Lab 3: Experiment 9

Assignment Expressions

Once we are able to declare variables, the next useful operation is to be able to change the value of those variables. We can change a value by reading in a new one from cin (as in Experiment #2), but we definitely do not want to get all of our information that way.

The value of a variable is changed most often through an assignment expression. Like an input or output expression, an assignment expression is complete in itself, so we can just tack on a semicolon for a statement:

Variable = Expression;

Here, Expression is any valid C++ statement and Variable is an identifier that has been declared as a variable, whose type matches that of Expression.

Looks familiar, doesn't it? It's a declaration without the data type.

When execution reaches this statement:

Expression is evaluated.
The value of the expression is stored in the memory associated with Variable.

To Declare, Not To Declare

For now, declare every variable once, just before you use it the first time. We'll see later on that there many situations where you'll use the same identifier in two or more declarations. We're not there yet. So, declare your variable once, and then use assignment statements to change the value.

Recognizing a declaration is simple: look for a data type. If you see a data type (e.g., int, double, char, etc.) in your statement, the compiler thinks you have a declaration.

Assignment Is an Operator

All operators in C++ actually evaluate to and return a value. Some operators also have a side effect, a change seen outside the statement. For example, the output operator as an operator evaluates to an output stream. Changing the contents of the screen is a side effect.

Similarly, an assignment operator has a side effect: change the contents of a variable. But it also has a value. Let's figure out what that value is by printing assignment expressions. Add this to your program, after your declaration of i:

cout << "555: " << ( i = 555 ) << endl
     << "-36: " << ( i = -36 ) << endl
     << "2+3: " << ( i = 2+3 ) << endl;

Question #3.9.1: What does this output statement display?

Assignment Chains

Because the assignment symbol = is an actual operator that produces a value in addition to making a side effect, we can chain it along:

i = j = k = 78;

This is known as assignment chaining.

But what are the values of i, j, and k after the assignment?

Modify your program to declare i, j, and k, initializing them to different values all less than 10. Right after this declaration, put in the assignment chain from above. Then print out the values of i, j, and k. Compile and run your program.

Question #3.9.2: What are the values of i, j, and k?

Surprised? Consider the implications of this result.

If = were left associative, the compiler would implicitly parenthesize the expression like so:

(((i = j) = k) = 78);

which seems a bit confusing. Sure, we'd start by assigning i the value in j, but then what? A tempting suggestion would be to assign the value of k to j, but that's not what the parenthesized code says. It turns out that this expression with these parentheses does compile in C++. Try it out.

Explicitly parenthesize the assignment chain as left associative. Recompile and execute the code.

Question #3.9.3: What are the values of i, j, and k?

Wacky. Doesn't seem particularly useful since there's a much simpler statement we could use instead.

Question #3.9.4: What's the simpler expression that would give us that same result?

Well, if the assignment operator is not left associative, it must be right associative ("middle associative" would be really wacky and too often ambiguous!):

(i = (j = (k = 78)));

Do the inner parentheses first: assign 78 to k. Sounds good. As an expression, that assignment evaluates to 78, so we can also assign that value to j. Sounds even better. Is i any different? Let's try it out.

Explicitly parenthesize the assignment chain right associative. Recompile and execute the code.

Question #3.9.5: What are the values of i, j, and k?

Terminology

assignment chain, assignment expression, side effect

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