Question

Mark the following statements as true or false. a. In a counter-controlled while loop, it is not necessary to initialize the loop control variable. b. It is possible that the body of a while loop may not execute at all. c. In an infinite while loop, the while expression (the decision maker) is initially false, but after the first iteration it is always true. d. The while loop: j = 0; while (j <= 10) j++; terminates if j > 10. e. A sentinel-controlled while loop is an event-controlled while loop whose termination depends on a special value. f. \(\quad\) A loop is a control structure that causes certain statements to execute over and over. g. To read data from a file of an unspecified length, an EOF-controlled loop is a good choice. h. When a while loop terminates, the control first goes back to the statement just before the while statement, and then the control goes to the statement immediately following the while loop.

Short answer

a: False, b: True, c: False, d: True, e: True, f: True, g: True, h: False.

Step-by-step solution

Understanding Statement a

In a counter-controlled while loop, the loop control variable must be initialized. This is because the loop relies on this variable to determine how many times to iterate. Without initialization, the loop control variable may have an undefined value, causing unpredictable behavior or even an infinite loop.

Analyzing Statement b

Indeed, it is possible for the body of a while loop to not execute. This happens if the condition of the loop evaluates to false the first time it is checked. In that case, the loop is skipped, and the control passes to the statement immediately following the loop.

Evaluating Statement c

An infinite while loop occurs when the while expression is always true after the first iteration. However, it does not mean the condition starts as false. If it were initially false, the loop wouldn't execute. Therefore, this statement is false.

Checking Statement d

The provided loop: `j = 0; while (j <= 10) j++;` keeps incrementing `j` until it becomes 11. At this point, the condition `j <= 10` becomes false, and the loop terminates. The loop terminates when `j` is greater than 10, thus this statement is true.

Understanding Statement e

A sentinel-controlled loop is a type of event-controlled loop where the end of data is indicated by a special value (a sentinel). This description is accurate, so the statement is true.

Clarifying Statement f

A loop is indeed a control structure that allows a block of code to be executed repeatedly. This is the fundamental characteristic of any loop construct, making the statement true.

Considering Statement g

For reading from a file with an unspecified length, an EOF (End-of-File) controlled loop is suitable as it will continue reading until the end of the file is reached, making the statement true.

Interpreting Statement h

When a while loop terminates, program control directly transfers to the statement immediately following the loop body. It does not go back to the statement before the while statement. Therefore, this statement is false.

Key concepts

Counter-Controlled Loop

A counter-controlled loop is a fundamental concept in programming. It uses a loop control variable to determine how many times the loop should run. This is often implemented using the `while` loop.

• Loop Control Variable: This variable must be initialized before the loop starts. This initialization is crucial because the loop depends on this variable to know when to stop.
• Condition Checking: The loop control variable is checked against a certain condition after every iteration. Typically, the loop will continue as long as this condition is true.
• Increment/Decrement: At the end of each loop iteration, the loop control variable is usually incremented or decremented to bring the loop closer to its end.

Imagine counting from 1 to 10. You start at 1 (initialize), and you count up by 1 each time (increment) until you reach 10 (condition). If your counter does not start at the right point or does not change, you might end up with an infinite loop.

Sentinel-Controlled Loop

A sentinel-controlled loop is a type of loop that stops execution when a particular value, called a sentinel, is encountered. Sentinels are especially useful for processing sequences of data when you don’t know the length.

• Sentinel Value: This is a special value that is used to detect when the loop should terminate.
• Checking for Sentinel: Each time through the loop, you check if the current data value matches the sentinel. If it does, the loop stops.
• No Fixed Number of Iterations: Unlike counter-controlled loops, sentinel-controlled loops do not know in advance how many iterations they will execute. This makes them flexible and adaptable for various situations.

Think about counting jelly beans from a jar, where the last bean is a special color. You keep counting until you encounter the colored sentinel bean, which tells you to stop counting.

EOF-Controlled Loop

An EOF-controlled loop is designed to read data from files where the length is unknown beforehand. EOF stands for End-of-File.

• End-of-File Indication: Operating systems provide a way to detect when the end of a file is reached. The loop uses this to decide when to stop reading data.
• Data Handling: The loop continues to read and process data records until it reaches this EOF marker.
• Applications: This type of loop is particularly good for text processing and file management tasks where files vary in length.

Imagine reading a book until you reach the last page. An EOF-controlled loop will keep reading until it detects that no more pages are left in the book.

Infinite Loop

An infinite loop occurs when a loop repeats indefinitely because its condition is always true. This is often a logical error and can have severe effects, like making a program unresponsive.

• Intentional vs. Accidental: Infinite loops can be intentional, such as in server processes that always run, or they can occur accidentally due to a problem in the loop's condition or its increment/decrement logic.
• Detecting Infinite Loops: Errors might show up as high CPU usage or the program becoming stuck, and debugging tools can help identify such loops.
• Preventive Measures: Often require careful logic design, ensuring conditions are correctly written, and loop control is accurately incremented or altered.

Think of a loop as a song on repeat. If the track never changes or stops, you might perfect your jam. But, in programming, infinite repetition usually signals you missed the stop button.

Loop Initialization

Loop initialization is a critical first step in setting up a loop. Whether it's a counter-controlled, sentinel-controlled, or EOF-controlled loop, initialization prepares the control variables and settings necessary for the loop to run correctly.

• Starting Conditions: Initialize variables that will be used in the loop, such as setting a counter variable to zero or preparing an input data stream.
• Setting Up Environment: This could involve opening files or establishing necessary data structures needed for the processing within the loop.
• Ensuring Correct Flow: Proper initialization can prevent issues like infinite loops or logic errors that can derail the intended completion of the loop.

Imagine preparing a race. Initialization is like setting up your track, starting blocks, and ensuring you're ready to sprint; without it, the race may not go smoothly.