Question

Suppose that the input is 38 35 71 14 -1. What is the output of the following code? Assume all variables are properly declared. cin >> sum; cin >> num; while (num != -1) sum = sum + num; cin >> num; } cout << "Sum = " << sum << endl;

Short answer

The output is "Sum = 158".

Step-by-step solution

Initialization

The input starts with the number 38, which is assigned to the variable `sum`. This means that initially:\[sum = 38\]

Reading the First Number

The next number, 35, is read into the variable `num`. The value of `num` is now 35.

First Iteration of the While Loop

Since `num` is not -1, the while loop executes. Inside the loop, the first action is to add `num` to `sum`:\[sum = 38 + 35 = 73\]Then, the program reads the next number, 71, into `num`. Hence, `num = 71`.

Second Iteration of the While Loop

Again, since `num` is not -1, the while loop continues. Add `num` to `sum`:\[sum = 73 + 71 = 144\]Read the next number, 14, into `num`: `num = 14`.

Third Iteration of the While Loop

Continue since `num` is not equal to -1. Increase the `sum` by the current value of `num`:\[sum = 144 + 14 = 158\]Read the next input, which is -1, into `num`: `num = -1`.

Exit the Loop

Since `num` is now -1, the condition for the while loop is not satisfied, and the loop exits.

Output the Result

The program outputs the current value of `sum`, which is 158. The statement printed is: "Sum = 158".

Key concepts

Understanding While Loops in C++

While loops are fundamental control structures in C++ programming. They allow you to repeat a block of code as long as a specified condition remains true. In our example, the condition is checking if a variable `num` is not equal to -1.

• A while loop begins with the keyword `while`, followed by a condition enclosed in parentheses.
• The code to be repeated is within curly braces `{}` that follow the while condition.
• If the condition is true, the loop executes the enclosed code. This repeats until the condition becomes false.

In this exercise, the loop processes each number and accumulates totals until the `-1` is encountered, which triggers the loop to stop. It's essential to ensure the loop condition eventually becomes false to prevent infinite looping.

Handling Input and Output in C++

Input and output operations are crucial in C++ for interacting with users. Generally, we use `cin` for input and `cout` for output. In this exercise, the program uses `cin` to read numbers and `cout` to display the sum.

• `cin` is an input stream used to take input from the standard input device, usually a keyboard.
• `cout` is an output stream used for displaying output to the standard output device, typically a screen.

The `>>` operator is utilized with `cin` to assign user input to variables. Meanwhile, `cout` uses the `<<` operator to print text or variable values. Correctly using these streams is fundamental for any interactive program.

The Role of Variable Initialization

Variable initialization in C++ is the process of assigning an initial value to a variable. It is crucial as uninitialized variables can lead to undefined behavior and errors. In our exercise, the number `38` is read into the variable `sum` as its initial value.

• Initialization happens at declaration, or at the first opportunity a value is assigned.
• Using clear initial values prevents errors and improves code reliability.

In many programming tasks, especially involving loops, correctly initializing variables ensures they behave as expected during execution.

Exploring Control Structures

Control structures like loops and conditional statements are the building blocks of decision-making in programming. They guide the flow of a program based on conditions.

• Control structures include if statements, loops, and switch cases.
• They determine which parts of code are executed, how many times loops run, and what actions to perform based on conditions.

In this exercise, the while loop serves as a control structure to repeatedly process input until a termination condition (`-1`) is met. Mastery of control structures is key to writing efficient and effective programs.