Question

Consider the following code (and assume that it is embedded in a complete and correct program and then run): string s1, s2; cout << "Enter a line of input:\n"; cin >> s1 >> s2; cout << s1 << "*" << s2 << "

Short answer

Answer: A*string<END OF OUTPUT

Step-by-step solution

Identify the input

For this exercise, we are given the input: ``` Enter a line of input: A string is a joy forever! ``` The two strings to be read by the program are extracted from this input.

Extract the strings

As `cin >> s1 >> s2;` reads two substrings separated by whitespace and stores them in s1 and s2, the input line "A string is a joy forever!" will be read into two parts: ``` s1 = "A" s2 = "string" ``` The rest of the input is read but not stored in s1 and s2.

Format the next line of output

With s1 and s2 identified, we can format the next line of output according to the given code: ``` cout << s1 << "*" << s2 << "<END OF OUTPUT"; ``` In this case, this line will print the following output, with s1 and s2 replaced: ``` A*string<END OF OUTPUT ``` So, the next line of output for the given input will be "A*string<END OF OUTPUT".

Key concepts

Stream Extraction Operator

In C++ programming, the stream extraction operator, denoted by >>, is commonly used for input operations, particularly with standard input objects. It's a bitwise operator that is overloaded to read data from an input stream. When used with the cin object, it allows the program to accept input from the user. Think of it like using a scoop to extract certain portions of your breakfast cereal from a bowl.

Using our example, cin >> s1 >> s2; performs a very specific task. This line tells the compiler to expect user input and split it by whitespace. It will then assign the first word to the string variable s1 and the second word to s2. The operation continues until it hits a whitespace character, which includes spaces, tabs, or new lines.

It's important to note that the stream extraction operator will not extract characters beyond this whitespace delimiter. In our exercise, even though 'A string is a joy forever!' was entered, only 'A' was stored in s1 and 'string' in s2, due to the presence of space after each word.

Cin Object

The cin object in C++ is an object of class istream and stands for 'character input'. This object is used alongside the stream extraction operator to read input from the standard input device, which is most commonly the keyboard.

In the context of our example, once the prompt 'Enter a line of input:' is displayed via the command line output, the cin object waits for input from the user. This input is then processed by cin with the help of the stream extraction operator.

Knowing how cin behaves can help you understand and predict how your program will handle user input. For instance, knowing that cin skips leading whitespace but stops reading input at the first whitespace character can guide you in formatting user prompts and processing input correctly.

Command Line Output

Command line output in C++ is usually handled by the cout object, which stands for 'character output'. The cout object, used with the stream insertion operator <<, sends data to the standard output device, which is typically the screen.

The exercise showcases the use of cout to display the strings stored in s1 and s2, separated by an asterisk, and followed by a unique marker for the end of output. The resulting output A*string<END OF OUTPUT provides a clear understanding of how the data is being manipulated and represented to the user.

Efficient use of command line output is critical for interactive applications. It allows you to provide necessary feedback to the user, making your programs more intuitive and user-friendly. Remembering to provide clear and concise output can significantly improve user experience.