Section 5: Functions

Now that you know what a function is, let's look at function syntax. We've already seen that a function can take some inputs, do some stuff, and then produce an output.

The basic form of a function definition is this:

output function_name ((input_1, input_2, input_3, input_...) {
   // code to execute inside function
}

At this point, let's refine our sample function definition. When programmers talk about functions, instead of the word input they usually use the word parameter. A parameter to a function is nothing more than an input to a function. At the same time, instead of using the word output, programmers generally refer to the return of a function. A particular function "returns" a value. So, here is our updated function definition:

return_type function_name ((parameter_1, parameter_2, parameter_3, parameter_...) {
   // code to execute inside function
}

Notice that in place of output, the function definition says return_type. That's because when we are actually writing a function definition, we'll put the return type there, immediately preceding the name of the function. The return type is nothing more than a plain old variable type, such as int, or double, etc.

Similarly, parameters use variable types also. If the first input to a function is an int, then the first parameter will be something like int my_number. We'll see what my_number does in just a moment.

A Real Function!

Enough dilly-dally, let's see a real, working, C++ function that actually does something! Suppose we need a function that, converts a temperature from Celsius to Fahrenheit. Here it is:

double 
convertToFahrenheit(double celsius){
    double fahrenheit = ((9/5) * celsius) + 32;
    return fahrenheit;
};

Seems simple enough. Now let's put together a simple program that outputs some corresponding values for fahrenheit and celsius.

//include this file for cout
#include <iostream.h>

// function for changing the value 
// of celsius to fahrenheit
double 
convertToFahrenheit(double celsius){
    double fahrenheit = ((9/5) * celsius) + 32;
    return fahrenheit;
}

void
convertToFahrenheitAndPrint(double celsius){
	double fahrenheit = convertToFahrenheit(celsius);
	cout << "Celsius: " << celsius;
	cout << " Fahrenheit: " << fahrenheit << endl; 
}

int main() {

	// boiling point
	cout << "Boiling Point of Water: ";
	convertToFahrenheitAndPrint(100);
	
	// normal body temperature
	cout << "Normal body temperature: ";
	convertToFahrenheitAndPrint(37);
	
	// freezing point of water
	cout << "freezing point of water: ";
	convertToFahrenheitAndPrint(0);

	return 0;
}

I have included another function called convertToFahrenheitAndPrint. This function takes as a parameter a double (decimal number), and prints "celsius:" then the number (the celsius value), then "fahrenheit:" then the converted value for fahrenheit that it gets from the function convertToFahrenheit.

Try it out. Do you notice a bug? If you're not up on Celsius Fahrenheit conversions the right output is as follows:

Boiling Point of Water: Celsius: 100 Fahrenheit: 212
Normal body temperature: Celsius: 37 Fahrenheit: 98.6
freezing point of water: Celsius: 0 Fahrenheit: 32

If you typed in the above program as listed the results that you get are:

Boiling Point of Water: Celsius: 100 Fahrenheit: 132
Normal body temperature: Celsius: 37 Fahrenheit: 69
freezing point of water: Celsius: 0 Fahrenheit: 32

Congratulations! You've found you're first bug. So what's going on here? My college physics book says that to convert from Celsius to Fahrenheit you multiply the Celsius value by 9/5 and then add 32. Looking at the incorrect results, I've noticed that the difference between the Celsius value and the Fahrenheit value is always 32. Then I remember something! Look at the line that does the conversion:

double fahrenheit = ((9/5) * celsius) + 32;

double fahrenheit = ((9.0/5.0) * celsius) + 32;