Question

What is the output of the following program segment? int count \(=5\) while \((--\text { count }>0)\) cout \(<<\) count \(<<\) " \("\) cout \(<<\) endl

Short answer

The output is "4 3 2 1" followed by a newline.

Step-by-step solution

Initialization

The program initializes an integer variable `count` to 5. This sets the starting point for the while loop.

Pre-Decrement and Condition Check

The while loop checks the condition `(--count > 0)`. This means `count` is decremented before the comparison. Initially, `count` is 5, so `--count` makes it 4. The comparison `4 > 0` is true, so the loop proceeds.

First Iteration Output

In the first iteration of the loop, `count` is 4 and this is printed to the output, followed by a space due to the syntax `cout << count << " ";` which results in `4 `. The loop then repeats.

Second Iteration

In the second iteration, `count` is decremented again to 3 (since pre-decrement is used). The condition `3 > 0` is true, so it prints `3 ` to the output and loops again.

Third Iteration

During the third iteration, `count` becomes 2 (after pre-decrement). The condition `2 > 0` holds true, printing `2 ` to the output, and the loop continues.

Fourth Iteration

The fourth iteration sees `count` decremented to 1. The condition `1 > 0` is true, so it prints `1 ` to the output.

Fifth Iteration and Loop Termination

On the fifth attempt, `count` turns to 0 after pre-decrement. The condition `0 > 0` is false, so the loop terminates. The program exits the loop. Finally, `cout << endl;` prints a newline character (end of line).

Key concepts

Pre-Decrement Operator

In C++, the pre-decrement operator `--` is an operation that decreases the value of a variable by one before it is used in any further expression. In the context of our program segment, the `--count` expression decreases the value of `count` from its current value by 1 before being evaluated for the condition check in the while loop. This is crucial because it means the decrement occurs before any comparison or output operation. For example, if `count` starts at 5, the pre-decrement results in `count` being 4 before the condition `(--count > 0)` is evaluated. This means that throughout the loop, we operate on the decremented value immediately, affecting each subsequent operation or condition based on this updated `count` value.

Output of Program

The program's output is determined by executing the while loop while a certain condition remains true. Each iteration prints the current value of `count` followed by a space until the loop concludes. When the program starts, `count` is initialized to 5. The loop prints:

• First, `4` is printed after the pre-decrement checks that count goes from 5 to 4.
• Next, `3` is printed after counting down from 4 to 3.
• Then, `2` is printed after the variable moves from 3 to 2.
• Finally, `1` is printed when the decrement changes count from 2 to 1.

On reaching the fifth iteration, the pre-decrement results in `0`, ending the loop as the condition `0 > 0` fails.

Loop Iteration

Each cycle of the while loop is referred to as an iteration. In our program, the iteration flow is controlled by the condition `( --count > 0 )`. With each iteration, the pre-decrement operator modifies `count`, and the current value of `count` is printed before checking the condition again. Let's go over the loop:

• 1st iteration: `count` is decremented from 5 to 4, `4 > 0` is true, and `4` is printed.
• 2nd iteration: `count` goes from 4 to 3, `3 > 0` is true, and `3` is printed.
• 3rd iteration: `count` reduces to 2, `2 > 0` holds, printing `2`.
• 4th iteration: `count` is now 1, `1 > 0` remains true, thus printing `1`.
• The 5th iteration decrements `count` to 0, and `0 > 0` is false, terminating the loop.

This systematic approach ensures a predictable and traceable flow of output.

Condition Check

The effectiveness of a while loop relies on its condition check, which determines whether the loop continues or stops. Here, the check `(--count > 0)` plays the critical role of deciding continuity. At the start of each iteration, before any print operation, `count` is decremented by 1, and the new value of `count` is evaluated against zero. The resulting boolean value—true if `count > 0` or false if `count <= 0`—dictates loop execution. Initially, the check `4 > 0` supports continuation, leading the loop through four successful shifts of decreasing values until `count` hits zero. At this failure point (`0 > 0` being false), the loop ceases its operation. Understanding this check underlines its impact not only on dynamics but also on the termination of repeated tasks.