Question

Given the following program segment: j = 2; for (i = 1; i <= 5; i++); { cout << setw(4) << j; j = j + 5; } cout << endl; write a while loop and a do...while loop that have the same output.

Short answer

Both loops execute the block once, outside the original for loop's iteration, due to the semicolon.

Step-by-step solution

Understand the For Loop

The given `for` loop with `i = 1; i <= 5; i++` does not actually iterate any actions due to the semicolon `;` after the loop, which means the loop body is empty. The code block that follows, incrementing `j` and printing it, is actually outside the loop, so it executes only once. This is a subtle nuance to recognize here.

Determine the For Loop Behavior

Although the loop condition is provided with the `for` loop, the actual behavior involves only initializing `j` to 2 and then executing the block `{ cout << setw(4) << j; j = j + 5; }`, resulting in printing 2, then increasing `j` by 5, just once.

Writing the While Loop

A `while` loop that mimics the behavior would require checking the same loop condition explicitly within the body. However, since the operation executes only once outside of the declared loop - effectively negating its repetition; the loop body only needs to mimic the same single execution. ```cpp j = 2; while (true) // Only to mimic a concrete loop step { cout << setw(4) << j; j = j + 5; break; // To ensure it runs only once as the original code } cout << endl; ```

Writing the Do...While Loop

For a `do...while` loop, we also mimic the operation of executing the block just once with similar structure. Remember, `do...while` always executes at least once. ```cpp j = 2; do { cout << setw(4) << j; j = j + 5; } while (false); // False to ensure singular execution cout << endl; ```

Key concepts

for loop

The `for loop` is one of the most versatile and commonly used looping constructs in C++ programming. It allows developers to repeat a block of code a specific number of times, usually when the number of iterations is known beforehand. The basic syntax of a `for loop` is as follows:

``` for (initialization; condition; increment) { // code block to be executed } ```

• Initialization: This part is executed once at the beginning of the loop. It is typically used to declare and set the initial value of a loop control variable.
• Condition: Before each iteration, this expression is evaluated. If it evaluates to true, the loop body will execute. If false, the loop terminates.
• Increment: This statement is executed after each iteration of the loop, effectively incrementing or updating the loop control variable.

In the exercise provided, the `for loop` had a semicolon immediately following it, which means the loop itself does not have any body and does not execute code, leading to the code under it running just once outside the loop. Understanding this nuance is essential because it highlights how critical syntax and structure are in programming.

while loop

A `while loop` is a control flow statement that allows repeated execution of a block of code as long as a specified condition is true. This type of loop is particularly useful when the number of iterations is not known ahead of time and is based on a condition.

Here's the general syntax of a `while loop`:

``` while (condition) { // code block to be executed } ```

• Condition: Before each loop iteration, the condition is checked. If it remains true, the loop continues to execute. If false, the loop stops.

In the problem, although we needed the loop to execute only once, we used a `while` loop to mimic the structure of a loop. In reality, since the intended behavior was to allow a single execution of the code block, the loop condition was set to `true`, and the loop was explicitly exited using a `break` statement. This construction demonstrates how `while loops` can be manipulated for specific purposes, even to run code just once by artificially constraining the exit condition.

do while loop

The `do while loop` is similar to the `while loop`, but with a key difference: it guarantees that the code block will be executed at least once. This is due to the nature of how the loop checks its condition.

The syntax of a `do while loop` follows this structure:

``` do { // code block to be executed } while (condition); ```

• Unlike the `while loop`, the condition is checked after the code block has executed. Therefore, the loop runs at least once, no matter what the condition initially evaluates to.

In the provided exercise, we demonstrated the `do while loop` to execute the print statement just once by setting the loop condition to `false`. The `do while loop` is beneficial when the code block needs to run initially regardless of condition checks, showcasing its use for executing initial actions before any condition checks.