What Gets Printed to the Screen?

//assume below statement are part of a main function

int oranges = 5;

int apples = 3;

int numFruit = apples + oranges;

cout << "Total Fruit: " << numFruit << endl;

apples = apples - 2;

oranges = apples;

cout << "Apples: " << apples << endl;

cout << "Oranges: " << oranges << endl;

Input and Output Cont'd

• Another way to enter data into a variable is to read it from the console
• The input console (keyboard) is called cin (console input)
• We use the >> operator with cin to send data to a variable
• For example:

  cin >> numberOfDragons;

• In this example, whatever valid integer number the user types is stored in the variable numberOfDragons

Output Using cout

• cout sends data to standard output, which is usually the terminal (console) window
• The "<<" takes data from the right-hand side and sends it to the console
• Any data can be output to the console
  • Variables
  • Literals
  • Expressions (which can include all of above)
• We can display multiple values in one cout
• Each data item must be separated with a << operator
• For example:

  int numberOfGames = 12;
  cout << numberOfGames << " games played." << endl;
  

• Three values are output:
  • The value of the variable numberOfGames
  • A literal string " games played."
  • An end-of-line (newline) character
• In the above example endl is a special word for an end-of-line (newline) character
• A second way to output an end-of-line (newline) is to use '\n'
• For example:

  int numberOfGames = 12;
  cout << numberOfGames << " games played.\n";
  

Prompting Users

• Good programming practice is to always "prompt" users for input like:

  cout << "Enter number of dragons: ";
  int numberOfDragons;
  cin >> numberOfDragons;
  cout << "You entered " << numberOfDragons
       << " dragons\n";
  

• Note that we do not put a newline after the prompt
• The prompt waits on the same line for keyboard input as follows:

  Enter number of dragons: _

• Where the underscore above denotes where keyboard entry is made
• Every cin should have a cout prompt before it

Activity 5.1: Calculating Your Weight on the Moon (10pts)

• The objective of our program is to calculate a user's weight on the moon.
• Find a partner and open up Eclipse.
  
• Create a new C++ project named Moon with a file named moon.cpp.
  
• Alter the block comment at the top of your program to include your name, your partner's name and the section information.

• Run your program to make sure it works and you get the output you expected.
• Now, we need another cout statement to prompt the user to enter his or her weight. Add this statement right below the lastcout statement.
  

• Make sure that you have left a space right before the second set of quotation marks to allow space for the user to type his or her answer.
• Now, run your program again to ensure you haven't made any errors.
• Next, we need to add a cin statement to capture the user input and store it in the weight variable. Be careful when typing the arrows that they are pointing in the correct direction (towards the variable).
  

• Using the weight entered by the user, we need to calculate the user's weight on the moon. To calculate his or her weight on the moon, we divide his or her weight on earth by 6. Write the following statement:

• Finally, we need to display our results to the user. Add a new cout statement like the one below:
  

• Run your program to calculate your weight on the moon!
• Submit your activity to Catalyst when you are finished. Note: Both partners need to submit for full credit.
  

• Within the curly braces of the main() function, declare two double variables named a and b, and assign them a value of 7 and2 respectively. For instance:

  double a = 7, b = 2;

• Add a line of code to display the arithmetic expression (a + b) and then recompile and run the program.

  cout << "a + b = " << a + b << endl;
  

  The output when you run the program should look like this:

  a + b = 9
  

  If you do not see this output, please ask a classmate or the instructor for help.

• Add three more lines of code like the previous one that computes the expressions: a - b, a * b and a / b. Compile and run your program again and make sure your program now displays the following output:

  a + b = 9
  a - b = 5
  a * b = 14
  a / b = 3.5
  

• The order of operations matters in C++ just like it does in algebra. Multiplication and division are performed before addition and subtraction. Add the following two statements to your program:

  cout << "a + b / 2 = " << a + b / 2 << endl;
  cout << "(a + b) / 2 = " << (a + b) / 2 << endl;
  

• Compile and run your program again and compare the output. Your program should now display the following output:

  a + b = 9
  a - b = 5
  a * b = 14
  a / b = 3.5
  a + b / 2 = 8
  (a + b) / 2 = 4.5
  

  Note how the output of the two statements is different. You can change the order of operation using parenthesis, just like in algebra.
  

  As you can see, arithmetic in C++ works much like you would expect. However, there are some mysteries when working with integer variables which we will now explore:

  • Truncation in integer division
  • Modulus (%) operator
  
  
• Truncation in integer division: Change the data type of the two variables from double to int, like this:

  int a = 7, b = 2;

• Compile and run your program again and compare the output. Note how the result of the division operation changed. What happened to the decimal part of the result?

  In programming terms, we say that the decimal part is truncated (cut short). You have to watch out for this in C++ programming or you may get unexpected results in your calculations. 
  

• Modulus (%) operator: Sometimes we want the integer remainder from an integer division. To see the integer remainder, we use the modulus (%) operator. Add the following statement to your program:

  cout << "a % b = " << a % b << endl;
  

• Compile and run your program again with this added statement. Your program should now display the following output:

  a + b = 9
  a - b = 5
  a * b = 14
  a / b = 3
  a + b / 2 = 8
  (a + b) / 2 = 4
  a % b = 1

• Mathematical functions: More complex mathematical operations require the use of a function in C++. one such function is sqrt(number) which calculates the square root of the number inside the parenthesis. Add the following statement to your program:

  cout << "sqrt(a + b) = " << sqrt(a + b) << endl;
  

• You program will not compile with this new statement because you must include a library of the mathematical functions. Add the statement: #include <cmath> to the top of your program like this:

  #include <iostream>
  #include <cmath> // math function library
  using namespace std;
  

• Compile and run your program again with this added statement. Your program should now compile and display the following output when run:

  a + b = 9
  a - b = 5
  a * b = 14
  a / b = 3
  a + b / 2 = 8
  (a + b) / 2 = 4
  a % b = 1
  sqrt(a + b) = 3
  

  For more information on mathematical functions see the section below.

• Save your program source code that displays all eight (8) expressions and submit it to Catalyst.
  

Completed Program

Once you are finished, your source code should look like the following: