int HEIGHT_FEET = 5;
    const int HEIGHT_INCHES = 8;
    double bmi = calculateBMI(HEIGHT_FEET, HEIGHT_INCHES, WEIGHT);
    cout << "Your BMI is: " << bmi << endl;

    return 0;
}

double calculateBMI(int heightInFeet, int heightInInches, double weight) {
    int heightInches2 = heightToSquareInches(heightInFeet, heightInInches);
    double bmi = weight / heightInches2 * 703;
    return bmi;
}

//helper function to convert the height from feet and inches to inches squared

int heightToSquareInches(int feet, int inches) {
    int heightInInches = feet * 12 + inches;
    int heightInInchesSquared = heightInInches * heightInInches;
    return heightInInchesSquared;

}

• Answer the questions from today's learning objectives