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 >> num; sum = num; while (num != -1) { cin >> num; sum = sum + num; } cout << "Sum = " << sum << endl;

Short answer

The final output is: Sum = 157.

Step-by-step solution

Initialize Variables

The code begins by reading an input value into the variable `num`. The value `38` is assigned to `num`, and then `sum` is initialized to be equal to `num`. Thus, initially, `sum = 38`.

Enter the Loop

The `while` loop checks if `num` is not equal to -1. Since `num` is currently `38`, the loop condition is true, allowing entry into the loop.

First Iteration

Within the loop, the next input value `35` is read into `num`. The `sum` is updated with this value, so now `sum = 38 + 35 = 73`.

Second Iteration

The loop checks the condition `num != -1` again, which holds true because `num` is `35`. The next input `71` is read into `num`, and `sum` is updated as `sum = 73 + 71 = 144`.

Third Iteration

The loop condition is checked, and since `num` is `71`, it is true. The next input `14` is read into `num`, and `sum` is updated to `sum = 144 + 14 = 158`.

Fourth Iteration

The loop condition `num != -1` is checked and holds true because `num` is `14`. The next input `-1` is read into `num`, and `sum` becomes `sum = 158 + (-1) = 157`.

Exit the Loop

Now, `num` equals `-1` causing the `while` loop condition to become false. Consequently, the loop exits without further iterations.

Output the Result

Outside of the loop, the code outputs the result with `cout << "Sum = " << sum << endl;`. Therefore, the final output on the console will be `Sum = 157`.

Key concepts

Variable Initialization

Variable initialization is a crucial first step in any programming exercise. In C++ programming, before you can use a variable, it should be declared, typically by specifying its data type and a name. However, initialization is just as important, which means assigning a value to the variable for the first time.
In the provided exercise, the program initializes two variables: `num` and `sum`. When the user inputs the first number, 38, it is stored in the variable `num`. This initial value is then assigned to `sum`, making the starting value of `sum` equal to `38`. Think of initialization as setting up your variables with starting values that the rest of your program can use to begin its calculations.

• Important: Proper initialization avoids undefined behavior, which can happen if you try to use a variable before giving it an initial value.
• Initialization helps control the logic flow in programs by setting expectations for the initial state.

While Loop

Loops are repetitive structures, and the `while` loop is one of the primary looping constructs in C++. A `while` loop repeatedly executes a set of statements as long as a specified condition is true.

In this exercise, the `while` loop begins checking if `num` is not equal to -1. This condition (`num != -1`) must be true for the loop to run. Upon entering, the program reads a new input for `num` and updates `sum` by adding `num` to it. The loop proceeds with these steps, requesting a fresh input each time, and continues looping until the input `num` is `-1`, which signals the end of input.

• A `while` loop tests the condition before each iteration, making it possible that a loop may never run if the condition is false initially.
• Be careful to ensure the loop will eventually terminate; otherwise, it could become an infinite loop.
• Understanding loop conditions and updates within the loop is crucial for predicting and controlling output.

Input Handling

Handling input effectively in C++ is important for user interaction and feeding data into the program's logic. Typically, input is managed through the `cin` object, which reads data from the standard input stream, usually the keyboard.

In this case, `cin >> num;` reads integers into the variable `num`. This is the primary way the program receives new numbers during each iteration of the loop. Good input handling ensures that the user's data is safely stored and used correctly throughout the program.

• It’s crucial to understand that `cin` will only work as intended if the user inputs data of the expected type; otherwise, errors may occur.
• In this program, entering `-1` acts as a flag to stop further data collection, effectively handling multiple entries until the endpoint is reached.
• Pay attention to input prompts and expected formats when writing or running C++ programs for effective input handling.

Code Execution Steps

Code execution steps are a sequence of actions that the CPU follows to execute a program. Understanding these helps developers predict and debug their software.
In this exercise, the code execution begins with initializing variables. Then, inputs are processed through a loop, updating values as specified.

Below is a simple breakdown of the execution steps:

• Input 38: The first input value initializes `num` and `sum`. After this setup, sum equals 38.
• Read and Accumulate: The next inputs are read into `num`, and each time `sum` is updated by adding the current `num`.
• Loop Control: The loop stops executing when the condition (`num != -1`) turns false. This efficient control prevents processing invalid input values.
• Output Result: After exiting the loop, the final result of `sum` is printed to the console.

By understanding these steps, programmers can better grasp how inputs are transformed through loops to yield the final output.