Examples: User-Defined Functions

The following examples will all have similar formatting: there will be a problem statement, an extraction of pertinent information, and finally the function itself with comments following.  While it is always more beneficial to implement functions interactively, this is difficult when reading notes.  Therefore, I stress that you read the problem statement, and try the problem yourself.  If you get stuck, take a look at some of the discussion and continue.  It will hurt you in the end to simply read this page in a non-interactive fashion.
 

Example 1: square()

Write a function that returns the square of a given double value.

The statement is quite clear in terms of the description of the function.

  • We are passed one double value.
  • The operation is simple: multiply the given value by itself and return it.
  • Since we are passed a double and the square of a double results in a double, we have our return type.
    •  With this information, we can write the function.

    // Version 1
    double square(double num) {
      double temp = num * num;
      return temp;
    }

    There is actually no need to assign to a temporary variable and then return the variable.  We can simplify this function a bit:

    // Version 2
    double square(double num) {
      return num * num;
    }

    In the second version, we utilize the fact that we can return an expression from a function.
     

    Example 2: maximum()

    Write a function that accepts two integers and returns the maximum value of the two integers.

    The statement is clear.

  • We are passed two integer arguments.
  • The operation that will be performed is returning the maximum of the two integers.
  • This requires a return data type of integer since we are passed integers.
    •  With this information, we can write the function.

    int maximum(int A, int B)
    {
      if (A > B) return A;
      return B;    //By default, if A is not max, then B is.
    }

    Note that the arguments passed are separated by commas with each having their data type specified.

    Some questions.

    What code is executed if the two values are equal?
    How would you change the above function to return the minimum of the two integers?
     

    Example 3: charToInt()

    Write a function that returns the integer value of a given character.

    Again, the statement is quite clear in terms of the description of the function.

  • We are passed a single character.
  • The operation is simply returning the associated integer value of the given character.
  • This requires a return data type of integer.
    •  We write the function.

    int charToInt(char c) {
      return (int)(c); // Only a cast is required here.
    }

    This is actually a trivial function, however, it is a good exercise.
     

    Example 4: power2()

    Write a function that returns the power of two value of a given integer exponent.

    Here the statement is a little ambiguous in terms of the return type.

  • We are passed a single integer.
  • The operation is to return the power of 2.
  • I will choose integer as a return type, although a double may also be used.
    •  We write the function.  The function body may be a bit difficult; a loop must be used.

    int power2(int exp) {
      int count;
      int result = 1;

      for (count = 0; count < exp; count++)
        result *= 2;

      return result; // Only a cast is required here.
    }

    The loop body shows that we continually multiply some result variable by 2.  The initialization of the result variable is very important; this logic will not work unless it is initialized to 1.
     

    Example 5: printError()

    Write a function that prints the message "ERROR: problem here." to the screen.

    This is a perfect opportunity to use a void function.

  • We are not passed any information.
  • The operation is simply to print a message.
  • Since there is no value that is returned, we choose the void return type.
    •  The function:

    void printError() {

      cout << "ERROR: problem here." << endl;

      return;
    }

    Note that I have included a return statement although it can be inserted at the end of the function body; remember it is optional in void functions.