Question

Rewrite the following program segment using a while structure rather than a repeat structure. Be sure the new version prints the same values as the original. num \(=100\) repeat: print (num) num \(=\) num \(-1\) until \((\) num \(>0)\)

Short answer

Translate repeat-until logic to precondition-based with 'while num > 0:'.

Step-by-step solution

Understand the Repeat Structure

The original segment uses a repeat-until loop, which will execute its body at least once and then continue as long as the condition is false. In this case, it prints the value of 'num,' decrementing 'num' each time, and stops when 'num' becomes less than or equal to 0.

Analyze the While Structure

A while loop executes its block as long as the provided condition is true. This means that the condition must be verified before the first block execution, which differs from the repeat-until loop structure.

Initialize the Variable

We maintain the initial assignment of 'num' before any loop begins. Therefore, start with: ```python num = 100 ```

Translate Repeat Loop to While Loop

Since the goal is to achieve similar behavior, convert the loop such that it runs while 'num' is greater than 0. The repeat-until condition ```(num > 0)``` prevents the loop from executing when false, so we'll use the opposite logic: ```python while num > 0: ```

Implement Loop Body

Copy the actions within the repeat-until loop into the while loop. These actions involve printing 'num' and then reducing 'num' by 1 for each iteration. The final code becomes: ```python while num > 0: print(num) num = num - 1 ```

Key concepts

Control Structures

Programming often requires executing different statements under specific conditions. These conditions are managed by control structures. They are the backbone of decision-making in programming languages.
Control structures allow developers to control the flow of execution in their programs by evaluating conditions and directing the program to execute certain blocks of code. These structures make software dynamic and responsive to various inputs or conditions the program may encounter. There are generally two types of control structures:

• **Conditional Structures**: These execute code based on certain conditions being true or false. Examples include `if-else` statements.
• **Looping Structures**: These repeat a block of code until a certain condition is met or no longer met. This is where loops like `for`, `while`, and `repeat-until` come in.

Knowing when and how to use these structures is critical for effective programming.

Repeat-Until Loop

The `repeat-until` loop is a control structure used to repeat a section of code until a specified condition becomes true. It's mostly found in languages other than Python, where its logic emphasizes executing a block at least once. In a `repeat-until` loop:

• The loop body executes before any condition is checked.
• The condition is checked after the loop executes, which is why it's always executed at least once.
• The loop continues repeating as long as the condition is false.

A typical usage might look like this: ``` repeat // code to execute until (condition is true) ``` This loop is useful if you need to guarantee that a block of code runs at least one time regardless of the conditions.

While Loop

In contrast to the `repeat-until` loop, a `while` loop checks the given condition before any code inside the loop runs. This means that if the condition is false from the beginning, the code inside the `while` loop might not execute at all. The basic structure of a `while` loop in most programming languages looks like this: ``` while (condition): // code to execute ``` In a `while` loop:

• The condition is evaluated before the loop’s code executes.
• The loop will keep executing as long as the condition is true.
• If a condition changes within the loop, that can affect future executions of the loop.

This makes `while` loops ideal for scenarios where the number of iterations is not known beforehand and depends on a dynamically changing variable.

Python Programming

Python is a high-level, versatile programming language. Known for its clear syntax and readability, Python has become a favorite among developers for scripting, web development, data analysis, artificial intelligence, and more. Key features of Python include:

• **Simplicity**: Python's syntax emphasizes readability and simplicity, making it an accessible entry point for beginners.
• **Versatility**: Python supports multiple programming paradigms, such as procedural, object-oriented, and functional programming. This makes it adaptable for various types of projects.
• **Extensive Libraries**: Python boasts a vast collection of libraries and frameworks, from data science packages like `numpy` and `pandas` to web frameworks such as `Django` and `Flask`.
• **Community Support**: A strong, active community supports the language, contributing to continuous improvement and resources for learning and troubleshooting.

For Pythonists dealing with loops, both `while` and `for` loops are fundamental, each suitable for various scenarios, and forming an essential part of the language's control structures.