Question

Suppose a, b, and c are int variables and a = 5 and b = 6. What value is assigned to each variable after each statement executes? If a variable is undefined at a particular statement, report UND (undefined). $$\begin{array}{l} a=(b++)+3 i & \text {___} & \text {___} & \text {___}\\\ c=2+a+(++b) i & \text {___} & \text {___} & \text {___}\\\ b=2 *(++c)-(a++) i & \text {___} & \text {___} & \text {___}\end{array}$$

Short answer

After execution: a = 10, b = 31, c = 20.

Step-by-step solution

Understanding the Execution Order

We'll begin by understanding the operations in the context of the expressions. In C/C++, the `++` operator has both a postfix and prefix form. `b++` increments `b` but returns its original value, while `++b` increments `b` and returns the updated value.

Executing the First Statement

Initially, `a = 5`, `b = 6`, and `c` is undefined. The first statement is `a = (b++) + 3`. Here, `b++` uses the current value of `b` (6), then increments `b` to 7. Thus, the expression becomes `a = 6 + 3`, which assigns `a` the value 9. After this statement, `a = 9`, `b = 7`, and `c` is still undefined.

Executing the Second Statement

The next statement is `c = 2 + a + (++b)`. Now, `++b` increments `b` to 8 and returns 8. Substitute the values: `c = 2 + 9 + 8`, giving `c = 19`. After this step, `a = 9`, `b = 8`, and `c = 19`.

Executing the Third Statement

The final statement is `b = 2 * (++c) - (a++)`. First, `++c` increments `c` to 20. The expression evaluates to `b = 2 * 20 - 9` since `a++` uses 9 and then increments `a` to 10 afterwards. This simplifies to `b = 40 - 9`, so `b = 31`. After evaluating this statement, the values are `a = 10`, `b = 31`, and `c = 20`.

Key concepts

Prefix and Postfix Increment

In C++, the `++` operator can be used in two different ways: as a prefix and as a postfix. Both are used to increase the value of a variable by 1. However, they differ in the timing of when the increment is visible. Understanding this difference is crucial for predicting the outcome of expressions.

• Postfix Increment (`b++`): This operation increases the value of `b` by 1, but not before the current value is used in the expression. For example, if `b` is initially 6, then `b++` in an expression will use the value 6, and only after that, `b` is incremented to 7.
• Prefix Increment (`++b`): This operation also increases the value of `b` by 1, but the increment occurs before the expression is evaluated. So if `b` is initially 6, `++b` will first increment `b` to 7 and then use this new value in the expression.

Being familiar with these subtleties ensures that you can correctly interpret expressions and understand the difference they can make in the sequence of operations.

Variable Assignment

Variable assignment in C++ is a critical concept where a value is assigned to a variable. Typically, an assignment expression, like `a = b++`, not only changes or initializes a variable but can also influence how the increment operators function. Given the exercise, understanding assignment helps discern why certain operations yield specific results.

• Order of Assigning: In the expression `a = (b++) + 3`, the assignment of `a` depends first on evaluating the value from the expression `(b++) + 3`. The postfix `b++` allows the current value of `b` to be used in the assignment, followed by an increment.
• Values Persisting After Operations: Assignments reflect the results of any operations that have taken place. After `a = 9`, if we continue with `c = 2 + a + (++b)`, `a` retains the value 9 while `b` is incremented prior to evaluation.

Recognizing the effect of operations on variable assignment prepares students to expect changes and maintain variable integrity throughout code execution.

Code Execution Order

The execution order in programming, particularly with complex expressions, affects the final output. It defines the sequence in which operations and evaluations occur.

• Order of Operations: Expressions in C++ follow a set precedence where certain operations are carried out before others. In `c = 2 + a + (++b)`, `++b` is executed before other arithmetic operations, affecting their outputs.
• Understanding Sequential Execution: Considerations must be made for each expression executed individually. For instance, the assignment `b = 2 * (++c) - (a++)` shows that `++c` happens first, followed by multiplication and subtraction, and finally, `a++` influences the overall result.
• Impact on Variables: Changes in variable values during execution can lead to different results if order is not carefully understood. Therefore, knowing when and where increments occur is crucial for predicting outcomes.

Grasping how the sequence of execution impacts results ensures students are capable of debugging and understanding sophisticated expressions.