Lab 2: Experiment 5

The Program

For this experiment, you will write code for string variables.

Strings in C++ are represented using the string class. Unlike integers, real numbers, and characters, strings are not built into C++. They're added to the language through a library. A class defines what strings are. The name of the class is also the data type we need for our variables.

Since the string type is defined by a class, we must load a library to use the string data type. The library is conveniently named string, so we must add

   #include <string>

A string literal begins and ends with double quotes, with any number of characters between them (e.g., "a string", "another string"). Incidentally, it isn't necessary to include the string library if we're only using string literals — as we've done already in our program when we've used "value1 is " and "value2 is " to label output — but there's no harm in doing so.

String Declarations

Write declarations for string variables str1 and str2, initializing them to "Samson" and "Delilah", respectively. Do not get rid of any code from earlier experiments. Compile and execute your code.

Question #2.5.1: What is the declaration that you added to your program?

Once you can compile and execute the code without problems, add a statement to output str1 and str2. The original program contained the statement

   cout << "value1 is " << value1 << "\n"
        << "value2 is " << value2 << endl;

Now compile and execute the program. Make sure it outputs the proper labels and values for the variables.

Question #2.5.2: What is the ouput statement you just added to your program?

Now, as before, you will experiment with these declarations.

String Initialization

In the integer experiment, we tried initializing an integer variable with various different types. Let's try this again for the string variables you just added to your program.

Initialize str1 to an integer. Compile your program.

Question #2.5.3: What is the first error or warning message that the compiler gives you? If it does compile (with or without warnings), what does it output for the new initialization when you execute it?

Initialize str1 to a double. Compile your program.

Question #2.5.4: What is the first error or warning message that the compiler gives you? If it does compile (with or without warnings), what does it output for the new initialization when you execute it?

Initialize str1 to a character. Compile your program.

Question #2.5.5: What is the first error or warning message that the compiler gives you? If it does compile (with or without warnings), what does it output for the new initialization when you execute it?

Initialize str1 to be equal to cin. Compile your program.

Question #2.5.6: What is the first error or warning message that the compiler gives you? If it does compile (with or without warnings), what does it output for the new initialization when you execute it?

It appears that string doesn't like any of these other types. You may find that your compiler gives very convoluted error messages since string is a class.

Change your program back so that str1 is initialized to a string.

String Literals

Escape Characters

In the previous experiment, we took a look at escape characters. These characters can also be used in strings; in fact, we used a string literal "\n" in output statements to advance output to a newline:

   cout << "value1 is " << value1 << "\n"
        << "value2 is " << value2 << endl;

But it, as well as other escape characters such as \t, can appear anywhere in a string literal. For example the statement

   cout << "\n\tJohn Doe\nt123 ABC St.\n\tSome, Where\n\t\t\12345\n"

        John Doe
        123 ABC St.
        Some, Where
                12345

Everything we learned about escape characters as chars also applies to them inside strings.

Number of Characters in a String Literal

We initialized str1 and str2 with strings that had several characters in them. The preceding output statement has an even longer string literals.

But how small can we make the strings? One character? Let's try it. Initialize str1 to "a", and add this output statement to the program:

   cout << "The number of characters in str1 is " << str1.size() << "\n";

Question #2.5.7: Make a prediction: will this compile? If so, what will it display? If not, what error message will you get?

Compile and execute the program (if you can).

Question #2.5.8: If it compiled, what did it display? If not, what error message did you get? Compare the result with your prediction.

Keep in mind that the char type stores exactly one character, but that's not the only way to store a character---a string can have just one character in it. The output statements in the program use the string literal "\n"; as seen in the previous experiment, this is an escape character which really counts as one character. We could replace the double quotes with single quotes, but we'd have to change them back if we wanted to add more characters to those literals; for example, we might want to change the output statement to

   cout << "str1 has " << str1.size() << " characters.\n"

What about no characters? Can a string be empty? To find out, initialize str1 to "".

Question #2.5.9: Make a prediction: will this compile? If so, what will it display? If not, what error message will you get?

Compile and execute the program (if you can).

Question #2.5.10: If it compiled, what did it display? If not, what error message did you get? Compare the result with your prediction.

The literal "" is known as the empty string and wlthough it seems rather useless now, we will see later how it can be used when constructing a string from smaller strings.