Question

Write a program that will read up to ten letters into an array and write the letters back to the screen in the reverse order. For example, if the input is abcd. then the output should be dcba Use a period as a sentinel value to mark the end of the input. Call the array letter_box. For this exercise you need not use any functions. This is just a toy program and can be very minimal.

Short answer

Question: Write a program that reads up to ten letters, stores them in an array called "letter_box", and then displays these letters in reverse order. The input will end with a sentinel value (a period).

Step-by-step solution

1. Initialize Variables and Read Input

First, initialize an array called "letter_box" which will store the letters, a variable called "i" that will act as a loop counter and a sentinel value that is a period (`.`). Then, create a loop that will read individual characters from the user input until either the sentinel value is encountered or we reach the maximum allowed number of characters (10). Be careful about checking for sentinel values. Use getchar() to get characters.

2. Store the Input in an Array (letter_box)

Inside the loop, store the read character in the "letter_box" array at the index i, and increment the loop counter i, which gives the total number of characters read.

3. Reverse the Order of the Array

Now, since we need to reverse the array, we will have two loop counters, one starting from 0 (j) and the other from total characters read minus one (k = i-1). Next, create a loop where the values at the two indices j and k are swapped, after which j is incremented and k is decremented. Continue this process until j is less than k.

4. Display the Reversed Array

After reversing the array, create a loop to iterate through the array "letter_box" and print each character using putchar().

Key concepts

C++ Program Structure

Understanding the structure of a C++ program is fundamental to learning how the language operates. A typical C++ program consists of one or more functions, with the main() function being the entry point. In the context of our array reversal exercise, the program does not use separate functions, keeping everything within the scope of main().

The program starts with variable declarations, including the array letter_box for storing characters. It then transitions into the input loop where characters are read, followed by the reversal logic, and finally, a loop to display the reversed characters. Each of these steps is organized sequentially, reflecting a simple and clear C++ program structure without external function calls or complex control structures.

Character Arrays

Character arrays in C++ are a way to store sequences of characters, making them useful for handling strings and input/output operations. In our exercise, letter_box is a character array used to store up to ten letters input by the user. When working with character arrays, it's important to note that C++ strings are typically null-terminated; however, in this minimal example, we are using the period as a sentinel to signify the end of input.

It's essential to handle these arrays with care to avoid common errors like buffer overflows, which can occur if the array boundaries are not properly respected. In this case, by limiting the array to ten characters and checking for the sentinel value, we ensure safe operation within the array's bounds.

Loops in C++

Loops are a vital part of programming in C++, allowing repetitive execution of a block of code. This exercise makes use of loops in several key areas:

• The first loop reads user input until the sentinel value is entered or the maximum array size is reached.
• The second loop is employed to reverse the contents of the letter_box array.
• The final loop is used to display the array's content in reverse order to the user.

These loops illustrate the use of while and for constructs in C++, with loop control variables such as i, j, and k guiding the flow of execution. It's crucial to manage these variables correctly to ensure that loops execute as intended without causing infinite loops or missing iterations.

Using getchar and putchar

In C++, getchar() and putchar() are simple input/output functions, useful for reading and writing single characters, respectively. In our array reversal exercise, getchar() is employed to capture individual characters entered by the user until the period is encountered. It is a standard function in C++ that reads the next available character from the standard input stream.

Conversely, putchar() is used to output the reversed characters stored in the letter_box array. This function takes a single character and writes it to the standard output stream, allowing us to display the result character by character. Both getchar() and putchar() are part of the cstdio library and offer a more granular level of control compared to streaming operations with cin and cout, which are more commonly used for input and output in C++.